A Reassessmment of Sorghum for Lager Beer Brewing

ELSEVIER P I I :S0960-8524(98)00047-9

Bioresource Technology 66 (1998) 253-261 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0960-8524/98 S--see front matter

A REASSESSMENT OF SORGHUM FOR LAGER-BEER BREWING

R. C. Agu* & G. H. Palmer

International Centre for Brewing and Distilling (1CBD), Heriot-Watt University, Edinburgh EH14 4AS, UK

(Received 17 December 1997; revised version received 4 February 1998; accepted 11 February 1998)

Abst ract

The out-of-steep moisture of sorghum is lower than expected (33-36%), but is adequate for enzymic modification of endosperm substrates of sorghum, producing sufficient amylolytic enzymes for brewing lager-type beers. The 65C standard mashing procedure fimited extract recovery from sorghum malt due to inadequate gelatinization of sorghum starch. The 65C mashing was, however, optimal for nitrogen solubilization and hydrolysis of the soluble proteins, yielding high levels of peptides and amino acids. In order to obtain optimal extract yield containing sufficient mash- tun sugar~protein extracts from sorghum malt, a controlled temperature mashing regime would be required. /t high germination temperature of 30C is required for optimal sorghum malt qualities, even though excessively high respiratory and carbohydrate malting loss occurred at 30C germination. This notwithstanding, sorghum germinated at 30C contained higher nitrogen materials in the embryo than in the roots. Also the presence of minerals in the embryo of sorghum may, in part, influence the enzyme- producing potentials of the embryo of sorghum. 1998 Elsevier Science Ltd. All rights reserved

Key words: sorghum, malting, lager beer, enzymes, extract yield.

INTRODUCTION

Sorghum, an indigenous African cereal, is very well adapted to the semi-arid and sub-tropical conditions prevailing over most of the African continent. Like rice and barley, sorghum belongs to the grass family - - the Gramineae. An advantage of sorghum is that it can yield a crop under harsh environmental stress, such as drought, where temperate cereals fail to grow. This property is important, especially in a world that is regarded in some quarters as getting hotter. Over 10000 cuitivars of sorghum exist and

*Author to whom all correspondence should be addressed 253

more are being developed of desired grain quality (Haln, 1966). However, the two most important species are Sorghum bicolor (L) Moench and Sorghum vulgare cv Fara fara. As a world food grain, sorghum is ranked fifth (Pomeranz, 1987), after wheat, maize, rice and barley. It is also the third largest cereal crop in the United States of America (Rooney, 1967), where it finds wide application in animal feed production (Rooney et al., 1986). Sorghum is also produced in large quantities in China, India, Argentina, Nigeria, Mexico and Australia.

Sorghum is the main food grain in parts of India and Africa, where it is mainly used in making bread, porridges and opaque alcoholic beers (Rooney et al., 1986; Serna-Saldivar et al., 1988; Bello et al., 1990; Mohammed et al., 1993). Although sorghum has been used for centuries to brew traditional (opaque) beer in Africa (Faparusi, 1970; Novellie, 1977; Ogundiwin and Tehinse, 1981), in recent times sorghum beer brewing has developed into a major industry. These types of beers differ from European (lager) types in that lactic acid fermentation also occurs during sorghum beer processing. Also, the alcoholic drink is consumed while still fermenting, and it contains large amounts of insoluble materials (Rooney and Serna-Saldivar, 1991). These are mainly starch fragments and dextrins which are not digested during mashing and fermentation (Glennie and Wight, 1986).

Although sorghum grain has always been a potential source of industrial brewing material, it was not until World War II, when brewing materials were scarce, followed by extensive discussion in 1943 on brewing with sorghum, that sorghum was offered as a brewing adjunct (Haln, 1966). Brewing with sorghum grits as adjunct faded out immediately with the end of World War II. The large quantity of sorghum grain and its usually low price (Haln, 1966) has enabled sorghum to remain as an important source of starch and protein.

