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Brewing beer with gluten free cereal has placed much emphasis on rice research in the beverage industry. Owing to the fact that there is diversity of rice cultivars; its global cultivation, and the physical characteristic such possessing husk, and rich starch endosperm makes rice a potential raw material for brewing gluten free beer. However, selection of rice cultivars for brewing is an ever-continuous studies with research themes centred on malting conditions; specialty rice malt production; mashing programmes suitable for rice; enzyme activities and physicochemical properties of malted rice; oxidation stability and organoleptic properties of beer produced from rice. Due to its inadequate free amino acids, limited enzyme activity, and large husk proportion, rice malt has drawbacks when used to make beer. Addressing these restrictions has inspired the creation of rice malt grist, the use of enzymes to boost free amino acids, and the addition of protein rice defatted seed meal to rice malt. This review article examines recent developments in the use of rice malt, and rice kernel as base raw material and adjunct, respectively, for beer brewing, and their effects on the quality of the wort, enzyme activity, phenolic acid, antioxidant activity, and organoleptic properties.
Faculty of Agro-Industry, Department of Biotechnology, Kasetsart University, Bangkok, Thailand
Faculty of Agro-Industry, Fermentation Technology Research Center, Kasetsart University, Bangkok, Thailand
Martin Zarnkow
Research Center Weihemstephan for Brewing and Food Quality, Technical University of Munich, Freising, Germany
Gavers Kwasi Oppong
Plant Sciences and the Bioeconomy, Rothamsted Research, Harpenden, UK
Future Food Beacon of Excellence and School of Biosciences, University of Nottingham, Nottingham, UK
Ulaiwan Withayagiat*
Faculty of Agro-Industry, Department of Biotechnology, Kasetsart University, Bangkok, Thailand
Faculty of Agro-Industry, Fermentation Technology Research Center, Kasetsart University, Bangkok, Thailand
*Address all correspondence to: hellie.go@ku.th, hellie.gonu@hotmail.com and fagiulw@ku.ac.th
1. Introduction
Beer is a global consumed beverage which can be produced from diverse cereals (rice, buckwheat, maize, sorghum, barley and wheat), and based on the geographical location, processing technique may vary [1, 2, 3, 4, 5]. Nevertheless, most cereals used to produce beer are dried germinated seeds. In countries where there are no legislations restricting the type of cereal used for beer production, rice is an essential raw material for beer production especially in Asia, the hub of rice production [6].
Rice is a global staple food raw material with a significant role in food nutrition and security. Hence, there are significant efforts to breed and release new cultivars to meet consumers’ nutritional demand such as antioxidant obtained from pigmented rice in beverages [7] or vitamin enriched precursor rice cultivar (golden rice) [8]. Adapting processing technique such as partial germination of unpolished rice to increase functional properties of the rice such phenolic acid content [9], and gamma aminobutyric acid, a neurotransmitter compound [10]. Beyond this nutritional importance of rice to the food industry, it’s an important material in the beverage industry.
Rice utilization in the beverage industry can be dated to prehistoric event to the Han Dynasty in China between 202B.C and anno Domini (A.D.) 220 [11]. The production of liquor such as sake a traditional Japanese beverage [12], makgeolli (rice wine) [13], and sato [14] of Korean and Thailand origin, respectively are rice based product. In addition, polished broken rice grains are used as supplementary ingredient (adjunct) in beer brewing [15] for esthetic value, improved organoleptic properties and functional properties [7, 16]. In recent times, partially germinated dried rice seed (rice malt) are used as base-raw material for the production of beer [6, 17] and syrup [18] for gluten intolerant consumers. Gluten is a plant protein and a precursor for allergy among gluten intolerant persons.
The race for gluten free beer from rice led to significant research in optimizing condition for malting rice cultivars [19], examining physicochemical attributes essential for beer brewing [20] as well as developing specialty rice malt for different sensory properties [21]. However, brewing beer with rice has limitations such as low protein and free amino-nitrogen content in the unmalted [15] and malted rice [6] as well as inadequate endogenous enzyme activity produced during malting process [22]. These limitations have not deterred the quest to use rice for brewing, but elevated research on rice cultivars selection for the purpose of brewing gluten free rice beer in different geographical region [6, 23, 24]. Compared to other tropical cereals, rice is cultivated worldwide and the most diversified cereal. For these reasons, the advancement of all rice beer brewing research has been enormous as well as its use as an adjunct. This reviewed article looks at a detailed use of rice in beer brewing process, considering cultivars, malting techniques, brewing condition, functional and organoleptic properties of rice beer.
Rice is the most globally cultivated cereal with continuous release of new varieties which comes with pigmented or non-pigmented phenotypes; aromatic or non-aromatic; long grain, short grain, and broad grain as well as hybrids. These phenotypic variations of rice are either of Oryza glaberrima L. or Oryza sativa genotype from which over 120,000 cultivars are being bred from [25]. Hence, providing many options for rice utilization in meeting nutritional needs in growing regions and outside growing areas.
2.1 Rice structure
Hulled rice can be referred to as rice seed consisting of husk, bran (pericarp and aleurone layer) and kernel which engulfs the embryo. The rice structure composition makes it an appropriate raw material for brewing especially the presence of the husk unlike pseudocereals which lack husk as illustrated in Figure 1.
Figure 1.
Illustrates the rice plant (A) and the matured rice seed structure (B).
2.1.1 The husk and functional property in brewing process
The husk serves as filtering bed material which enables the separation of liquid phase to form the solid matrix in malt extract production by creating micro-sized pores which enable liquid to sieve through. For this reason, in the production of wheat, maize, sorghum malt beverage, rice husk is incorporated to aid in creating a space in the grain filtration bed formed from the endosperm of the cereal in a mash (water and milled cereal mixture) to prevents the formation of a gummy or porridge like matrix [26, 27]. The rice husk constitutes about 16–28% of the rice weight [28]. The husk is made from the modification of two leaves thus the palea (dorsal) and the larger lemma (ventral). The matured palea and lemma are responsible for the roughness and toughness of the rice husk. The rice husk is made up of hemicellulose (10–18%), lignin (25–29%), cellulose (33–45%) [29, 30] and an estimated 20% of silicon dioxide nanoparticle [31]. The hemicellulose is a polymer made from varying sugar linkages such as a xylose the main structure, arabinose, mannose or galactose. The cellulose structure is composed of polymerized glucose structured and lignin an integral part of the cell is formed via polymerization of phenylpropanoid monomer (guaiacyl units, p-hydroxyphenyl units, and syringyl units) [32]. Unlike many cereals such as wheat, sorghum, rye and maize, the rice husk is a key feature for its potential as a raw material for brewing.
The rice husk thickness and its weight fraction are relatively higher than those of barley seed (which is ideal for brewing purpose) and contains polyphenols comprising of flavonoids (3.08mg catechin equivalent/g), and phenolic acids (14.90 mg gallic acid equivalent (GAE)/g) which exist in both free and bound forms. These compounds provide an astringent feeling in rice malt extract and beer. Hence, to overcome this drawback, the proportion of rice husk prior to brewing process can be reduced. Some phenolic acids including chlorogenic acids pyrogallol, coumaric, sinapic acids and ferulic acids, p-hydroxybenzoic acid, protocatechuic acid, vanillic, syringic, caffeic acids, and gallic acids are present in the free and bound phenolic extracts of rice husk [33, 34, 35]. However, the amount of rice husk phenolic acids is dependent on the cultivar variety and phenolic extraction method. This seed structure has abundant coumaric acids relative to other phenolic acids [33], and also possess flavonoids such as myricetin and apigenin [35, 36]. Comparatively, the total flavonoid content in rice husk is about 3–6 folds lesser than total phenolic acid content [34, 36].
