A. FOOD PRESERVATION AND MICROORGANISMS
1. Appertization or Canning
Appertization is a heat-preservation process developed for foods packed in hermetically sealed containers. Canning combines thermal processing with sealed packaging to achieve commercial stability and extend shelf life.
2. Pasteurization of Milk
Pasteurization is a controlled heat treatment of milk designed to destroy pathogenic microorganisms of public-health significance and reduce the overall microbial load. It does not make milk commercially sterile.
3. Microbial Food Spoilage
Food spoilage may be caused by bacteria, yeasts, and molds. Their growth and metabolic activities can produce undesirable changes in odor, flavor, color, texture, and appearance.
4. Role of Microorganisms in Food Deterioration
Microorganisms are major biological agents responsible for food deterioration. Their ability to grow depends on factors such as temperature, pH, water activity, nutrients, oxygen availability, and food composition.
5. Beneficial Use of Microorganisms
Microorganisms are not only associated with food spoilage. Selected microorganisms are intentionally used in fermentation and food production to develop desirable flavor, texture, acidity, aroma, and other characteristics.
6. Lactic Acid Bacteria
Lactic acid bacteria are important microorganisms in fermented dairy products and cheese manufacture. They convert fermentable carbohydrates into lactic acid and contribute to product flavor, acidity, and preservation.
7. Yeast in Bread Making
Baker’s yeast, principally Saccharomyces cerevisiae, is used for dough fermentation. It metabolizes fermentable sugars and produces carbon dioxide, which causes dough expansion and leavening.
B. BACTERIAL GROWTH AND CLASSIFICATION
8. Bacterial Growth Curve
The typical bacterial growth curve consists of four phases:
- Lag phase: cells adapt to the new environment.
- Log phase: cells multiply rapidly at an approximately exponential rate.
- Stationary phase: cell multiplication and cell death approach a balance because nutrients become limiting and metabolic by-products accumulate.
- Death phase: the number of viable cells decreases.
9. Classification of Bacteria by Cell Shape
Bacteria can be classified morphologically according to their cellular shape. Major forms include cocci, bacilli, vibrios, and spiral-shaped bacteria.
10. Major Bacterial Cell Shapes
- Cocci: spherical or approximately spherical cells
- Bacilli: rod-shaped cells
- Vibrios: curved or comma-shaped cells
- Spirilla: rigid spiral-shaped cells
11. Bacterial Endospores
Some bacteria, particularly species of Bacillus and Clostridium, produce endospores. Endospores are highly resistant to heat, desiccation, radiation, and other environmental stresses and are generally more resistant than vegetative bacterial cells.
12. Microbial Growth in Acidic Foods
Most pathogenic and spoilage bacteria are less tolerant of acidic conditions than yeasts and molds. Consequently, yeasts and molds can be important spoilage organisms in acidic foods.
13. Classification by Oxygen Requirement
Microorganisms can be classified according to their relationship with oxygen. Important groups include obligate aerobes, which require oxygen, and obligate anaerobes, which cannot grow in the presence of oxygen. Facultative organisms can grow under both conditions.
14. Yeasts
Yeasts are unicellular fungi that occur widely in nature. They are important in food fermentation and may also cause spoilage of foods containing sugars and acids.
15. Conditions Favoring Yeast Growth
Many yeasts grow effectively in environments containing fermentable sugars and sufficient available water. Compared with many bacteria, yeasts generally tolerate lower pH and lower water activity.
16. Molds
Molds are multicellular filamentous fungi composed primarily of hyphae. Their growth often produces a visible fuzzy, cottony, or powdery appearance on food.
17. Temperature and Microbial Growth
Microorganisms have species-specific minimum, optimum, and maximum growth temperatures. Therefore, a single temperature range should not be treated as the universal optimum for bacteria, yeasts, and molds.
18. Psychrophiles
Psychrophiles are microorganisms whose optimum growth temperature is low and that are capable of growth at refrigeration or near-refrigeration temperatures.
19. Thermophiles
Thermophiles are microorganisms adapted to relatively high temperatures and have optimum growth temperatures substantially above those of mesophiles.
20. Mesophiles
Mesophiles are microorganisms that grow best at moderate temperatures. Many important foodborne pathogens are mesophilic organisms.
