26.1 Microbes and how they feed

Two kinds of microbe

The microbes that make food fall into two big groups. Scientists call bacteria prokaryotes. Yeasts and other fungi, such as molds, belong to the second group, the eukaryotes. Each microbe used in food belongs to one group or the other. Among them are Saccharomyces, Lactobacillus, Leuconostoc, Streptococcus, Penicillium and Acetobacter.

Of these, Lactobacillus and Leuconostoc are bacteria that make lactic acid. Saccharomyces is a yeast.

Sugar: what microbes eat

Microbes run on carbohydrates, and the one they use most is glucose, a simple sugar. Every feeding route described below starts from glucose.

Sugars come in three sizes.

• Monosaccharide means "one sugar". These are the building blocks: glucose, fructose, galactose and others.
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• Disaccharide means two sugars joined. Maltose, the sugar of grain, is two glucose units. Lactose, the sugar of milk, is galactose plus glucose. Sucrose, table sugar, is glucose plus fructose.
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• Polysaccharide means a long chain of hundreds or thousands of sugar units. Starch in wheat, rice and barley is a mix of two of them: amylose, a straight chain, and amylopectin, a branched one. Cellulose, the stiff material of plant walls, is another.
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A two-sugar or many-sugar molecule cannot be used whole. It must first be split into single sugars by a matching enzyme — a protein that drives one particular chemical change. Lactase splits lactose; maltase splits maltose; amylase cuts starch chains. This splitting matters in beer, cheese and yogurt. Mammals cannot digest cellulose at all unless helpful bacteria in the gut do it for them.

The grains that supply starch are taught with grains, pasta and noodles. see chapter 16: What a grain is

Turning sugar into energy

A cell burns glucose in stages. The first stage is glycolysis: a chain of enzyme steps that breaks one glucose into two smaller pieces called pyruvate. Along the way the cell gains ATP, the small molecule that carries energy around inside a cell, and NADH, a carrier loaded with spare electrons.

When oxygen is plentiful, pyruvate goes on to a second stage, the Krebs cycle (also called the citric acid or TCA cycle), which ends by turning the carbon into carbon dioxide. The NADH then passes its electrons down a chain of proteins to oxygen, which becomes water, and that flow makes much more ATP. Using oxygen this way is called aerobic respiration.

Life without oxygen

Many microbes cannot use oxygen like that. Some lack the oxygen; some lack the genes for the parts of the machinery. Their problem is the NADH. Glycolysis uses up a carrier called NAD+ and leaves NADH, and unless the cell can empty those carriers again, glycolysis stops. Fermentation is the cell's way of emptying them without oxygen.

• A facultative microbe can live either way. It uses oxygen when it has it, and ferments when it does not.
• An obligate anaerobe cannot use oxygen at all, even when it is present, because it is missing parts of the Krebs cycle or the electron chain. Many bacteria are like this.

Lactic acid fermentation

This earns the cell no extra ATP, so it is a poorer living than breathing oxygen. A facultative microbe will switch back to oxygen when there is enough. But the acid is a weapon. It makes the food sour, slows down other living things and drives out competing microbes. People noticed long ago that food fermented this way lasts longer.

There are two styles.

• Homolactic fermentation makes lactic acid and nothing else. Lactobacillus delbrueckii and Streptococcus thermophilus, the bacteria used for yogurt, work this way.
• Heterolactic fermentation takes a side route through the sugar (called the pentose phosphate pathway) and makes a mix of lactic acid and ethanol, the alcohol in drinks. Leuconostoc mesenteroides works this way, and it is used to sour cucumbers into pickles and cabbage into sauerkraut.

Acid is one of the six conditions that control the growth of bacteria. Bacteria do not grow in acid, which is why acidic foods such as vinegar and lemon juice serve as preservatives. The full list of what bacteria need is with the rest of food safety. see chapter 2: What bacteria need: FATTOM