2.1.2 The aleurone layer
The aleurone layer is a key functional structure during seed dormancy and germination in malting process. The aleurone layer is a sheet of cells which are stacked to form 3–4thick cell layers that surrounds the endosperm of the rice [37]. The aleurone structure is reported to exist in two forms; aleurone that engulfs the endosperm and aleurone around the embryo. The aleurones differ based on cytoplasmic cell arrangement, density and rice cultivar [38, 39, 40]. During partial germination of rice seed, the deoxyribonucleic acid present in the aleurone expresses hydrolyzing enzyme; alpha amylase, beta amylase, limit dextrinase, and protease [19, 22, 41]. The synergistic interaction of the secreted enzyme and phytohormone carry out the breakdown of the starchy kernel to simple forms (glucose, maltose, oligosaccharides) [42] and protein to amino acids or dipeptides be utilized by the developing embryo. In addition, the aleurone layer contributes significant proportion of antioxidant, minerals (magnesium, potassium, sulfur, phosphorus, calcium, zinc, manganese, and iron), vitamins, soluble and insoluble dietary fiber in rice-based products [37]. It is enriched with phenolic acids (800–1243 mg/g), tocopherols (0.35–0.77 mg/g), γ-oryzanol (0.56–1.08mg/g); protein content between 0.11–0.16 mg/g [43]; oleic acid linoleic acid, and palmitic acid which constitute 36, 37, and 23%, respectively. Because of these attributes, the rice bran is used as a specialty material in gluten free beer production to improve foaming, color, and beer mouth feel (full-bodied beer) [44].
2.1.3 Rice endosperm
The removal of the rice husk and bran exposes the endosperm which is a rich starch compartment structure (Figure 1), and it makes up approximately 90.5% of rice weight [28]. The starch contained by the rice endosperm is dependent on the type of rice cultivar, geographical location, climate and period [45]. Rice cultivar designated as waxy rice (sticky rice) has an endosperm containing high proportion of amylopectin (96–98%) with very low amylose (<4%) while non-waxy rice (non-sticky rice) contain fraction of amylose (12–29%) and amylopectin (80–85%) [45, 46]. The proportion of amylose and amylopectin influence starch composition, as well as the temperature and time required for complete gelatinization of rice starch [47]. The starchy endosperm is the source of reducing sugar and oligosaccharides when hydrolysed by endogenous enzyme during germination process [24].
2.1.4 The embryo
The embryo is a minute structure embedded in the endosperm and constitute approximately 2–3% of the total weight of the rice seed. It comprises of the plumule, radicle, scutellum, coleoptile, and epiblast (Figure 1) [48]. The scutellum serves as a nutrient reservoir (simple sugars) needed for embryo development during germination while signaling the aleurone cell via gibberellic acid. The development of the embryo leads to shoot and root projecting out of the rice husk, which originate from the plumule and radicle, respectively. The coleoptile of the embryo provides a covering for the emerging plumule during germination [38]. Polyphenols such as anthocyanin and phenolic acids have been identified in the rice embryo of white, red, and black rice kernel, and were higher than endosperm anthocyanin and phenolic acids but lower than the rice bran content [49].
2.2 Rice cultivars malted for brewing purposes
2.2.1 Rice cultivars
Rice cultivars in the world can be classified as Oryza glaberrima of Africa origin, or Oryza sativa with subspecies belonging to japonica, indica or javanica of Asia origin [50, 51]. The subspecies of O. sativa thus japonica and javanica are often regard as sticky rice as well as the O. sativa var. glutinosa cultivar, while O. sativa var. indica is a non-sticky rice cultivar. Furthermore, cultivars of indica possess long kernel (long grain rice) relative to japonica cultivars (short grain rice). There are few cultivars of japonica that possess long kernel. Nevertheless, the O. sativa species is the most cultivated and studied rice cultivar.
The numerous rice varieties make selection of rice seeds a great challenge for the food and beverage industries (Table 1). Mayer et al. [24] studied rice malt from 10 different Italian rice cultivar of O. sativa L.; eight belonging to the subspecies japonica and two to the subspecies indica; only two cultivars, Balilla and Centauro had physicochemical characteristics suitable for an all-rice beverage (Table 1). In rice-growing hubs such as Thailand, two pigmented rice cultivars of indica subspecies were also suitable for all rice beer [19]. Elsewhere, pigmented rice such as Indian black, Riceberry rice provide an alternate material for functional beer due to the high oxidation stability it provides [6, 7]. West African rice cultivars of Nigeria and Ghana origin were also investigated for rice beverage quality [54, 56]. Among the numerous rice cultivars studied for brewing purposes, majority of the rice cultivars are characterized as waxy rice (non-glutinuous rice/non-sticky) with few cultivars used for brewing as non-waxy rice (glutinuous rice) as described in the section endosperm 2.1.4.
Rice cultivars used as adjunct in brewing process are broken fraction of waxy rice kernels which are produced during rice milling process (polishing). The use of polished broken rice to substitute barley malt for beer brewing can be in the range 30–60% [16, 52, 53]. The broken polished rice fraction are preferred for brewing because, it is cheaper relative to whole grain polished rice, which are premium for their esthetic value. The broken polished rice contains low lipid content compared to other adjunct such as maize, sorghum, and oat. Polished broken rice as adjunct can further be processed either by extrusion or torrefication into pregelatinized rice flakes, and also, the broken rice is better for grist mill because of less abrasion. However, extruded rice as adjunct is characterized as possessing low soluble protein limits because of protein denaturation via thermo-mechanical effects which results from the synergist effect of force, heat, and pressure [63]. Hence, it is vital to select processed rice as adjunct based on function.
2.3 Rice malting process
Dried germinated cereals with preserved enzymes activity are often regarded as malt. Malts are produced from three processes; steeping, germination, and kilning or drying which is collectively known as malting (Figure 2) [64, 65]. Each stage of malting requires an optimum temperature control, aeration condition and moisture content. These factors have been integrated in optimizing rice malting conditions.
Figure 2.
Illustrates the three stages of malting process (steeping, germination and kilning).
2.3.1 Rice steeping condition and water uptake
The rice seed has thick and tough husk which is a limiting factor for water absorption or imbibition. The significance of steeping is to enable seeds to absorb water to obtain a moisture content between 40 and 45% to initiate germination, growth hormone distribution and enzyme activities within the seed as germination commence. Bewley [66] proposed that three stages of water imbibition by dry seed exist, thus, rapid imbibition phase of water by the rice seed within 12–20 h (stage 1), followed by a stable imbibition which lasts for 30 h (stage 2). At stage 2, the coleorhiza (protective tissue layer that surrounds the radicle) development is seen. The final stage exhibits a rapid uptake of water which is linked with radical projection with water uptake lasting for 20 h. The water uptake by rice seed suggests that a minimum of 70 h will be required for a dry seed to be saturated with water via steeping [67]. However, the amount of water uptake by rice seed is dependent on the steeping period and type of steeping schedule.
2.3.1.1 Continuous steeping and discontinuous steeping on rice seed imbibition
The method of steeping rice seed in water significantly influences the rate of water absorption prior to germination. Two steeping methods have been applied to rice thus, continuous steeping and discontinuous steeping.
Continuous steeping implies immersing rice seeds in water for a prolonged period and changing water at certain intervals. The water absorption and pregermination metabolism according to Bewley [66] and Yang et al. [68] suggest a mechanism associated with continuous steeping schedule. Continuous steeping of rice seeds for 24 h, 48 h, and 72h for malt production, Usansa et al. [22] and Owusu-Mensah et al. [56] reported an increase in seed weight (degree of steeping) between 28 and 36% which was below the required steeping degree for malting cereal. Notwithstanding, steeping degree of 36% can initiate germination but the endosperm modification via enzyme activity will not be sufficient. When steeping degree is below is 40%, the seeds are either sprayed with mist during germination period or immersed in water to achieve 40% steeping degree. The continuous steeping, is not suitable for water sensitive rice cultivar. Thus, rice seeds that tend to have low germination rate because of excess water or moisture during steeping. The inability of obtain of steeping degree of 40–45% via continuous steeping led to the adaptation of discontinuous steeping of rice [24].