254 R. C. Agu, G. H. Palmer

It is noteworthy that in countries like Mexico, Cuba, Nigeria and Israel, the commercial value of sorghum is shifting from a food source for humans and animals to the raw material for the industrial production of European-type lager beer (Palmer et al., 1989). This recent development in the commercial value of sorghum in lager-beer brewing should encourage radical agricultural mechanization in the growing of sorghum (Palmer et al., 1989). This is more so for a country like Nigeria where only sorghum and maize are used to brew large volumes of European-type lager beer. For example, prior to the commercial use of sorghum in lager-beer brewing in Nigeria, the annual production figure for sorghum for 1967/70 was 4 million tonnes (Olayide et al., 1972), rising to between 4-8 and 5 million tonnes in 1988/89 (Aisien, 1988; Palmer et al., 1989).

In recent times, however, a dramatic change in the trend of sorghum production in Nigeria has been reported, probably reflecting the commercial use of sorghum in beer production in Nigeria (Anon, 1993). Recent reports show that the supply and demand statistics for sorghum grown in Nigeria in 1991 was estimated at 10 and 12million tonnes, respectively. These figures are significantly higher than the production figures reported for 1970 or 1989. Notwithstanding this rise, these production figures are far below the 21 million tonnes produced in the United States of America (Palmer et al., 1989).

The literature is filled with brewing-related studies on sorghum, especially with a view to substi- tuting barley malt with sorghum malt (Skinner, 1976; Okafor and Aniche, 1980; Aisien and Muts, 1987). However, some studies have suggested that the use of sorghum malt in lager-beer brewing is not likely to succeed because of some inherent problems associated with sorghum (Aisien, 1982, 1988; Glennie et al., 1983; Glennie, 1984; Morrall et al., 1986; Dale et al., 1989, 1990; Bajomo and Young, 1990; EtokAkpan and Palmer, 1990a,b; Palmer, 1991). The present purpose is to report further research findings on the use of sorghum and malt in brewing clear lager beers. Some important physiological differences between sorghum and barley will also be highlighted, which should be of value for both the food and brewing industries.

wine made from barley, while liquor made from corn and water was brewed in Germany and Spain. Although brewing was introduced into ancient Greece and Italy, it does not appear to have ever been used extensively in these countries. The manufacture of ale is an old tradition in England. Other countries too, including Russia and some African countries, have ancient records of the preparations of intoxicating liquors from a variety of cereals.

"All the nations" says Pliny, "who inhabit the west of Europe have liquor with which they intoxicate themselves, made from corn and water (fruge madida). The manner of making this liquor is somewhat different in Gaul, Spain and other countries, and it is called by many various names; but its nature and properties are everywhere the same. The people of Spain, in particular, brew this liquor so well that it will keep good for a long time. So exquisite is the ingenuity of mankind in gratifying their vicious appetite, that they have thus invented a method to make water itself intoxicated".

It is clear that from the onset the brewing industry depended to a great extent on cereal grains, the primary raw material. In modern brewing, however, barley has acquired an international acceptance as the brewing grain of choice, probably due to some inherent special qualities it has, or due to extensive research conducted on barley which has resulted in a considerable improvement of the brewing qualities associated with barley. Beer is therefore defined as a beverage obtained by alcoholic fermentation of malted cereal, usually barley malt, with or without other starchy materials and to which hops have been added (Jasper and Philip, 1974). Hoyrop (1978), on the other hand, defined lager beer as a brew from barley malt, which is stored for a period of time for maturing. But beer is also considered as the generic term for liquors made from malted barley, variously called beer, ale, stout, porter and lager. Lager and ale are usually produced using bottom- and top-fermenting yeasts, respectively.

USE OF VARIOUS CEREALS AND POTENTIALS OF SORGHUM IN MODERN BREWING

BRIEF HISTORY OF THE GENERAL USE OF GRAINS IN BREWING

The manufacture of alcoholic liquor is at least 6000years old. Some of the oldest records of brewing are from Egypt, China, Greece and Rome (Jasper and Philip, 1974). The manufacture of alcoholic drink was practised at least 5000 years ago by the Egyptians (Anon, 1973). In the middle ages, brewing was associated with the monasteries. Horodotus (450 BC) recorded that Egyptians drank