C. FOODBORNE MICROORGANISMS AND THERMAL CONTROL
21. Bacterial Foodborne Disease
Bacteria can cause foodborne disease through different mechanisms. Foodborne infection results from ingestion of viable pathogenic microorganisms, whereas foodborne intoxication results from ingestion of toxins that are already present in the food.
22. Clostridium botulinum and Botulism
Clostridium botulinum is an anaerobic, Gram-positive, spore-forming bacterium that produces botulinum neurotoxins. Consumption of food containing botulinum toxin can cause botulism, a serious neurological disease.
23. Thermal Death Time
Thermal death time is the time required at a specified temperature to achieve a defined reduction in the viable population of a microorganism. Thermal death relationships are used to evaluate the effectiveness of heat treatments.
24. Microbial Inactivation by Temperature
Microbial populations can be controlled by thermal or cold treatments. Heat generally causes microbial inactivation, while refrigeration and freezing primarily slow microbial growth and may cause injury or death in some microorganisms.
25. Major Heat Treatments
Two important thermal preservation approaches are:
- Pasteurization: designed primarily to destroy vegetative pathogens and reduce microbial populations.
- Commercial sterilization: a more severe treatment intended to achieve commercial stability by controlling microorganisms capable of growing under the intended storage conditions.
26. Cold Preservation
Refrigeration and chilling slow microbial growth by reducing temperature. Freezing converts available water into ice and greatly restricts microbial growth, although freezing does not necessarily destroy all microorganisms.
27. Important Foodborne Pathogens
Important bacterial foodborne pathogens include Salmonella, Shigella, pathogenic Escherichia coli, Campylobacter, Bacillus cereus, Vibrio species, and Clostridium species. The specific hazard depends on the organism, food, and route of contamination.
28. Drying as a Preservation Method
Drying reduces the amount of water available to microorganisms. Reduction in water activity inhibits microbial growth and can substantially increase food stability.
29. Water Activity
Water activity (aᵥ) is a measure of the availability of water in a food for microbial growth and chemical reactions. It is different from total moisture content.
30. Water Activity and Bacteria
Most bacteria require relatively high water activity for growth. The minimum water activity varies considerably among species; therefore, a single value should not be used as a universal bacterial growth limit.
31. Water Activity and Yeasts/Molds
Many yeasts and molds can grow at lower water activities than most bacteria. Some xerophilic molds and osmophilic yeasts can grow at particularly low water activity.
32. Effect of Sugar and Salt
High concentrations of sugar or salt can reduce water activity through osmotic effects. This reduction in available water can inhibit the growth of many microorganisms.
33. Smoking
Smoking is a traditional preservation method that exposes food to smoke generated from controlled combustion or smoldering material. Smoke can contribute antimicrobial and antioxidant effects while also producing characteristic flavor and color.
34. Chemical Preservatives
Food preservatives can inhibit or retard microbial growth. Examples include benzoates, sorbates, propionates, and sulfur dioxide/sulfites. Their effectiveness depends on factors such as concentration, pH, food composition, and the target microorganism.
D. IRRADIATION AND FERMENTATION
35. Food Irradiation
Food irradiation uses ionizing radiation to reduce or eliminate microorganisms and insects and can extend food shelf life. Approved food-irradiation sources include gamma radiation, X-rays, and electron beams.
36. Irradiation as a Cold Process
Food irradiation is often described as a non-thermal or “cold” preservation technology because it can achieve microbial reduction without the substantial heating associated with conventional thermal processing. It should not be confused with conventional thermal sterilization.
37. Food Fermentation
Food fermentation uses selected microorganisms or microbial cultures to transform food components. Applications include bread, beer, fermented dairy products, cheese, alcoholic beverages, and various traditional fermented foods.
38. Baker’s Yeast
Saccharomyces cerevisiae is widely used as baker’s yeast. During fermentation, it converts fermentable sugars into carbon dioxide and ethanol; carbon dioxide is primarily responsible for dough leavening.
39. Brewer’s Yeast
Yeast cultures, commonly strains of Saccharomyces cerevisiae or related brewing yeasts, ferment sugars in wort to produce ethanol and carbon dioxide during beer production.
40. Intermediate-Moisture Foods
Intermediate-moisture foods have sufficient moisture to retain a soft or semi-soft texture but have reduced water activity that limits microbial growth. Water activity, rather than moisture content alone, is the more useful parameter for predicting microbial stability.
