NOTES ON FOOD MICROBIOLOGY:

Food as substrate for microorganisms:

Microbial growth in food depends on following factors:

1. Intrinsic : These are the factors which are inherent to the food i.e., the physical and chemical properties of food. It includes pH, moisture content, oxidation, reduction potential, nutrient content, antimicrobial constituents and biological structure.
2. Extrinsic: Include environmental condition where the food is stored. (like temperature of storage, relative humidity of environment, presence and concentration of gases, presence and activity of other m/o)
3. Implicit: These are the physiological properties of micro-organisms (like hydrolytic activity).
4. Process factor: These factors will determine spoilage (vulnerability and type of m/o and preservation method)

INTRINSIC FACTORS:

pH: most m/o have an optimum range of pH for growth which is between 6.6-7.5.

• Growth range of m/o:  

1. BACTERIA : they do not grow below range pH 4.5. food which has pH more than 4.5 are more likely to be spoiled by bacteria. They were also grown in alkaline solution of egg white.
2.  FUNGI : There is no actual range. All food are prone to spoilage by fungi. they have to compete from bacteria for growth therefore mainly spoiled by acidic food.  

• pH range of foods depends on the acidity of food. They are classified as
• first low acid foods (above pH 5.3) - e.g., meat, fish, milk
• medium acidic food (pH 5.3 to 4.5) e.g., spinach, pumpkin.
• Acidic food (pH 4.5 to 3.7) e.g., tomato, pineapple.
• Highly acidic food (pH below 3.7) e.g., berries.

CATEGORIES: 

1. Fruits and Vegetables : Fruit (2-5 pH) and vegetables (pH- 4.2- 6). both have low pH.  However,  vegetables have higher pH compared to fruits. Therefore, fruits are liable to spoilage by fungi and yeast and vegetables are equally spoiled by bacteria.
2. Meat and fish : pH is above 5.6, can be spoiled by both bacteria and molds but bacterial spoilage is more dominant. The keeping quality of meat and fish :  Meat of fatigue animal spoil faster than rested animal. This is because a final pH attained upon completion of rigor mortis (stiffening of the joints and muscles a few hours after death). In case of fatigue animal, glycogen can utilized completely whereas in case of rested animal glycogen is 1% remain. It is converted to lactic acid. Therefore, pH of blood fluids drops to 5.6 from 7.4. This is responsible for increasing its keeping quality.
3. Milk and milk products : The pH of milk is around 6-6.5. Butter (6.1 to 6.4) Dahi (4.2-4.5).
4. Some foods have inherent buffering capacity. Meat is more highly buffered than vegetables as they have proteins. Milk is buffered due to the presence of casein. Buffers permit fermentation to go on longer with greater yield of products. Vegetables have low buffering capacity permitting decrease in pH by small amount of acid by lactic acid bacteria during early part of fermentation. E.g., in case of Sauerkraut fermentation (finely cut cabbage fermented by various lactic acid bacteria) - this results in suppression of competing organisms and rapid succession (due to low pH) .

MOISTURE CONTENT : 

m/o require moisture for growth. The water requirement of m/o is described in terms of water activity (aw) in the environment. Water activity = p/p0, where, p = vapor pressure of solution p0 = vapor pressure of solvent (water).

Water activity is defined by the ratio of water vapor pressure of food substrate to the vapor pressure of pure water at the same temperature. For pure water, aw = 1. For 1M solution of NaCl, aw = 0.98.

Relative humidity (RH) = 100 * aw 

Lowest  water activity values permitting growth of spoilage organism as follows:

 for most spoilage bacteria = 0.90,

 For most spoilage yeast = 0.88

For most spoilage mold = 0.80

For most spoilage halophilic = 0.75

Foe Xerophilic = 0.61

For Osmophilic yeast = 0.61

Yeast and fungi resists better aw values. Bacteria grow if high moisture content is available. For E.coli , aw is 0.96. Clostridium botulism = 0.93. Exception : Staphylococcus aureus and micrococcus are able to survive at low aw of 0.86.

If we are able to use freely available water by adding solutes like salt/sugar etc. It has drying effect, it can even take out water from the microbial cells. Thus, the oldest food preservation method i.e., drying and desiccation are based on lowering aw. Water can be made unavailable by :

• addition of solutes and ions that tie up water molecules out of food.
• hydrophilic colloids like gels etc. that make water unavailable. e.g., jelly has high conc. of gelatin.
• Water of crystallization i.e., ice is unavailable to m/o in a food as more ice is formed, the conc. of solutes in unfrozen water is increased lowering its aw.

OXIDATION - REDUCTION POTENTIAL O/R (Eh):

m/o display varying degrees of sensitivity to the O/R potential of their growth medium. The O/R potential of a substrate may be defined as " the ease with which the substrate losses or gains electrons." When electron are transferred from one compound to another, a potential difference is created between the two compounds. This difference can be measured in millivolts. More highly oxidized a substance the more positive will be its electrical potential, it will favor the growth of aerobic m/o. More reducing the substance, more negative will be its electrical potential, less O2 is available. Therefore, it favors the growth of anaerobic microbes. Hence, aerobic m/o requires +ve Eh for growth and anaerobic m/o requires -ve Eh.

The O/R potential of a food is determined by the :

• Characteristic potential
• Poising capacity : resistance to change in the potential of the food.
• O2 tension of atmosphere around the food.
• Access that the atmosphere has to the food.