The chemical compositions of cereal grains are similar, suggesting their general use in brewing. This is illustrated in Table 1. The similarity of grain composition notwithstanding, some factors and/or variations in different parameters may, however, limit the grain for use in brewing. For example, although rice is slightly higher in starch due to the low protein and fat content, its value as a staple food in most countries of the world will limit expanded use of rice as a brewing cereal. Wheat is characterized by its high content of expandable

Reassessment of sorghum .for lager-beer brewing 255

Table I. Percentage average composition of cereal grains (dry basis)

Sorghum Corn Wheat Barley Rice

Starch 7 l" I 72" l 69-8 67"11 73-0 Protein 10-5 9.5 13-2 12.7 9-2 Fat 3-0 4.5 1-9 1.9 1.4 Fibre 2-0 2.0 2-6 5.4 2-7 Ash 1-5 1 "5 1-8 2.8 1-8

Source: Haln (1966).

proteins (Briggs et al., 1981; Palmer, 1989) and this may present a serious problem during the brewing process, especially with regards to lautering and haze. Corn is used internationally as a brewing adjunct. Sorghum is similar to corn in chemical composition (Table 1), except that the fat content of sorghum is lower than corn, which is advantageous in brewing.

As regards fermentable carbohydrate extracts, sorghum is similar to the corn and rice used internationally as brewing adjuncts. The high starch content of sorghum, if converted to sugars, and the low price of sorghum (Haln, 1966), would offer not only a rich source of fermentable extracts, but would reduce brewing costs. Although, unlike barley, sorghum does not contain a husk, a disadvantage in standard brewhouse mash filtration, this would not present any problem with modern mash filters. It is interesting to note that a number of large breweries have used endosperm grits of the type shown in Table 2 for the production of lager beer, with good results (Haln, 1966; Canales and Sierra, 1976; Canales, 1979; Palmer, 1989). From the brews of each type of adjunct and malt and the wort fermented separately, the data given in Table 3 were obtained. It is clear from Table 3 that sorghum furnished a higher amount of ~-amino nitrogen than other adjuncts (Haln, 1966). The importance of this is that a-amino nitrogen is required by yeast during fermentation so that yeast can grow and produce alcohol and flavor compounds. The use of cereals other than barley in brewing is becoming widely accepted. It is interesting to note that wheat is used in the production of Weissbier (wheat beer).

Table 2. Properties of brewer's grits (dry basis)

Sorghum Corn Rice

Moisture (%);' 11.7 1 i.6 12-2 Oil (%) 0.72 0-92 0-78 Extract (%) 91.4 91.1 93'6 Protein (N 6.25) 10.40 9.65 Fibre (%) 0'75 0'80 Ash (%) 0"30 0"35

Source: Haln (1966). "Grain moisture reflects the level of moisture at which the grain will keep safe without deterioration when stored.

USE OF SORGHUM MALT IN MODERN BEER BREWING

Enzyme generation by sorghum malts One serious problem usually highlighted in experi- mental studies of brewing with sorghum malt is insufficient enzyme levels. Early studies (Kneen, 1944, 1945) led to the incorrect view that because sorghum malt contained little or no fl-amylase for saccharification it was unsuitable for brewing lager- type beers. Other workers (Novellie, 1959, 1960a,b, 1962a,b; Okon and Uwaifo, 1984, 1985) argued that sorghum has sufficient amylolytic enzymes but the conventional methods of enzyme assay for barley were unsuitable for research studies on sorghum. In support of this view, Dufour et al. (1992) reported that although brewers' specifications for barley malt are fairly established, this is far from the case with sorghum malt. When different methods (South African Bureau of Standards, 1970) were used for the determination of sorghum malt diastatic power, Taylor and Robbins (1993) reported that ungerminated sorghum grain exhibited no fi-amylasc activity, but when malted, sorghum had //-amylase activity of less than 25% of the level in barley malt. Because neither reducing agent nor papain increased the level of //-amylase enzymes, these workers concluded that /~-amylase in sorghum was not in the bound form, unlike in barley. However, as in barley, sorghum /~-amylase was more temperature-labile than its ~-amylase (Taylor and Robbins, 1993).