Discontinuous steeping involves a cycle of soaking seed and an air-rest period [69]. Adapting this steeping method, Mayer et al. [24] steeped rice seeds with a steeping schedule of 8 h steeping and 8 h air-rest cycle for 72h led to a steeping degree of 38–42% which enables rapid seed metabolism for a well modified seed kernel during germination [70, 71]. The high steeping degree recorded in discontinuous steeping suggested that the air-rest period enabled water imbibed by the seed to dissipate evenly and the seed exposure to air leads to partial drying of seed which creates a gradient for rapid water absorption in subsequent steeping. This steeping method is suitable for water sensitive rice seed. The multiple resteeping cycles in discontinuous steeping method remove microbial spores on seed hence improving malt quality. Hence, the discontinuous steeping schedule has been adapted in malting different rice cultivars with varying steeping schedule [6, 23].
2.3.1.2 Steeping temperature on water uptake by rice seed
Steeping rice seed in malting process is significantly influenced by temperature. The steeping temperature influences the rate of water uptake by rice seed [22] and microbial population of malt [72]. Conventionally, rice seeds are steeped in water between 28 and 30°C in tropical regions [56]. Steeping temperatures that promote adequate germination of rice seed is between 20 and 35°C [6, 22, 24]. According to Usansa et al. [22], increasing steeping temperature from 20–30°C was directly proportional to steeping degree. And temperature below 20°C is associated with low rate of water uptake. At low temperature between 23 and 25°C, discontinuous steeping of rice seed resulted in a steeping degree between 38 and 40% [24] but required steeping period of 72h. Meanwhile steeping rice at 30°C via discontinuous steeping, reduced steeping period to 24 h because of rapid water uptake [6] but the steeping degree was not reported. However, continuous steeping of non-glutinous rice seeds at 30°C had a steeping degree less than 40% [22]. Temperatures between 35°C and 40°C used in steeping rice had higher imbibition water rate but temperatures beyond 35°C the steeped rice seed developed odor which can affect the organoleptic properties of derived rice malt product [23].
Despite high steeping temperatures (30–35°C) accelerate water imbibition by rice seed, it has the potential to cause over steeping, thus, water absorbed by the rice seed exceeding the maximum steeping degree (>45%) which leads to rapid metabolic activities within the seed hence, the potential of a high malting loss (>10%) during germination. Therefore, caution is needed to monitor steeping degree when using warmer steeping conditions but this phenomenon is rice cultivar dependent [6, 22, 23, 24]. Hence, Moirangthem et al. [6], proposed 24 h discontinuous steeping period constituting 6 h soaking and 3 h air-rest of 3 cycles at 30°C.
2.3.1.3 Steeping period on water uptake by rice seed
The rate of water uptake by rice seed depends on steeping time either by continuous steeping or discontinuous steeping coupled with low to high steeping water temperatures. The minimum and maximum steeping period for rice seed are 24 h and 72h, respectively. The steeping period is influenced by steeping temperature and rice cultivar. Steeping time of 72 h is suitable for rice with steeping temperature of 25–28°C [56] and also for rice cultivars with thick husk [73]. This is because, steeping for 72 h enables sufficient kernel endosperm modification because of the gradual imbibition and dissipation of water in the rice kernel for sufficient enzyme activity relative to seeds steeped for 24 h. Malted rice seeds steeped for 24 h would require a final high temperature during mashing process for residual starch gelatinization.
2.3.1.4 Accelerating water uptake during steeping
In other to overcome the limitation of water uptake by rice seed within 24 h of steeping, rice seed were primed with photosynthesized silver nanoparticle solutions (AgNPs) at 10 and 20 mg/L. This led to rapid water uptake between 15.6–19.1% and 16.6–19.3%, respectively, within for 4 h and 24 h compared to the control (9.1–14.8%) [74] however, this improvement has not been adopted in the rice malting process.
2.3.1.5 Effect of aerobic and anaerobic steeping condition on rice
Rice seed can be steeped in water with aeration condition or under anaerobic condition. Rice seeds are capable of germination either under hypoxia or anoxia condition. This is partly because of the different types of alpha amylase enzymes regulated gene that can be expressed under varying conditions. Hence, rice seeds can germinate under aerobic condition and anaerobic.
Aeration or oxygenating during steeping is the most adaptive method malting process because it contributes to high enzyme activity. This steeping condition is characterized by protrusion of both shoot and root. These structures grow excessively if oxygenation is not regulated during steeping hence resulting to high malting loss. Aerating rice seed during steeping enables the growth hormone gibberellic acid to be produced and stimulate amylase production a metabolic pathway associated to a specific amylase gene which are described in details by Damaris et al. [75]. Anaerobic steeping of rice can result to two types of conditions thus hypoxia which implies little amount of oxygen availability in the steeping water, and anoxia which is a state of complete depletion of oxygen in steeped water. These condition induces elongation of coleoptile which is a protective sheath around the shoot and delay formation of root [76]. The inadequate oxygen supply and prolong steeping period (72 h) limit the rate of starch hydrolysis of the rice endosperm there for low enzyme activity. Rice seed germination under anaerobic condition is regulated by a group of amylase gene classified as Amy 3.
2.3.1.6 Effect of steeping on hormonal release
Water uptake by the steeped rice induces growth-promoting hormones such as gibberellic acids (GA), a response triggered by the viable embryo. GA diffusion from the scutellum into the aleurone layer promotes the formation of the enzyme α-amylase which commences starch hydrolyses. The total activation of the aleurone layer is proposed to be progressive hence as steeping progresses more enzymes are activated [77]. Prior to steeping, α-amylase in paddy rice is absent until germination commences.
2.3.2 Germination period
The Figure 3 illustrate the stages of rice seed (Figure 3a) germination during malting process. Germination period commence at the end of steeping. However, the chitted rice seeds are hydrated via spraying with water or resteeping the chitted seed for short period to maintain the steeping of 45% [19]. In malting process, a germinated cereal can be referred to as chitted seeds, thus the formation of a white visible dot or tip known as coleorhiza observed especially a day or two steeping (Figure 3b) [78]; protrusion, when there is an extension of the coleorhiza in the absence of root (Figure 3c), and emergence which is a seedling stage of root and shoot formation (Figure 3d) which further increase in length as germination period increases (Figure 3e and f). The development of coleorhiza to sprout are visible with rice cultivars during germination period of 1–5 days with shoot and roots extending unlike germinating barley in malting process (the shoot protruding from the kernel remains beneath the husk). Germination period in malting rice requires 4–5 days for sufficient starch modification at temperature between 20 and 25°C [22, 24]. The indigens of Nagaland, India who are noted for rice beer production relied on the prevailing weather to determine the germination period of rice seed during malting, thus during summer, rice was germinated for 3–4 days [79]. Which implied that germination temperature would be estimated to between 30°C and 35°C for 3 days germination [6]. At this temperatures rice germination is rapid, a condition referred to as accelerated germination [80]. Accelerated germination conditions requires caution to minimize high malting loss and control of bacteria or fungi development because of the warm condition. During winter in Nagaland, rice seeds were germinated for 7 days [79]. Germination period between 5 and 7 days implied temperatures were below 14°C. The end of germination period, the sprouted seed is regarded as green malt.
Figure 3.
Paddy rice at different stages of shoot and root development during malt. (a) Rice seed with no visible shoot extension, (b) rice seed about to chit after steeping, (c) extension of the rice shoot, (d) shoot and root development.
