A Bacterial Biomarker of pH

Fermented food items are a staple in many cultures. They are popular for great taste and health benefits. The only disappointment is that fermentation is a complex cellular process and replicating it in a kitchen requires expertise. Keeping our recent enthusiasm in food science, I will discuss biomarkers in the fermentation of milk.

Fermentation of milk is the process in which Lactobacillus species convert lactose into lactic acid. This causes the pH of the milk to decrease. Casein, the protein component of milk coagulates in low pH environments. This seems easy and simple, except when things go wrong. Yogurt, for example is best made from a fresh culture of Lactobacillus not more than two days old. Once the culture gets older than that, the resulting yogurt tastes excessively sour and later, even bitter. When acid begins to accumulating in the yogurt, bacteria sense it as a sign of stress. Stress metabolism triggers a complex process that leads to sporulation – the bacterial equivalent of hibernation. Normal growth in such bacteria can be revived only when the conditions are favorable – plenty of sugar to feed on.

Sporulation can be identified with a microscope. Sporulating bacteria take up less stain and show encapsulated spores inside them.

lacto spores
Gram stained sporulating Lactobacillus
Gram stained Lactobacillus

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