Extract yields Limited endosperm cell wall degradation, with thc inherent economic drawbacks of low extract yields, poor wort separation and poor beer filtration (Aisien, 1982, 1988; Glennie et al., 1983; Glennie, 1984; Okon and Uwaifo, 1985; Morrall et al., 1986; Aniche and Palmer, 1990a; Bajomo and Young, 1990; EtokAkpan and Palmer, 1990a,b; Palmer, 1991) are other obstacles reported when sorghum malt is used in lager-beer production. To overcome the problem of low extract yield from sorghum malt, Skinner (1976) adopted a very extensive mashing procedure involving a three-stage decoction technique to extract sorghum malt. The extract yield obtained from this mashing regime was lower than the levels usually obtained from a well-modified sample of barley malt.

Although Dufour et al. (1992) obtained extract recovery of 82-7% from sorghum malt, which is higher than values reported elsewhere (Skinner, 1976; Okon and Uwaifo, 1984, 1985), this value was again lower than that from barley malt. The higher extract yield of 82-7% and reasonable attenuation limit achieved from sorghum malt (Dufour et al., 1992) led these workers to conclude that it is feasible to produce sorghum malts of a quality similar to barley malt if the right sorghum cultivars

256 R. C. Agu, G. H. Palmer

Table 3. Average wort properties of the brews

Adjuncts Plato" pH Total nitrogen a-Amino nitrogen Diacetyl Antho-cyanogen (ppm) (ppm) (ppm) (ppm)

Sorghum grits 12"0 5-30 782 317 0"14 17-1 Corn grits 12.0 5.35 820 251 0" 15 19.4 Brewer's rice 12' 1 5-35 812 239 0" 15 25.4 Corn syrup 12.0 5"30 785 181 0.16 25"0 Cane sugar 12.1 5-40 775 188 0-14 46.1 Barley malt 7"2P 7"2 5.45 835 185 0-11 44.9 Barley malt 12P 12.0 5.40 1345 415 0.12 84.2

Source: Haln (1966). "Plato is a measure of extracts recovered as excess gravity of water.

are selected. This supports earlier studies (Glennie et al., 1985). In further support of this view, Palmer (1989) reported that high extract yield could be obtained from sorghum malt by means of a three- stage mashing procedure, but the corresponding fermentable extracts were significantly lower than they were in barley malt worts.

Fermentable sugars Regarding the relative amounts of fermentable sugars in sorghum and barley worts, the major difference was found to reside in the glucose content (Palmer, 1989; Dufour et al., 1992; Taylor, 1992; Byrne et al., 1993). While some workers found barley malt worts to contain more maltose than glucose (Briggs et al., 1981; Dufour et al., 1992), others reported that sorghum malt worts contain similar levels of glucose and maltose (Taylor, 1992; Byrne et al., 1993). The difference observed in the proportions of glucose and maltose sugars in sorghum and barley malt worts was attributed to the low levels of/I-amylase enzymes in sorghum malt, in contrast to the high levels of this enzyme in barley malt (Palmer, 1989). Other workers (Byrne et al., 1993; Taylor and Dewar, 1994) attributed the high level of glucose found in sorghum malt wort to the catalytic activity of ~-glucosidase in hydrolysing maltose to glucose in sorghum malt wort. When high levels of glucose are present in fermentable extract, some yeast strains may lose their capacity to ferment maltose (Griffin, 1970; Lovegren and Hautern, 1977). The risk is real for sorghum wort if glucose syrup is used to supplement sorghum malt wort (Dufour et al., 1992).

The drawbacks highlighted above in using sorghum malt in lager-beer brewing led some workers to propose that since enzymes were also required when malted sorghum was used alone for brewing, the best practical approach was to use commercial enzymes to convert unmalted sorghum grains (Macfadden, 1989; Dale et al., 1989; Bajomo and Young, 1993). However, mashing experiments performed using unmalted instead of malted sorghum and exogenous enzymes are associated with processing difficulties (Albini et al., 1987; Aisien, 1988; Dale et al., 1989, 1990; Ugboaja et al., 1991).