2.3.3 Kilning or drying of germinated rice
Kilning germinated rice seed enables the reduction of moisture content in the ranges 4–6%. The low moisture content slows the development of mold because of the low water activity. Drying condition of the green malt is critical to preserve the activated endogenous enzyme in the kernel which would be relied on for further starch hydrolysis during brewing process known as mashing. Kilning of green rice malts varies with user’s objective. There are two types of kilning method thus fixed or single temperature–time cycle and stepwise multiple temperature–time cycle. Stepwise multiple temperature–time cycle is often used to kiln green rice malt with reported conditions as follows: 45°C, 12h; 50°C,12 h; 55°C, 13.5 h; 70°C, 6 h; and 1 h cooling [6, 24] or drying at 50°C, 18 h and 70°C for 6 h [7]. The initial low temperature of a stepwise kilning cycle enables low degradation rate of endogenous enzymes and a high final temperature is important for the removal of undesirable aromatic compounds such as dimethylsulphide, and impact color on malt. Kilning of green rice malt with a fixed drying temperature either at 50°C [22] or 60°C [57] are used to obtain malt with high enzyme activity or diastatic power. Temperatures for kilning rice green malt are below 80°C because of the less thick rice endosperm relatively to barley. Temperature above 80°C can be used in kilning base malt but will reduce amount of enzyme activity and impact more color on malt extract [81]. Ceccaroni et al. [21] produced specialty rice malts by drying green malt at an initial temperature of 100°C and final temperatures of 145°C, 152°C or 160°C to obtain three different caramelized rice malt whiles temperatures of 176°C and 193°C were used to produce dark rice malt. These specialty rice malts do not possess enzyme activities but are meant to impact color, and aroma.
2.4 Effect of germination on endogenous enzyme activation and starch hydrolysis in malting process
The ultimate goal of malting cereals is to reduce starch molecules to oligosaccharide, disaccharide, and monosaccharide. The hydrolysis of starch is carried out by enzymes collectively known as carbohydrase which consists of α-amylase, β-amylase, limit dextrinase, and α-glucosidase [64, 82]. Other enzymes such as lipase [83], protease [84], and phenylalanine ammonia-lyase [85] have been reported in germinated rice. Each group of enzyme contributes to the modification of the rice kernel and solubilized solutes in wort during mashing.
2.4.1 Enzyme activation and expression
Enzymatic activation and production commence with steeping. The amount of enzyme increases as germination progresses via deoxyribonucleic acid (DNA) expression, which is translated to the production of hormones and enzymes. Prior to starch degradation by enzymes the embryo activation, signaling and development is associated with the pre-existing sucrose in the embryo as energy source [86].
2.4.2 Effect of malting on rice α-amylase
In rice, its estimated that over eight α-amylase genes exist and are responsible for the expression of α amylase during germination. Some α-amylase genes studied include, Amy1A, Amy1C, Amy2A, Amy3A, Amy3B, Amy3C, Amy3D, and Amy3E. These genes can be grouped into three categories thus, Amy1, Amy2, and Amy3 [87] and are collectively responsible for α-amylase production in rice seed to withstand long period of steeping under aerobic and anaerobic without impeding the degradation of the starchy kernel to produce maltose and glucose. The α-amylase activity is reported to below detection level or absent in the rice seed [18, 22, 75] however, its synthesis is associated with gibberellic acid dissipation during steeping. The onset of steeping and germination period (1–7 days) are positively correlated with increasing amount of α-amylase activity [18, 22]. However, germinating rice for more than 5 days leads to significant malting loss despite having α-amylase activity. Starch is hydrolysed by α-amylase by cleaving off glucose and maltose from the reducing end and non-reducing end on the α(1–4) glycosidic bonds of the starch and dextrin molecules. According to Usansa et al. [22], the amount of alpha amylase activity is dependent on rice cultivar as shown in Table 2, and influenced by aerobic and anaerobic condition. The later leads to low alpha amylase activity relative to aerated condition during malting. The α-amylase activity in rice malt is low relative to α amylase activity in barley malt (104–305 CU/g dw) [89] and oat (86–110 CU/g dw) [90].
Types of enzyme activities in malted rice for brewing.
2.4.3 Effect of malting process on rice β-amylase
The β-amylase is present in many cereals’ aleurone cells and does not depend on gibberellic acid for its activation during malting [91, 92]. The amount of β-amylase present in the rice seed can be influenced by storage condition of rice seed and aging. Despite present in the rice seed before germination [92], β-amylase activity initiates the early phase of germination during steeping period. The β-amylase activity reduces in rice seed as germination period progresses from day 4–5 [22]. This report corroborated with Wunthunyarat et al. [18] findings on β-amylase activity in germinated brown rice under aerobic condition. Germinating rice in an anaerobic condition, β-amylase activity in brown rice malt increased with germination period but was below values recorded under aerobic condition. Nevertheless, Wunthunyarat et al. [18] and Usansa et al. [22] concluded that β-amylase acitivity in malted rice was cultivar dependent. According to Yamaguchi et al. [93] the expression of β-amylase during steeping and germination period was absent in the radicle, plumule and embryo but present in the endosperm after 96 h germination which suggested the β-amylase was activated in-situ, and proposed to be as a result of glucose presence and further suggested that, some rice cultivars had no β-amylase activity present due to the lack of messenger RNA or the promoting gene for the β-amylase was blocked. The β-amylase activity in the malted rice seed (Table 2) is relative lower to β-amylase activity in 94 commercial barley malt which ranges between 420 and 1125 BU/g dw [89]; oat malt (105–189.2 BU/g dw) [90]. The β-amylase activity in malting produces maltose which is released only from the non-reducing ends of amylose and amylopectin. The release of maltose unit from amylose as substrate is possible when the glucose unit of the amylose structure is an even number [94]. Also, this enzyme produces maltose from amylopectin and other soluble starch as substrate but this process stops when the β-amylase approaches the α-1 → 6-D-glucosidic bond, leaving a starch known as the β-limit dextrin [95, 96] or reduces when exposed to maltoheptaose as substrate [94].
2.4.4 Effect of malting process on rice limit dextrinase activity
The limited dextrinase or α-dextrin endo-1,6-α-glucosidase is the sole enzymes responsible for cleaving the amylopectin structure at the α-1-6 glycosidic bond of starch, and produce dextrin during seed germination [97] hence referred to as starch debranching enzyme. And it is vital in starch assembling during seed development [98]. The limited dextrinase has more affinity to hydrolyse amylopectin-oligosaccharides which has substantial amount of α-1-6 glycosidic bond linkages during starch degradation in malting process [97]. The amylopectin-oligosaccharides structure is either produced by α-amylase or β-amylase activities and are referred to as α-limit dextrin [99] or β-limit dextrin, respectively [95]. In unmalted rice seed the limited dextrinase activity is higher than both α-amylase and β-amylase [100]. The limited dextrinase activity further increases with malting process. This enzyme is expressed in both non-waxy rice (non-glutinous rice) and waxy rice (glutinous rice) which contain different amylose and amylopectin composition as discussed in previous paragraph on some rice cultivars malted for brewing purposes. The limited dextrinase activity in Thai malted glutinous and non-glutinous, and in malted Italian glutinous rice (Centauro 2 and Balilla) is approximately 10 folds [19, 24] (Table 2) higher than value recorded for 94 commercial barley malt (300–749 U/kg) [89]. The high limited dextrinase activity in rice seed and malt make rice a suitable material to source for limited dextrinase with potential commercial application via enzyme immobilization [100].
2.4.5 α-Glucosidase
Furthermore, another enzyme associated in the production of glucose during germination phase of malting is α-glucosidase. This enzyme reduces maltose to glucose hence α-glucosidase is described to have strong affinity with glucose liberation than oligosaccharides [101]. The α-glucosidase activity is present in rice seed and increase 3 folds during after germination in the malting process [19] as shown in Table 2.
2.4.5.1 Rice starch structure and morphological changes during malting
The rich starch endosperm of rice contains tightly packed starch granules which possess a polyhedral or polygonal structure (Figure 4). The integrity of the rice starch granules can be influenced by storage period, thus, rice stored for 2 years had the straight-chain starch content decreasing as shown in Figure 4. This structural shape may vary among cultivars thus rice designated as nonwaxy rice may possess polyhedral irregular, oval, angular shape while waxy rice starch granules could consist of either an angular or irregular shapes. The variation in starch granule shape may contribute to the wide distribution of rice starch granule size in the ranges 3–8 μm [88, 103, 104, 105]. The tightly packed starch granules are partially loosed during germination phase of malting [106]. This is conspicuous around endosperm structure close to the aleurone layer but structure distant from the aleurone are less loose or modified.
