Microorganisms in Food: Production and Safety

The primary application of yeast is alcoholic fermentation, where they convert sugars into alcohol and carbon dioxide ($CO_2$). This is the fundamental process used to leaven bread and produce beverages like wine and beer. For example, Saccharomyces cerevisiae can produce $CO_2$ through the breakdown of sucrose.

Mold

Unlike bacteria and yeast, molds are multicellular and filamentous. They reproduce by producing spores and are capable of growing on dry foods like cereals and grains, as well as at both room and cold temperatures. Molds require oxygen for growth.

In food production, molds are used to create specific textures and flavors in products like soy sauce (using Aspergillus oryzae) and various cheeses, such as Roquefort, Gorgonzola, and Camembert.

The Science of Fermentation

Fermentation is the biological breakdown of carbohydrates (CHO) by microorganisms. This process is used not only to create food but also to preserve it by reducing pH, producing alcohol, or reducing oxygen levels. Fermentation can significantly improve the flavor, texture, and nutritional value of food.

There are several key types of fermentation used in food science:

1. Lactic Acid Fermentation: Converts sugars to lactic acid (e.g., yogurt, pickles).
2. Alcoholic Fermentation: Converts sugars to ethanol and $CO_2$ (e.g., wine, beer).
3. Acetic Acid Fermentation: Converts ethanol into acetic acid ($CH_3COOH$) in the presence of oxygen. $\begin{aligned} C_2H_5OH + O_2 \rightarrow CH_3COOH + H_2O \end{aligned}$
4. $CO_2$ Fermentation: The production of carbon dioxide, essential for the rising of bread dough.

Microbial Growth and Food Safety

While microbes can be beneficial, they can also cause food spoilage or illness. Spoilage is indicated by changes in taste, smell, sight, or texture, such as the smell of ammonia or rotten eggs, or a slimy, mushy texture.

Bacterial Growth Calculations

Bacteria multiply through binary fission, where one cell divides into two. This leads to exponential growth. To predict how much bacteria will be present after a certain time, we use the concept of generation time (GT), which is the time it takes for the population to double.

Worked Example: A small piece of chicken (100 g) has an initial E. coli load of 800 CFU/g. If the generation time is 20 minutes, what is the total bacteria count after 2 hours?

1. Convert total time to minutes: $2 \text{ hours} = 120 \text{ minutes}$.
2. Calculate the number of generations ($G$): $G = \frac{120 \text{ min}}{20 \text{ min/gen}} = 6 \text{ generations}$
3. Calculate the final count per gram using the formula $\text{Initial Count} \times 2^G$: $\begin{aligned} 800 \times 2^6 &= 800 \times 64 \\ &= 51,200 \text{ (or } 5 \times 10^4) \text{ CFU/g} \end{aligned}$

Managing Risks

Food-borne illnesses can be caused by bacteria (like E. coli), viruses, or parasites. Some molds even produce dangerous toxins, such as Aspergillus flavus producing aflatoxin.

To prevent illness, strict food handling is required:

• Workers with communicable diseases or skin lesions must not handle food.
• Frequent hand washing and wearing clean clothing/head covers are essential.
• Proper temperature control is vital. For example, the "Danger Zone" for food is between $40^\circ\text{F}$ and $140^\circ\text{F}$ ($4^\circ\text{C}$ to $60^\circ\text{C}$).

Critical Concept: Pathogens may be present in food in numbers too low to cause disease initially. However, if food is mishandled or left at room temperature, these microorganisms can grow to dangerous levels.