Most important is poor foam retention (Dale et al., 1989). Studies show that the use of exogenous enzymes in mashing raw sorghum reduced foam head retention because commercial proteolytic enzymes destroyed foam proteins (Agu and Palmer, 1998). Although it was argued that the best practical approach was to mash raw sorghum with commercial enzymes, one major advantage in favour of using malted sorghum as a percentage of the raw material in lager-beer brewing is that the proteolytic enzymes of the malted grain produced sufficient free a-amino nitrogen (FAN) for efficient buffering capacity and optimal yeast performance (Haln, 1966; Palmer, 1989; Bajomo and Young, 1993).

Physiological differences of sorghum and barley in relation to malting Recent studies on malting of sorghum show that a germinative energy at 99% level can be achieved with some sorghum varieties (Table 4). This suggests that even germination will occur during malting of sorghum. Germination of grains is an essential part of the malting process because when grains do not germinate, or germinate poorly, they do not contri- bute to the enzyme development of the malt and uneven modification of the malt occurs. Sufficient enzyme modification of the endosperm substrates will not be achieved and will result in sub-optimal extract development. Also important is that ungerminated grains have been shown to be ready sources of microbial infection during malting of grains [Iygor, 1987; Agu and Palmer, 1997c (unpub- lished data)]. This could lead to the production of malt with potentials to develop aflatoxins during the brewing process.

Protein content, grain hydration The protein value of sorghum, 8-11% (Table 4), is of an acceptable level for effective proteolysis during malting of sorghum (Palmer, 1989). High nitrogen levels may limit the extent of proteolysis in cereal grains to be malted, and hence may limit the release of starch and protein reserves of the grain. It is interesting to note that when sorghum grains are steeped in water, a preparatory stage for germina-

Reassessment of sorghum for lager-beer brewing

Table 4. Properties of sorghum and sorghum malt and extracts, malting temperature 30C

257

Sorghum Malt Extract

Moisture (%) G.E. (%) 4 ml test Total nitrogen (%) Protein (N x 6-25) Out-of-steep moisture (%) Malting loss (%) a-Amylase (U/g) fl-Amylase (U/g) Wort colour (L) HWE (l/kg) a TSN (%)b FAN (mg/l) " Ninhydrin assay FAN (mg/1) TNBS assay TRS (/~g/ml) Glucose (mg/l) Maltose (mg/l)

9"4-13"0 80-99

1"47-1"74 9"2-10"9 33-36

21"3-28"5 39-135 80-168

5"5-12"0 270-327 (89-196) '~ 0"5-0'7 (0"3-0"6) d 135-316 (94-216) d 164-412 (172-350)" 98-188 24-220 68-245

~HWE (hot water extract) is a measure of soluble materials leached into water under mashing conditions at 65C. "TSN (total soluble nitrogen) is a measure of nitrogen materials solubilized as a result of proteolysis during malting, which are extracted during mashing. ~FAN (free amino nitrogen) is a measure of hydrolysed portions of the soluble proteins during mashing. dValues in parenthesis are from 65C mashing regime.

tion, the maximum hydration obtained from sorghum is usually 33-36% (Evans and Taylor, 1990; Agu and Palmer, 1996) (see also Table 4). This value of out-of-steep moisture for sorghum is lower than 44-46% in the case of barley (Baxter, 1978; Baxter and O'Farrell, 1980; Baxter et al., 1980). It is not clear if the pericarp of sorghum behaves in a similar manner to that of barley, e.g. permeable to water but semi-permeable to some salts (Palmer et al., 1989). However, the limited level of water, 33-36%, imbibed during steeping of sorghum, suggests limited permeability of the pericarp, or poor hydration potentials of endosperm, of sorghum. Adequate hydration is important for maximum enzymic modification of endosperm substrates during germination (malting). This highlights one important physiological difference between barley and sorghum, and may in part account for the differences in the mode of enzymic modification of the endosperm reported for barley and sorghum.