Figure 4.
Effect of different storage times on rice starch granules as shown by scanning electron microscope; (a1,a2) are starch granules under storage year-0 2 K, 6 K lens; (b1,b2) are starch granules under storage year-1 2 K, 6 K lens; (c1,c2) are starch granules under storage year-2 2 K, 6 K lens [102].
2.5 Germination period effect on phenolic acids in malting process
Phenolic acids are integral part of seed cell wall with functional properties of antioxidation when consumed in foods and beverages. Germinating process in malting rice seeds activates phenolic synthesizing and cleaving enzymes (feruloyl esterase) [23, 107]. The feruloyl esterase in rice liberates bound phenolic acids which are not readily available hence increasing free phenolic content in the germinating paddy rice. Jirapa et al. [108] observed an increase in phenolic acid content in germinated paddy rice after 24 h steeping and 12h germination of 5 paddy rice cultivars resulted in an increase of the total phenolic acids by 3–4 folds, but decline after 48 h germination to an estimated 0.9–2 fold higher than the ungerminated paddy rice [108]. This is because embryo developing into shoot and roots induces reactive oxygen species (ROS) release in the form of H2O2 and O2− within 36 h after imbibing water [109], however, the activities of ROS are inhibited due to the production of ROS inhibition or reducing enzymes in the form of catalase, superoxide dismutase (SOD), and ascorbate peroxidase (APX) [110] which are associated with the synthesis and cleaving of phenolic acids in the formation of shoot and root cell structures.
Pale malted paddy rice (Centauro variety) produced using equal steeping and air-rest period for 72 h and 120 h air-rest of germination had total phenolic acids (TPA) approximately 0.89 mg GAE/ g d.w higher than non-pale paddy rice malt which were steeped for 48 h [21]. However, pale paddy rice malt which were roasted at 176°C and 193°C recorded TPA content in the ranges 1.5–2.2 mg GAE/g d.w. The synergistic effect of steeping (3 days, room temperature) and germination (37°C) to attain root and shoot length of 15–25 mm [111] which is equivalent to germination period of about 4–6 days at 20°C [22] increase phenolic content from 4.11 mg GAE/ g in ungerminated rice to 9.82 mg GAE/g after germination [111] which is higher than values reported by Ceccaroni et al. [21]. Malting condition did not necessarily increase phenolic acid because with rice cultivar such as Pong Ell had lower phenolic acids than ungerminated rice seed [108]. The results suggested that the increase of TPA in the germinated paddy rice cultivars was not necessarily proportional to the period of germination [108, 112], but an influence of rice cultivar and malting conditions [113, 114]. The increase in phenolic acids in malted rice according to Gonu et al. [23] is attributed to bound phenolic acids or esterified phenolic acid [62] because some soluble phenolic acids (free phenolic acids (FPA)) are released during steeping process. However, as germination progressed from 24 h to 120 h, the FPA content increases but was below the FPA in unmalted rice [23]. The variation in TPA content reported in malted rice for brewing (Table 3) is because of the difference in phenolic acid extraction. Methanol, acetone and ethanol have been used as solvent to extract phenolic acids. PA solubilized in these solvents are referred to as FPA. The FPA content can be increased significantly when the organic acids are acidified before extracting FPA as observed in Gonu et al. [23] relative to dos Santos et al. [57] and Jirapa et al. [108] report. Nevertheless, the variation in TPA content in malted paddy rice is significant hence require appropriate standardized data collection and analytical method. Notwithstanding, the increase in phenolic content after germination suggest the benefits of using malted rice for beverage or food processing.
Total phenolic acids content in malted paddy rice.
2.6 Sugar profile of malted rice
The Table 4 highlights the sugar profile in rice malt extracts. The hydrolysed starchy endosperm during malting produces glucose, fructose, sucrose, maltose, and oligosaccharides, in the form of maltotriose, maltotetratose, maltopentaose, maltohexaose and maltoheptaose [17, 24, 42] however other oligosaccharide can be present because as starch is reduced during germination different types of oligosaccharides are produced. Rice malt possess higher glucose content than maltose or similar concentration contrary to the sugar profile in malted barley which has more maltose content than glucose. This might be because of the synergistic interaction between limit dextrinase and β-amylase. Thus, as the amylopectin branch is cleaved of from the main amylose chain by limited dextrinase and the non-reducing ends are hydrolysed by β-amylase into glucose molecules.
2.7 Total protein content, total nitrogen value and free amino nitrogen (FAN) content and amino acid profile of rice malt
For the purpose of brewing, protein content is essential to improve foaming properties in both alcoholic and non-alcohol beer. Malted rice cultivars have been reported to have protein content of approximately 7.0–7.6% dry basis [20, 56] which is below the protein content of wheat malt (11.4–13.5% dry basis) [118] and barley malt (10.0–12.5% dry basis) [119] which are preferred by brewers which indicates the potential amount of FAN that can solubilize in malt extract. In brewing beer, protein is a key component that helps yeast activity and the esthetic character of beer via foaming and haze as well as for flavor and color. The protein isolates from rice are categorized as albumin globulins [120], prolamins [121], and glutelins [122], which are water soluble, soluble in salt solution, alcohol soluble and soluble in weak acid/alkali solution, respectively. Malted rice extract studied have been reported to contain FAN in the ranges 146.33–154 mg/L [123] or 144–166 mg/L [24] which is below FAN content of barley malt extract (202 mg/L) [124] hence using rice malt solely for brewing provide insufficient FAN to support yeast health during fermentation which is dependent on FAN and soluble amino acids. Therefore, it is more suitable to complete rice malt with cereal rich in FAN such as barley, or wheat when producing beer without considering gluten allergy. However, protein rich pulse extract, yeast extract [125] or soybean meal can be used to improve FAN and amino acid content [126]. Soybean meal added to rice malt mash was subjected to exogenous enzymes (neutrase) to improve soluble FAN and amino acid content [126].
Soluble amino acids in malt extract (wort) are essential for functional beverage, and also influence yeast metabolism during wort fermentation. The metabolism of amino acids by yeast activity and preference group amino acid into fours grouped as shown in Table 5. Group A and B amino acids are absorbed by yeast rapidly and intermediately, respectively [127, 128, 129]. However, this grouping might not be absolute for all brewing yeast and also the yeast studied is not a true representation of yeast strain available. Nevertheless, the significance of amino acid to yeast metabolism cannot be understated. For instance, insufficient amount of valine in wort can result to the production of off-flavor compounds to such as diacetyl [130]. The low amino acid content in rice malt wort led to the quest for low diacetyl producing yeast (Saccharomyces cerevisiae strain LPST01) [126].
2.8 Mashing condition and wort production using broken polished rice
2.8.1 Mashing with broken polished rice as adjunct
The use of rice in brewing has mainly been applied as a complementary cereal to substitute malted barley. Conventionally, the rice used are broken grains produced during rice milling process (polishing) and are often of non-waxy rice cultivars. The polished broken rice as adjunct can sometimes be further processed either by extrusion [131] or torrefication into pregelatinized rice flakes [16].
The polished broken rice grist is a key material in producing gluten free extract for gluten beer which is categorized as non-malt beer in jurisdiction such as Japan. The use of commercial brewing enzymes has significantly paved the way to increase rice grist proportion in brewing process. The percentage of polished rice in gluten free beer brewing can range from high composition between 60 and 100% w/w with other cereal such as sorghum, buckwheat, teff and pseudocereal. Meanwhile in non-gluten free beer made with barley or wheat malt, percentage of polished rice addition can be as low as 10–60% [16, 52, 53] and high, above 60% w/w [132]. As a substitute for barley malt in brewing process, the use of polished broken rice requires special processing or handling prior to mashing. Mashing implies the solublization of phytocompound via endogenous enzymes present in malted cereal using specific temperature–time schedule.