E. FERMENTED FOODS
41. Yogurt Starter Culture
Traditional yogurt production uses a symbiotic starter culture of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. These bacteria ferment lactose and produce lactic acid, causing acidification and development of yogurt structure and flavor.
42. Bacillus in UHT Milk
Some Bacillus species are important spoilage organisms in UHT and other heat-processed milk because their endospores can survive thermal processing. Certain strains can subsequently grow or produce spoilage enzymes during storage.
43. Kefir
Kefir is a fermented milk product produced using a complex microbial community containing lactic acid bacteria, yeasts, and, in some cultures, acetic acid bacteria. The exact microbial composition depends on the kefir grains and production system.
44. Fermented Soybean Products
Tempeh, sufu, and Tao-si are traditional fermented soybean products. Different microorganisms and fermentation conditions are used in their manufacture, producing characteristic changes in flavor, texture, and composition.
45. Saccharomyces cerevisiae in Whisky Production
Saccharomyces cerevisiae is an important fermentation yeast used in whisky production. It converts fermentable sugars into ethanol and other fermentation metabolites that contribute to the characteristics of the fermented wash.
46. Leuconostoc and Idli Fermentation
Idli batter undergoes mixed microbial fermentation involving lactic acid bacteria and yeasts. Leuconostoc species may contribute to acidification, gas production, and flavor development during fermentation.
47. Humectants
Humectants such as glycerol and sorbitol bind or retain water and can reduce water activity in food systems. Their use can improve texture, stability, and microbial shelf life. Glycols may have specific regulatory limitations depending on the intended food application and jurisdiction.
F. MYCOTOXINS AND BACTERIAL CLASSIFICATION
48. Mycotoxins
Mycotoxins are toxic secondary metabolites produced by certain filamentous fungi. Important examples include aflatoxins, citrinin, and ochratoxin A. They can contaminate agricultural commodities and processed foods.
49. Gram-Positive and Gram-Negative Bacteria
Gram staining separates bacteria into Gram-positive and Gram-negative groups based on differences in cell-envelope structure and their response to the Gram-staining procedure. Gram-positive bacteria generally possess a thick peptidoglycan layer, whereas Gram-negative bacteria possess a thinner peptidoglycan layer and an outer membrane.
50. Foodborne Infection
Foodborne infection occurs when viable pathogenic microorganisms are consumed with contaminated food and subsequently multiply, invade, or otherwise cause disease in the host. Important examples include infections caused by Salmonella, pathogenic E. coli, Campylobacter, and Shigella. Bacillus cereus can cause both toxin-mediated food poisoning and, in some circumstances, other forms of foodborne illness; therefore, it should not simply be classified as a food infection in every case.
HIGH-YIELD CORRECTIONS FROM THE ORIGINAL NOTES
| Original statement | Scientifically correct form |
|---|---|
| Pediococcus botulinum | Clostridium botulinum |
| “Bacteria cannot grow in acidic medium” | Many bacteria are inhibited by acidity, but acid tolerance varies by species. |
| “16–38°C is optimum for bacteria, yeasts and molds” | Growth temperatures are species-specific; no single universal optimum applies to all three groups. |
| “Thermophiles grow at high-temperature storage” | Thermophiles are microorganisms adapted to relatively high growth temperatures. |
| “Irradiation uses X-rays, microwaves, UV and ionizing radiation” | Food irradiation for microbial control primarily uses ionizing radiation, including gamma rays, X-rays, and electron beams. |
| “Irradiation = cold sterilization” | Irradiation is a non-thermal preservation technology; some high-dose applications can achieve sterilization, but not every irradiation treatment is sterilizing. |
| “Water activity is amount of water available” | Water activity measures the thermodynamic availability of water, not simply the amount of moisture. |
| “Most bacteria require aᵥ 0.90–1.00” | Many bacteria require high aᵥ, but minimum values vary by species and conditions. |
| “Some yeasts/molds grow at aᵥ 0.65” | Some yeasts and molds can grow at low aᵥ; the exact minimum is organism-specific. |
| “Bacillus cereus causes food infection” | B. cereus is primarily associated with toxin-mediated food poisoning, including emetic and diarrheal syndromes. |
| “Lactobacillus bulgaricus” | Current nomenclature: Lactobacillus delbrueckii subsp. bulgaricus. |