Notwithstanding, this low out-of-steep moisture is optimal for the germination of sorghum. The additional water required to maintain enzyme modification of the endosperm during malting of sorghum is supplied by spraying on limited quantities of water. When barley is germinated at a higher temperature than the optimal malting temperature of 16 or 17C, excessive moisture is lost. The usual practice of adding water to sorghum at 25C during germination, for effective enzymic modification of endosperm substrates, was not effective for barley (Agu and Palmer, 1997b). This highlights another important physiological difference during malting of sorghum and barley. It is noteworthy that an increase in germination temperature of 3-4C

limited enzymic modification of the endosperm of barley (Agu and Palmer, 1997b). These observations show that the malting procedures developed for barley are not likely to work for sorghum because of differences in the physiology of sorghum and barley grains.

Malt ing response When sorghum was malted at different temperatures, highest respiratory and carbohydrate malting losses occurred at the higher germination temperature of 30C (Table 4). This high malting loss notwithstanding, optimal malt qualities for sorghum were obtained when the malt was made at 30C (Novellie, 1962a; Okafor and Aniche, 1980; Pathirana et al., 1983; Maileshi and Desikachar, 1986; Onwuama and Okafor, 1991; Ratnavathi and Ravi, 1991; Demuyakor and Ohta, 1992; Lasekan et al., 1995; Agu and Palmer, 1996). This probably reflects the tropical physiology of sorghum. Barley responded differently, producing optimal malt qualities at more temperate germination temperatures of 16-17C. This observation further highlights physiological differences between sorghum and barley.

It is worth noticing that the high malting loss of sorghum germinated at 30C was associated with higher levels of nitrogen in the embryo than in the roots of sorghum when malted at 30C than at 20C (Agu and Palmer, 1996). Corresponding high levels of nitrogen materials were found in the embryo of barley germinated at 17C. It is interesting to note that although high nitrogen concentrations were found in the embryos of sorghum and barley during germination (malting), the enzyme production sites of both cereal grains differ.

258 R. C. Agu, G. H. Pahner

Location of enzymes The embryo is the major site for the synthesis of many hydrolytic enzymes in sorghum. In contrast, the aleurone layer is the major production site for hydrolytic enzymes in barley (Varner and Chandra, 1964; Macleod et al., 1964; Aisien et al., 1983; Mundy et al., 1983; Palmer, 1989; Aniche and Palmer, 1991)b). The difference between the production sites of these hydrolytic enzymes in the two grains, barley and sorghum, further suggests that different physiological factors may be playing a key role in the observed difference in the enzyme production sites of both cereals. In this regard, it is worth noting that whilst gibberellic acid is required for enzyme synthesis and release in barley, this hormone plays no such role in enzyme development in sorghum. Phosphate is an important mineral of the aleurone in barley - - in sorghum this mineral is mainly located in the embryo (Palmer et al., 1989), and in part, may account for differences in the enzyme-producing potentials of the aleurone of barley and the embryo of sorghum.

Enzyme development and soluble carbohydrate and protein-extract recovery in sorghum malt Recent studies (Agu and Palmer, 1996, 1997a,b,c) show that starch hydrolysing enzymes, ~- and /#amylase, which developed during malting of sorghum appeared to be in low activities (Table 4), when assayed using new standard methods (McCleary and Sheehan, 1987; McCleary and Codd, 1989). These seemingly low enzyme levels of sorghum malt are nevertheless sufficient to produce commercially acceptable yields of extract. However, an adapted mashing procedure developed for extracting sorghum malt (Agu and Palmer, 1996, 1997a,b), which gelatinized sorghum starch and protected the enzymes of sorghum malt, must be used to extract sorghum malt if equivalent starch extract to that achieved for barley malt is to be realized. Table 4 shows that extract recovery rates similar to/or higher than those of well-modified barley malt were achieved with different sorghum varieties using this decantation mashing regime (Agu and Palmer, 1996, 1997a,b,c). In contrast, the 65C standard mashing regime used for barley malt, which did not gelatinize sorghum starch, produced extract yields which were low (Okon and Uwaifo, 1984, 1985; Dufour et al., 1992; Agu and Palmer, 1996). It is clear from these results that the main reason for the low carbohydrate extract yield usually reported for sorghum malt is caused by inadequate gelatinization of sorghum starch rather than inadequate levels of hydrolytic enzymes.