Figure 5 illustrate the mashing process when using polished rice. Polished broken rice for brewing are treated in a special way relative to malted grains because of the absence of enzymes such as α-amylase and β-amylase which are required to hydrolyse the starch rich grist. Hence, two methods are adapted thus thermal treatment of rice grist slurry (gelatinization) with/without the addition of enzyme. Gelatinization without enzymes implies cooking rice in cereal cooker to breakdown the starch between 85 and 100°C prior to its addition to the mash (water and milled malt regulated at temperatures suitable for enzymes within the malt) [16, 133, 134] for 30–60 min (Figure 5). Meanwhile it’s worth noting that the gelatinization temperature of rice is dependent on cultivar type thus polished broken rice with high amylose content tend to have higher gelatinization temperature, and vice versa for rice cultivars with low gelatinization temperature (<70°C). This implies such polished broken rice might not require a pre-gelatinization process is an ideal brewing temperature regime (60–78°C) which can gelatinize the starch. To minimize the time required for complete gelatinization and increase extract yield of broken polished rice grist, exogenous enzymes such as TermamylSC® (E.C.3.2.1.1) which is a heat stable α-amylase at 90°C [15, 126], is added to the cooking rice to accelerate gelatinization process and increase extract yield. This leads to the production of high reducing sugar which can further be hydrolysed by endogenous amylase enzymes in the malted grain at mashing phase to glucose, maltose and maltotriose.
Figure 5.
Illustrates how broken rice is handled prior to mashing and the brewing process: 1. Malted barley; 2 = milling; 3 = mashing of malts using specific temperature and time schedule; 4 = broken rice; 5 = cooking and boiling of polished broken rice slurry; 6. Mashing of malted barley and gelatinized rice; 7 = filtration and lautering; 8. Boiling of wort and hop addition; 9 = whirlpool; 10 = heat plate exchanger; 11 = fermentation tank of boiled wort with selected yeast; 12 maturation; 13 filtration system; 14 = bottling; 15; plate heat exchange; 16 = keg.
Further, in other to produce all polished broken rice extract for beer, a two-step process known as liquefaction (gelatinization with enzyme, as explained in processing polished rice) followed by a saccharification phase at 50–60°C via fungal α amylase to obtain glucose, maltose, and maltodextrins or by a β-amylase extracted from malted cereal. The derived rice extract is a rich source of fermentable sugar with limited nutrients to support yeast growth hence soluble nutrients such as yeast extract, diammonium phosphate and diammonium sulphate are added to the rice extract [132] or hydrolysed protein rich materials such as defatted seed cake of soy or sunflower via enzyme activity such as Neutrase® at 50°C [126].
2.8.2 Polished broken rice-barley malt mash
In conventional brewing process, polished broken rice is used as adjunct after gelatinization. Often, gelatinized rice is added to the malted barley slurry mash in percentage proportion discussed earlier, which increases the temperature to 60–68°C suitable for α-amylase or β-amylase activity from the malted grain subsequently increased to 75–78°C to complete mashing and inactivate enzyme activity. Gluten rich mash and low gluten mash with rice has been possible using malt above 50% w/w and below 40% w/w, respectively. However, for brewing consistency and reproducibility processed polished rice such as extruded rice [52] and flaked torrefied rice [16, 135] are used as adjunct (30–60%w/w) with malt directly for brewing. However, the type of rice adjunct used in brewing process may be influenced by the existing technologies in a geographical location. In countries where there is no restriction on external enzyme usage in brewing, thermal stable alpha amylase is added to the cooking phase or mashing out phase to improve malt extract yield [15].
2.9 Mashing conditions with malted paddy rice
Malted cereals for the production of malt extract are often checked for brewing performance via standard congress mash according to the European Brewing Convention method (EBC analytica 4.5.1) or Central European Commission for Brewing Analysis method (MEBAK R-206.00.002) [136], and America Society of Brewers Chemist Method (Malt-4). Per these analytical procedures rice malt after mashing possesses a poor filtration ability, inadequate FAN content, and total nitrogen [24]. Hence, mashing rice malt is associated with modified mashing temperature and time schedule (Table 6) based on the congress mash which makes it feasible for the production of rice malt extract and beer. The temperature and time schedule of obtaining rice malt extract is highlighted in Table 5. In order to improve the FAN content of paddy rice malt extract protein rest is often carried out for 30 min at 45°C but this condition is also suitable for the release of the malt secondary metabolites such as polyphenols [137]. Furthermore, to enhance FAN and nitrogen related content during rice malt mashing, exogenous protease enzyme (Neutrase® 0.8 L (E.C.3.4.24.28)) [15] as explained earlier.
Temperature and time schedule for protein and starch hydrolysis of rice malt mash.
Brewing gluten free beer from rice, researchers are focused on using malted paddy rice thus non-waxy paddy rice which contain both amylose structure and amylopectin structure. Hydrolysis of starch in malted paddy rice occurs using mashing temperature of 60–65°C and 70–74°C to activate β-amylase and α-amylase, respectively. In order to completely hydrolyse traces of rice starch, thermal hydrolysis of starch (final mashing temperature or mashing out temperature) is carried out at temperatures above 75°C [6, 7] which also inactivated amylase activities. Final mashing temperature below 75°C of rice malt will lead to some amount of retrogradation which is a phenomenon in which rice some starch molecules reconstitute after heating hence retaining some amount of water molecules which result to low sugar extract yield. As a result, temperatures above 75°C are used at the end of mashing for period of 10–60 min. However, this condition is dependent on the malting period of rice, thus, shorter germination period (3 days) would require prolong mashing out condition [6] and rice cultivar.
Rice malt extract yield can range from 71.7–72.3% DM for pale rice malt; caramelized rice malt, dark rice malt range between 60.6–67.6% and 44.6–44.9%, respectively [21]. Noonim and Venkatachalam [123] recorded an increasing rice malt extract yield of 61.3, 63.6, and 65.6% from rice malt germinated for 3, 5, and 7 days respectively. Malt extract yield of day 6 and 7 were within rice malt extract yield (64–66%) reported by Mayer et al. [24] after 5 days germination, but differed from reports of Ceccaroni et al. [21]. Extract yield of rice malt is significantly influenced by germination period during malting, rice cultivar and mashing method.
3.1 Fermentable and non-fermentable sugars in rice malt extract
Rice malt extract has fermentable sugar (glucose, fructose, sucrose, maltose, and maltotriose) which can be assimilated by profile brewing yeast, and non-fermentable sugars which cannot be metabolized by the brewing yeast. Table 7, highlights both fermentable and non-fermentable in rice malt extract.
In rice malt wort, solubilized bioactive phytocompounds collectively known as polyphenols have been reported [7, 23]. Polyphenols in rice malt beer or wort have been less researched. Polyphenols comprise of phenolic acids, flavonoids and anthocyanin. These compounds are solubilized in wort during initial mashing temperatures in the ranges 40–45°C, release significant amount of polyphenols relative to higher initial mashing temperatures (50–60°C) [138]. Polyphenols in wort is as a result of free phenolic acid solubilizing in water and the liberation of phenolic acids from the seed cell walls during malting by the enzyme feruloyl esterase [107] and mashing at lower temperature (40–45°C) [138]. The solubilized polyphenol decreases in malt extract during boiling especially compounds classified as flavonoids. Boiling wort leads to the formation of polyphenol-protein complex (macro-molecule) which precipitate to forms hot trub. More polyphenol-protein are precipitated when cooling the boiled wort which is referred to as cold trubs [139]. Phenolic acids are the most prominent and persistent compounds in wort. These compounds are benzene structures containing hydroxyl groups and carboxylic groups. The phenolic acids are subdivided into hydroxycinnamic acids (ferulic acid, coumaric acid, sinapic acid, and caffeic) and hydroxybenzoic acids (p-hydrobenzoic acid, vanillic acid, and gallic acid) (Table 7). These compounds are of key interest to brewers since they can be metabolized by some yeast strains to produce flavor compounds.
