brewing yeast and fermentation Table 2.5 Dextrin spectrum of a 12°Plato lager wort (from Enevoldsen, 1974).

Composition (degree of polymerisation) g 100 ml 1 wort

1 [non-fermentable sugars] 0.03

2 [non-fermentable sugars[ 0.07

3 [non-fermentable sugars] 0.12

10 0.07

13 0.12

14 0.11

15 0.08

Dextrins (4 or more glucose units) [% total wort extract) 22.1

Non-fermentable sugars [% total extract] 2.0

Fermentable sugars [% total extract] 66.9

Non-carbohydrate [% total extract] 9.0

Degree of polymerisation

Fig. 2.14 Distribution of dextrins in malt wort (drawn from data in Table 2.5).

a-(l->6) linkages. Obviously linear dextrins can have one structure only, whereas in the case of larger dextrins several branched structures are possible. Predictably, the proportion of branched dextrins increases with the number of glucose units such that those with four glucose units contain 30% branched molecules; 70% in the case of those with seven glucose units and 100% at ten glucose units and above (Enevoldsen and Schmidt, 1973; Enevoldsen, 1974). The same authors concluded that this was evidence that the majority of a-(l->6) linkages of malt amylopectin survive wort production intact. Furthermore, the relative abundance and patterns of branching in wort dextrins was a reflection of the structure of the starch of malt.

Concentrations of (3-glucans [(1 ->3)(1 ->4)-p-D-glucans] in worts are dependent on the type of malt used and the mashing regime. Aastrup & Erdal (1987) reported levels of 560-620 mgl 1 in sweet worts of 15.7°Plato made with brewing malt. Using the same wort production regimes non-brewing malt produced sweet worts containing 1000-1300 mgl 1 p-glucan. (3-glucans are derived from malt endosperm cell walls where they exist as polymers with molecular weights between 50000 and 200000 (Schur et al., 1974). Complete degradation yields glucose, although during malting endo-b-(l->3)(l->4) glucanases yield saccharides of varying chain-lengths and the disaccharides cellobiose and laminoaribiose. The proportions of degradation products depend on the mashing temperature, since malt (3-glucanases are heat labile, or whether or not (3-glucanases are added during wort production as process aids.

p-glucans, together with pentosans, contribute to wort viscosity and their occurrence at high concentrations in certain barley varieties is one of the factors which makes them unsuitable for malting for brewing purposes (Vietor et al., 1993). In malts some 73% of pentosans are in the form of arabinoxylans which consist of a polymer of |3-(l->4) linked xylose units with side chains of single p-(l->3) linked arabinose molecules. These polymers are associated with the aleurone layer cell walls of barley grains. During mashing little degradation occurs and the polysaccharides persist in worts at concentrations of the order of 2.0 g per litre of wort (Vietor et al., 1991).

2.4.2 Nitrogenous components

Nitrogenous components of wort account for 4-5% of the total dissolved solids. The bulk (85-90%) of the total is in the form of amino acids, small peptides and proteins (Enari, 1974; Hudson, 1974; Jones, 1974; Lie et al., 1974; Moll et al., 1978). The relative proportions of each of these groups of nitrogenous components depend on the composition of the grist and the conditions of wort production. Thus, controlling factors are the nitrogen content of the malt, dosage rates and type of adjunct, the extent of proteolysis during mashing and precipitation and removal during the wort boil and clarification steps.

Worts can contain more than 1000 mgl 1 total nitrogen, depending on the materials used and the process conditions, although 700-800 mgl 1 is more typical (Jones, 1974). Clapperton (1971) fractionated the soluble nitrogen of wort and reported that 20% was protein, 22% polypeptides and 58% peptides and free amino acids. Free amino acid concentrations are within the range 150-230 mgl 1 in a wort of 10.5°Plato (Lie et al., 1974). Recommended free amino nitrogen concentrations of wort is of the order of 150-200 mg 1 1 where oxygen is the limiting substrate. A third to a half of the amino acids in wort arise from the action of proteases (mainly car-boxypeptidases) during mashing, the remainder being derived directly from malt and formed during malting. Malt carboxypeptidases have maximal activity at temperatures between 40 and 60°C and are inactivated at 70°C (Jones, 1974). It may be appreciated, therefore, that the temperature at which mashing is conducted has a crucial impact on the free amino nitrogen content of worts. All free amino acids can be assimilated by yeast during fermentation, under appropriate conditions, other than proline, which requires oxygen (see Section 3.3.2). Some 40% of the small peptides are also utilised (Clapperton, 1971).

The free amino acid spectra of worts are relatively constant (Table 2.6). Total concentrations arising in worts are influenced by raw materials and wort production techniques. Jones (1974) reported a-amino nitrogen contents of barley as varying between 592 and 946 (ig per grain. Abrading and gibberellic acid treatment during malting both increase a-amino nitrogen contents of malts, whereas there is a negative correlation between nitrogen level and curing temperature. The temperature of mashing has a most significant impact on a-amino nitrogen concentration (Tables 2.7a and b). The data presented here, taken from Chen et al. (1973), shows that a mashing temperature close to 60°C produced a greater yield of a- amino nitrogen than 40 or 70°C. Use of low nitrogen adjuncts, in addition to malt, predictably reduces wort nitrogen content (Table 2.8).

The protein and polypeptide fraction of worts are a diverse group of molecules with molecular weights within the range 5000-100000 (Enari, 1974). The various protein fractions have importance with respect to their contribution to head retention of beers and ability to interact with polyphenols and form beer hazes.

Table 2.6 Amino acid content of worts (mg 100 ml 1 wort of 10°Plato) (from Otter and Taylor, 1976).

Amino acid

All malt wort

Brewery wort

Malt + 25%

Malt +25%

Acidified mash

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