The results presented in Table 4 show that the malting and brewing biochemistry of sorghum is quite different from the malting and brewing biochemistry of barley. It is, however, interesting to note that although the extract recovery from the 65C mashing was low, nitrogen solubilization and

hydrolysis of the soluble nitrogen in sorghum malt were effective in the 65C mashing procedure (Table 4). Mashing of sorghum malt at 65C rather than the decantation method resulted in the production of high levels of peptides (Agu and Palmer, 1996, 1997a,b). Although yeast metabolizes small peptides, it is not clear how yeast will behave in a medium containing high peptide levels. This requires further investigation. However, the result highlights that whilst a different mashing procedure is required for efficient extraction of the carbohydrates of sorghum malt, soluble protein extraction of sorghum malt can occur at similar mashing procedures to those routinely used for barley malt. Brewing with sorghum malt would therefore require the development of a suitable mashing regime which would be quite different from that of barley malt (Dufour et al., 1992).

Low maltose yield of sorghum Regarding the ratios of glucose and maltose sugars present in sorghum malt wort, different propositions have been put forward (Palmer, 1989; Dufour et al., 1992; Byrne et al., 1993; Taylor and Dewar, 1994). While some workers attributed the low maltose and high glucose sugars of sorghum malt wort to low fi-amylase enzyme activity (Palmer, 1989), other workers attributed this to the role of a-glucosidase enzyme of sorghum malt in hydrolysing maltose to glucose (Byrne et al., 1993; Taylor and Dewar, 1994). However, recent studies (Agu and Palmer, 1997c) showed that there is no direct relationship between ~-glucosidase enzyme levels of sorghum or barley malt and the maltose to glucose ratios found in their worts. It is worth noting that barley malt developed a higher level of ~-glucosidase enzymes than did sorghum malt, but produced less glucose and several times more maltose in the wort than in sorghum wort (Agu and Palmer, 1997c).

It is important to note that the limitation to maltose production in sorghum malt wort was caused by inadequate gelatinization of sorghum starch (Agu and Palmer, 1996, 1997a,c). Although the different sorghum varieties malted and mashed under similar conditions showed wide variations in the sugar profiles in their worts, the variations found in the sugar profiles could be due to seasonal and processing differences, but variations caused by variety and malting temperature do not alter the greater influence which starch gelatinization has on the sugar profile of sorghum worts than on those of barley.

CONCLUSION

This report has shown that sorghum malt can produce sufficient amylolytic and proteolytic enzymes for brewing lager-type beer. The low amylolytic enzyme levels usually reported for

Reassessment of sorghum for lager-beer brewing 259

sorghum malt seem to relate to the method of enzyme assay. It is known, however, that a-amylase attack on sorghum starch during malting could be greater than that which occurs in barley during malting (Palmer, 1989). This suggests that the failure of a-amylase to hydrolyse sorghum starch in the 65C mashing regime is caused by factors affecting starch gelatinization problems rather than deficien- cies in enzyme level.

This paper has also highlighted some important physiological differences between sorghum and barley. Like other cereals used in modern brewing, sorghum undergoes similar physiological changes when malted. However, in order to obtain optimal extract yield from sorghum malt, a controlled temperature mashing procedure rather than the conventional mashing at 65C would be required. In general, recent studies (Agu and Palmer, 1996, 1997a,c) show that this decantation mashing procedure was very effective in extracting sorghum malt, producing sufficient mash-tun sugar/protein extracts for optimal yeast fermentation.

Apart from sorghum being a cheap source of brewing raw material, the high temperature (25-30C) required to malt sorghum is advantageous because refrigeration is expensive. Also as a crop, sorghum is more likely to survive than other cereals in dry tropical conditions and is likely to play an important role as a food source in a world that may be getting warmer. More scientific knowledge is required about sorghum, not only in terms of germination and food mobilization (modification), but also in terms of the physiological principles which ensure that the grain will germinate effectively in the field. For example, sorghum malt has very low levels of /3-glucan cell wall material, but /3-glucanase appears to be required to assist filtration during mashing (Aisien and Muts, 1987). The reason for this may be in the differences in the structures of the endosperm cell walls of sorghum and barley malts. Sorghum can compliment the use of other cereals as food for humans and as raw material for brewing and distilling.

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A Reassessmment of Sorghum for Lager Beer Brewing