Few research articles highlight the polyphenol profile in malted rice wort (Table 7). According to Table 7, malted rice wort produced from pigmented rice had higher total phenolic acid content (TPC) than non-pigmented malted rice wort [6]. This is because of the presence of anthocyanin in pigmented malted rice [57]. In addition, wort produced from pigmented malted rice such as Riceberry rice malt [7], and malted Indian black rice [6] had higher vanillic acid in additions to the hydroxycinnamic acids in but low in non-pigmented rice as shown in Table 7.
Studying the dynamics of hydroxycinnamic acids in producing malted rice wort, Gonu and Withayagiat [7] reported higher ferulic acid (2.56 mg/L) relative to coumaric acid (2.23 mg/L) which supports earlier report by Moirangthem et al. [6] acid (Table 7) but were lower than ferulic acid (3.2 mg/L) in malted barley wort [7]. Gonu and Withayagiat [7], further reported that malted rice wort contained higher coumaric acid content than in malted barley wort and suggested that substituting barley malt with rice malt would increase coumaric acids content whiles reducing ferulic acid. This phenomenon was reported using broken rice grist as adjunct in wort production by Vanbeneden et al. [138]. When the malt extract is boiled, the ferulic acids, coumaric acid, and sinapic are decarboxylated to 4-vinylguaiacol, 4-vinylphenol and 4-vinylsyringol, respectively. Collectively, these derived phenolic acids are responsible for the clove, spicy notes in the boiled malt extract [140].
The production of all paddy rice malt beer and non-malt rice beer which are regards as gluten free beer is feasible. Both ale and lager beer have been produced using malted paddy rice [1, 17] be it pigmented or non-pigmented rice [57, 123]. Ale rice beers produced using top fermenting yeasts have wort conditioned in the range 18–20°C. Commercial brewers’ dry yeast (Saccharomyces cerevisiae) used for rice beer include Nottingham ale dried yeast; Safbrew T-58; SafeAle US-05; LalBrew Belle Saison™ and LalBrew BRY 97™. Active/primary fermentation (18–20°C) of rice wort with top fermenting yeast takes about 4–6 days. This is subsequently followed by yeast flocculation phase for 4–6 days at lower temperatures of 4°C to −2°C [1, 6, 57]. Whilst, lager beers are produced using bottom fermenting yeast (Saccharomyces pastorianus; Saflager S-23) at temperatures between 10 and 13°C for 6–10 days and a flocculation condition between 4°C to −2°C.
4.2 Microbial contaminant and metabolites
Even when general management procedures are followed, various anthropogenic factors and the quality of the raw materials can compromise the integrity of the fermenting wort and the activity of the yeast. Microbial metabolites such as Aflatoxin has been reported in commercial beer. Aflatoxin is toxin metabolites that is produced by molds such as Aspergillus species and Fusarium species. This metabolite is reported to contribute to gushing phenomenon. Furthermore, in the brewing set for commercial production and craft brewing set-up, microbial contamination are often associated with bacteria in the genus Lactobacillus and Pediococcus, wild yeast such as S. cerevisiae var. diastaticus and other non S. cerevisiae which are exhaustively discussed by Kordialik-Bogacka [141].
4.3 pH kinetics of fermenting rice wort
4.3.1 pH kinetics of fermenting paddy rice malt wort
Regardless of the fermentation condition and yeast strain, fermentation of rice wort is characterized by rapid decline in specific gravity of wort within 48–72 h of fermentation. As a result, an increase in organic acids such as lactic acid, acetic acid and succinic in addition to other pre-existing organic acid prior to fermentation (citric acid, malic acid, fumaric acid and formic acid) [142] which accounts for a low pH between 3.5–3.9 in fermented rice wort (green rice beer). This is because of low buffering capacity compounds such as amino acids; peptides and gums in fermenting rice wort which are abundant in barley malt wort. However, during fermentation very low pH also indicates the potential of acid producing microbial contamination.
4.3.2 pH kinetics in fermenting wort containing polished broken rice as adjunct
Fermenting composite rice-barley malt wort possesses a stable steady decline in pH. Lager beer produced from a composite of torrefied flaked rice (30–60%w/w) and barley malt had a pH range of 4.0–4.2 [16] and using gelatinized rice of 15–45%w/w and barley malt, the beer had a pH of 4.2–4.5 [142]. However, lower pH (3.5–3.8) in beer has been reported using polished broken rice above 40%w/w [53] because the buffering capacity compounds decreases with increase rice proportion in brewing process.
4.4 Effect of fermentation on free amino acid (FAN) and constituent in paddy rice malt wort
FAN content is vital for yeast metabolic process during fermentation. The kinetics of FAN in fermenting rice wort for lager beer revealed that about 50–60% of FAN content was utilized by yeast after 6 days [17] whilst in ale beer Ceccaroni et al. [21] reported about 75% depletion of FAN. The high depletion of FAN in ale beer is a factor of temperature and high metabolic process relative to lager fermenting yeast. Nevertheless, rice cultivar is a key factor on FAN metabolism by yeast, thus about 89% FAN was depleted after producing rice beer with malted paddy rice with white kernel and 55% FAN reduction in beer produced with malted paddy black rice [6] despite using same yeast strain (Nottingham ale dried yeast). It’s been reported that pigmented compounds such as anthocyanin or flavonoids can suppress the metabolic activities of yeast during fermentation. Contrary to Moirangthem et al. [6], Noonim and Venkatachalam [123] observed a 50% decline in FAN content in finished malted paddy rice (white kernel) beer using Safbrew T-58 yeast regardless of the period of malting. This implies that, the amount of FAN available to yeast during fermentation might depend on its bio-availability and yeast strain. The bio-availability of FAN to yeast will be dependent on rice cultivar and processes involved in malting, and mashing conditioning to solubilize FAN.
According to Table 8, amino acid in final rice beer declined significantly. It’s worth noting that amino acids have buffering capacity effect and essential for yeast metabolism. Hence, low amino acid during fermentation can contribute to the low pH in both ale and lager rice beers.
Amino acid (mg/L) profile of malted paddy rice beer.
4.5 Effect of fermentation on polyphenols content in rice beer
Table 9 highlights some polyphenol content in wort and beer produced from rice. Despite some flavonoids such as catechin, and phenolic acids as mentioned in Section 2.8 are present in wort prior to fermentation. The final amount of polyphenols in beer is influenced by the initial amount of polyphenols present in wort prior to fermentation, type of yeast for fermentation, cooling, and filtration of beer [145]. Beer produced by some top fermenting yeast (Saccharomyces cerevisiae) or bottom fermenting yeast (Saccharomyces pastorianus) may have the ability to utilize phenolic acids. These yeasts have the phenolic acid decarboxylase (PAD1) gene which expresses the enzyme, phenolic acid decarboxylase [106, 140, 146]. This enzyme cleaves off the carboxylic function group on the hydroxycinnamic acids thus ferulic acid, coumaric acid, and sinapic acid to produce 4-vinylguaiacol, 4-vinylphenol and 4-vinylsyringol, respectively [140, 147]. Usansa [106], reported an increase in 4-vinylguaiacol in both ale and lager rice beer which suggests that a reduction in ferulic acid but the TPC content were not reported in the beers produced.
Total phenolic acid, flavonoid, anthocyanin and phenolic acid profile in rice malt wort and beer.
