2. 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)
3.
4. 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
5. CONTD…
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
6.
7. 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.
8. 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)
9.
10. 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
11.
12. 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.
13. 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.
14.
15. Heating foods may result in the formation of inhibitory substances. e.g.,
heating lipidsmay increase auto-oxidation and make them inhibitory.
Browning concentration of sugar syrups may result in production of furfurals
which are inhibitory to fermentationorganisms.
NISIN produced by Streptococcus lactis may be useful for inhibiting Clostridia
in cheese.
16. 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
17. 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.
18. 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.
19. 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% CO2atm. 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.