Different m/o have different pH requirements, aerobes require oxidized environment, e.g., Bacillus. Anaerobic require reduced environment. e.g., Clostridium.. Microaerophiles (require air but a low O2 tension) slightly reduced environment (Lactobacillus). Facultative anaerobes can grow in any condition.
O/R potential of food: fruits and vegetables : Eh of -100mV; low O/R potential inside however, higher O/R in the outer layers. They have presence of reducing sugars, ascorbic acid. Therefore, O/R is low inside and resist a change in O/R. In case of fruit juice, Eh = +400mV because of free diffusion of O2. Clearing of fruit juices leads to removal of oxidizing and reducing substances. Therefore, juice present in pulp is less susceptible to growth of aerobic m/o than clear juices. In case of meat, intact big chunks of meat have an Eh of -100mV, interiors have low O/R potential because of presence of more proteins and sulfur-hydryl groups. Exterior have high O/R. Therefore, outer layer susceptible to spoilage by aerobic m/o whereas interiors may be undergoing Putrefaction. by anaerobic m/o. Minced meat has +ve Eh of 200mV because surface area increases. Now, they can be spoiled by aerobic m/o.
Techniques to control O/R potential of food:

• Vacuum packaging
• Skin tight packaging 
• Canning
• Addition of anti-oxidants (natural oxidant- ascorbic acid ; synthetic oxidant- propyl gallate (PG))

NUTRIENT CONTENT:

m/o require following : 1) Water (aw)   2) Source of energy which comes from sugar, aa etc.

3) Source of nitrogen (amino acid, protein )   4) Vitamins and growth factors

Nutrient requirement are least for molds followed by yeast, gram negative bacteria and gram positive bacteria. Fruits tend to be lower in vitamin B than meat and this fact  along with usual low pH and +Eh helps to explain the usual spoilage of fruits by molds rather than bacteria.

ANTIMICROBIAL CONSTITUENTS OF FOOD:

Eggs contain lysozyme in egg white and conalbumin which is responsible for binding albumin making it unavailable for m/o. Milk has lactoferrin which is able to bind iron making it unavailable for m/o. Lactoperoxidase   system which also has antimicrobial effect. Spices has essential oils that possess antimicrobial activity. e.g., Eugenol in cloves, allicin in garlic, turmeric, cinnamic aldehyde in cinnamon. 

Fruits and vegetables has organic acid derivative e.g., hydroxycinnamic acid derivative which has both anti bacterial and antifungal activities. m/o growing in food may produce inhibitory substances e.g., propionic acid produced by Propioni bacterium in Swiss cheese is inhibitory to molds. NISIN produced by Streptococcus lactis may be useful for inhibiting Clostridia in cheese.

Heating foods may result in the formation of inhibitory substances. e.g., heating lipids may increase auto-oxidation and make them inhibitory. Browning concentration of sugar syrups may result in production of furfurals which are inhibitory to fermentation organisms.

BIOLOGICAL/OUTER STRUCTURE:

The natural covering of some foods provide protection against the entry and damage by spoilage organisms. e.g., shells of eggs, peels of fruits, hides of animals, layer of fat over meat.

EXTRINSIC FACTOR:

TEMPERATURE OF STORAGE OF FOOD:

M/O grow over a wide range of temperature. Psychotropic optimize15-30C e.g., Pseudomonas, Alcaligenes Enterococcus. These m/o grow well at refrigerator temperature and cause spoilage of eggs, meat, fish etc. Thermophiles (55-65C)  e.g., Clostridium. the quality of food product must be taken into account in selecting storage temperature. e.g., banana are kept better if stored at 13-17C than 5-7C. Vegetables at 10C if stored at normal temperature they are susceptible to Mesophiles. In pasteurization, it is important to immediately chill the food.\

RELATIVE HUMIDITY OF ENVIRONMENT:

When food with low aw values are placed in an environment of high relative humidity, the food pick up moisture from environment which can lead to surface microbial growth. Foods with high aw, lose moisture when placed in an environment of low humidity.

Foods that undergo surface spoilage from molds, yeasts and certain bacteria should be stored under low relative humidity. In properly wrapped meats undergo surface spoilage in the refrigerator due to high relative humidity of the refrigerator.

PRESENCE AND CONC. OF GASES IN THE ENVIRONMENT:

This storage of food in atmosphere containing increased amount of CO2 up to 10% is referred to as a controlled atm or CA storage. It is done for fruits such as apples pears. CO2 retards fungal blotting it acts as competitive inhibitor of ethylene. Ethylene acts as an senescence factor in fruits and its inhibition has the effect of maintaining the fruit in a better state of natural resistance. Steaks (meat product) are stored 100% CO2 atm. fish in 80% CO2 atm. The inhibitory effect of CO2 increases with decrease in temperature due to increasing solubility of CO2 at lower temp. Therefore, they are more effective during cold storage.

Ozone also has preservative effects as it is a strong oxidizing agent. It has been used to retard surface spoilage of beef.

PRESENCE AND ACTIVITY OF OTHER M/O:

E.g., Bacteriocins, organic acids, antibiotics etc are produced by food-borne organisms that are lethal to other m/o.

Lactic antagonism- inhibition of food spoilage organism is due to NISIN produced by lactic acid bacteria, lowering of pH, production of organic acids, diacetyl etc.

REASONS FOR INTERFERENCE ARE :

• Competition for nutrients
• Rendering the environment unfavorable