Malted rice beer produced by Moirangthem et al. [6] and Ceccaroni et al. [21] had lower TPC content in beer relative to the wort (Table 9). The reduction of TPC content is attributed to the cooling of the fermented wort at 2–4°C for 4–6 days which allows yeast cells to flocculate and causes polyphenol-protein aggregation and precipitation. Brewer’s yeast used by Moirangthem et al. [6] and Ceccaroni et al. [21] were Nottingham ale dried yeast (Danstar; Lallemand Inc., Montreal, Canada) and SafeAle US-05, Fermentis, which do not metabolize phenolic acids. In addition, some polyphenols adhered to the cell membrane of the yeast during fermentation [148] can contributes to TPC reduction in beer.
5. Functional properties of rice malt-based beer and functionality
Wort and beer functionality are attributed to the presence of antioxidants (Table 8), dietary fiber and minerals. The presence of antioxidant and mineral influence the oxidation stability of wort and beer which intends inform the possible shelf life of non-alcoholic and alcoholic beer [6]. Few literatures have in-depth report on oxidation stability of rice malt-based beverage. Nevertheless, all rice malt beer produced with black rice malt was reported to possess higher oxidation stability relative to all barley malt beer via electron spin resonance spectroscopy [6]. The oxidation stability of malted beverage is dependent on factors such as the availability of pro-oxidant which includes elemental composition and the type of polyphenols present in brewing raw materials and in extract. In germinated rice, the elemental composition was as follow potassium (K) > calcium (Ca) > zinc (Zn) > manganese (Mn) > iron (Fe) [149]. These elements were present in rice beer produced from black rice malt and white rice malt. Moirangthem et al. [6] reported elemental composition concentration as follows Mn > K > Ca > Cu > Zn > Fe in black rice beer and in white rice beer Mn > K > Cu > Ca > Fe > Zn, relative to barley malt beer, Cu, Fe and Zn concentration were higher. Both Fe and Cu have unpaired electrons hence can form radicals easily via Fenton reaction and oxidation reaction respectively [137, 150]. Hence the low concentration of these elements in rice may account for the higher oxidation stability in rice beer [6]. Furthermore, the use of pigment rice malt for brewing also increase the functional properties of malt extract and beer via antioxidant activity [1, 7]. Pigmented rice malt extract has higher functional attributes due to the presence of trace amount of pigmented compounds such as anthocyanin [57] hence higher antioxidant activity in pigmented rice malt extract and beer [7, 21] as shown in Table 9. There are many discrepancies in the antioxidant activities related to rice beer. This is because, the method of analysis has noted been standardized. For instance antioxidant activities in beer are determined either by using whole samples or extracting phenolic compounds from the beer for analysis.
6. Sensory and volatile aromatic compounds in beer
The sensory test of products can be categories into two major groups thus using a selected panel which could be made up of trained and untrained personnel, and using high throughput machines such as olfactory machines. At times these methods are integrated to evaluate the sensory properties of the sampled product. The use of waxy and non-waxy rice malt or polished broken fragments have been feasible in the production of gluten. Using a five hedonic scale and experienced panels of 8 from Germany, who are familiar with beer evaluated ale and lager beer produced from waxy and non-waxy rice malt as having a liked appearance (3/5); beer aroma, flavor, mouthfeel and flavor were within the scale of 2 (normal). Despite, all the rice malt beers were drinkable, they possess low mouth feel (watery) hence the tasting panelists tend to prefer a glass [106]. The thin mouth feel associated with rice beer is because of the low nonfermentable sugar profile in rice beer such as maltotetraose, maltopentaose, maltohexaose, maltoheptaose or dextrin; and gum in the beer. In assessing the organoleptic properties of rice beer from different pale rice malt produced from different rice cultivars; it was noticed that each cultivar had a low mouth feel attribute (3/9) on a 9 hedonic scale coupled with less cereal (3/9) and malty character between the scale of 2–3 [17]. These attributes of the rice beers were observed to be dependent on the rice cultivar [17, 106]. However, other aroma character of beer such a fruity or estery is influenced by yeast metabolic activity and hop addition. Improving the mouthfeel of beer is linked with the additions of specialty malt (roasted malt or caramelized malt) because of the presence of unfermentable sugar rice in the form of melanoidins. Using descriptive sensory analysis of 9 hedonic scale, the addition of specialty rice malt in brewing rice beer resulted in an amber colored beer with a more pronounced maltiness (5/9), cereal notes (3.8), expressing caramel (4.5/10), burnt (3.2/10) and bitterness taste of beer [21].
6.1 Volatile aromatic compounds in rice beer profile
The volatile aromatic compounds in beer are produced from yeast metabolism of solubilized phytocompounds derived from malt during mashing. In addition to ethanol and carbon produced by yeast during fermentation, other derived alcohol compounds are produced which impact different aroma and flavor. These aroma inducing compounds are regarded as volatile compounds. These compounds are classified as higher alcohols or fusel alcohol, esters (ethyl esters), carbonyl compounds, aldehydes and vicinal diketones (Table 10). Collectively, these compounds influence the aroma and flavor properties of beer. The production of each class of aromatic compound is dependent on yeast type, temperature, the chemical profile of the wort such as the amount of FAN, fermentable sugars and storage condition of beer. All paddy rice malt beer possesses the aroma inducing compounds as shown in Table 10.
One major esthetic value of beer is foaming stability. The foaming properties of beer is dependent on protein content in malt as highlighted in Section 2.6. Evans and Sheehan [152] highlighted that protein responsible for foaming can be grouped into two based on molecular weight and ability to cause foaming thus, i) high molecular weight protein (HMW, MW 35,000–50,000) made mainly of protein Z and low molecular weight protein (LMW, MW 5000–15,000) comprising of lipid transfer proteins (LTP 1). According to Usansa [106], rice wort contains HMW protein of 47,000 Da but was not present in boiled wort. Meanwhile LPT 1 protein of 11,000 Da was present in both boiled wort and beer which is described to have a poor foam-stabilizing properties. Beer produced from different rice paddy malt cultivar having similar LPT 1 protein molecular weight varied in foaming and stability probably due to the absence of HMW (Table 11) [106, 152].
The use of cereal for brewing purposes is dependent on its chemical composition such as protein content, extract yield and enzyme activity. These chemical composition in cereals have evolved through breeding. The use of rice for malt extract and brewing beer is feasible. However, current malted rice cultivars are characterized as having low FAN, low enzymes activity of α and β amylase. Hence breeding programmes to improve existing cultivars to possess higher FAN, and enzyme activity will enable the use of rice for brewing. And using green rice malt for brewing to improve enzyme activity, potential increase in FAN while minimizing energy consumption in kilning process. Increase in rice malt extract yield would require brewing methods that would remove some fractions of the rice malt husk during milling via dehusking prior to mashing to limit the effect of husk on low extract and on rice beer astringency however this implementation in rice brewing is limited in research. The production of rice malt extract result in the production of malt spent grain which contains significant amount of husk compared to barley hence further studies are required to optimize the brewers spent grain utilization.
The use of rice in brewing serves as an alternate material for gluten free beer and as an adjunct for the production of light beer. The numerous rice cultivars present suggest the availability of varying rice malt extract, specialty rice malt and beer with different organoleptic attribute and functional properties. Despite its low attributes such as enzyme activities and nitrogen content, it’s appropriate to use rice malt either with cereals rich in protein or by adding yeast extract or use exogenous enzymes to improve nitrogen content, extract yield and fermentability. Intense research on rice in brewing will reduce significant reliance on barley and wheat in the production of cereal-based beverages.
The Office of the Ministry of Higher Education, Science, Research and Innovation, Thailand Science Research and Innovation through the Kasetsart University Reinventing University Program 2022.
Mark-Jefferson Buer Boyetey, and Emmanuel Odame for prof-reading, and Nicholas Jon Martinez for assisting in the artwork.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of interest
The authors of this article have not conflict of interest related to finance or authorship.
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Written By
Hellie Gonu, Martin Zarnkow, Gavers Kwasi Oppong and Ulaiwan Withayagiat
Submitted: 23 January 2023Reviewed: 09 February 2023Published: 05 April 2023
Open access peer-reviewed chapter
