Microbes in Food Processing

Fermentation: The Basic Process

Fermentation, in the biological sense used throughout this chapter, is the metabolic process by which microorganisms break down organic molecules -- most commonly sugars -- to obtain energy, typically without requiring oxygen. Depending on the organism and the substrate, fermentation yields different end products: lactic acid, ethanol and carbon dioxide, or various organic acids. It is these end products, and the changes they produce in taste, texture and preservability, that make fermentation so valuable in food processing.

Curd (Dahi) From Milk

The transformation of milk into curd is perhaps the most familiar example of microbial food processing in Indian households. Fresh milk contains the sugar lactose. When a small amount of already-set curd -- containing a live, active population of lactic-acid bacteria (LAB), principally species of the genus Lactobacillus -- is stirred into warm fresh milk as a starter culture, these bacteria begin multiplying rapidly, doubling in number every twenty to thirty minutes under favourable conditions. As they grow, the LAB ferment the lactose into lactic acid. The steadily accumulating lactic acid lowers the pH of the milk; once the pH drops far enough, the milk proteins (chiefly casein) can no longer stay dissolved in their normal form and instead coagulate, causing the liquid milk to set into the semi-solid gel we recognise as curd. Beyond the change in texture and the characteristic tangy taste, this bacterial fermentation also increases the food's nutritional value: LAB synthesise vitamin B12, so curd made this way generally contains more of this vitamin than the milk it was made from.

Leavening Bread

Bread-making relies on a very different microorganism -- the yeast Saccharomyces cerevisiae, commonly called baker's yeast. When yeast is mixed into wheat-flour dough along with sugar and kneaded, it ferments the available sugars, releasing carbon dioxide gas and a small quantity of ethanol as by-products. Because the dough is elastic (thanks to a protein network called gluten), the carbon dioxide gas gets trapped inside as countless tiny bubbles rather than simply escaping. This trapped gas is what makes the dough rise, or "prove," before baking. During baking, the heat expands these gas bubbles further and then sets the structure permanently, producing bread's characteristic light, porous, spongy texture -- very different from the dense, flat result of baking unleavened dough. The small amount of ethanol produced evaporates away during baking.

Idli and Dosa Batter

Idli and dosa, staple South Indian foods, are made from a batter of rice and dehusked black gram (urad dal), soaked, ground and then left to ferment, typically overnight. This fermentation is carried out by a natural mixed population of bacteria present on the rice and lentils themselves -- most notably lactic-acid bacteria such as Leuconostoc mesenteroides, along with other naturally occurring bacteria and, in some traditional accounts, yeasts. As these microbes ferment the sugars in the batter, two visible changes occur: the batter increases substantially in volume as carbon dioxide gas is trapped within it (much like the rise seen in bread dough), and its texture and flavour change, developing a mild sourness from the organic acids produced. The fermented batter, once steamed (for idli) or shallow-fried on a griddle (for dosa), yields a soft, slightly tangy, easily digestible food -- and fermentation also improves the batter's nutritional profile, since some of the fermenting bacteria synthesise B-group vitamins. …