The main leavening agent is yeast. When yeast was not available, sourdoughs were employed. Their active constituents were in part not only endogenous yeasts but also heterofermentative lactic acid bacteria.

Baker's yeast is of course Saccharomyces cerevisiae. It is a top fermenting organism, cultured on molasses in aerobic, fed-batch culture so as to maximise yield. Growth is optimal at 28-32°C and within the pH range 4-5.

The bread mix will comprise 1-6% yeast depending on the weight of flour and some other factors. The yeast is most commonly employed as a compressed cake of 28-32% solids. The cake can be stored at 4°C for 6-8 days and may be mixed with water before use. The yeast may also be in the form of a cream, which is a centrifuged and washed suspension of approximately 18% solids. This is shipped as needed to bakeries for use within the day. For logistical reasons, there is increasing use of ADY, a dehydrated form of 92-96% solids. It can be stored for upwards of a year. It is re-hydrated prior to use.

Sourdough starter cultures typically comprise 2 x 107 to 9 x 1011 per gram bacteria and 1.7 x 105 to 8 x 106 per gram yeasts. The precise populations are frequently ill defined, but Lactobacilli are prevalent (Table 12.1). The organisms are either anaerobes or microaerophiles, are either homofermenta-tive or heterofermentative, and are acid tolerant. The acid produced by these organisms results in bread with good grain texture and an elastic crumb. The heterofermentative organisms tend to give preferred organoleptic characters to the product. Thus, San Francisco sourdough employed chiefly the hetero-fermentative Lactobacillus sanfranciscensis and the yeasts Torulopsis holmii, Saccharomyces inusitus and Saccharomyces exiguous.

Table 12.1 Sourdough starter organisms.

Homofermentative organisms

Lactobacillus acidophilus Lactobacillus casei Lactobacillus farciminis Lactobacillus plantarum Heterofermentative organisms Lactobacillus brevis Lactobacillus brevis var lindneri Lactobacillus buchneri Lactobacillus fermentum Lactobacillus fructivorans Yeasts Candida crusei Pichia saitoi

Saccharomyces cerevisiae Torulopsis holmii

Chemical leavening agents tend to be employed for sweet goods and cakes. A combination of carbonate and acid when heated generates carbon dioxide. Thus, a mixture of baking powder (sodium bicarbonate) and tartaric acid or citric acid achieves widespread use. Baking powder may also be used to support the leavening power of yeast. Similarly, lactic acid bacteria may accompany baking powder.

Leavening may also be achieved by physical treatments - that is, the beating in of air. Egg whites may be added to underpin foam formation.

One example of mechanical leavening involves the retention of steam between thin sheets of dough and intervening fat layers, namely puff pastry.

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