Factor affecting growth of microorganisms in food

How microbes spoil foods?

• Food is a collection of essential nutrients of animals or plants origin.
• In most cases, micro-organisms utilize food as a source of nutrients for their growth.
• This can result in spoilage of food.
• Micro-organisms spoil food by:
  • Increasing their number
  • Utilizing or consuming the nutrients
  • Producing enzymatic change in food
  • Producing various metabolites that give bad odour or taste.
• However, growth of microbes in food is not always harmful, sometimes it is beneficial. For example; Many microbes are inoculated in food to make various fermented food products. Many factors affect growth and activity of microorganism in food.
• This is the reason why-
  • Some foods are very stable to microbial spoilage and other are spoiled faster.
  • Different types of foods are spoiled by different types of mos.
  • Microbial growth in some food is beneficial and harmful in other foods.

Factors affecting microbial growth in foods:

• Factor affecting growth of microorganisms in food are divided into two types:
  • Intrinsic parameter or factors
  • Extrinsic parameter or factors

1. Intrinsic parameter:

• Inherent or natural characteristics of food that affects growth of microbes in it are called intrinsic parameter of food. They include:

i. pH of food:

• Mos are significantly affected by pH of medium in which they grow.
• In general, yeast and molds are more acid tolerant than bacteria.
• Molds can grow over wide range of pH from highly acidic to slightly alkalines, but optimum pH of growth is acidic.
• Most fermentative yeast grow best at 4-4.5 pH.
• Most bacteria grow best at neutral pH, but few are favored by slightly acidic (e.g. Lactobacillus) or by slightly alkaline (e.g. proteolytic bacteria) pH.
• Different food differs in their pH as given below:
  • Highly acidic food (pH below 3.7): e.g. lemon, apple etc.
  • Acidic food (pH 3.7-4.6): e.g. tomato, pineapple, strawberry etc.
  • Medium acidic food (pH 4.6-5.3): e.g. banana, pumpkin etc.
  • Low or non-acidic (pH over 5.3): e.g. milk, meat, cheese etc.
• Food with low pH (below 4.5 are usually spoiled by mold.
• Food with neutral pH are usually spoiled by bacteria but all three groups of microbes can grow in it.
• Therefore, food with neutral pH like meat are microbiologically less stable.
• On the other hand, excellent keeping quality of some food like pickles is mainly due to their pH because, most bacteria cannot grow in acid pH.

ii. Moisture content or water activity (A_w):

• Microorganisms require sufficient moisture.
• Water requirement of microbes in food is expressed in term of water activity or available water (A_w).
• A_w of distilled water is one, various solute present in food bind water in the form of shell of hydration and make it unavailable for microbial growth.
• Some of them include salt, sugar, gel etc. so when concentration of solutes increases amount of available water decrease.
• Some important group of food and their A_w value includes:
  • 0.98 to above- Fresh meat, shell fish, fresh vegetable etc.
  • 0.93 to 0.98- Bread, tomato paste, evaporated milk.
  • 0.60 to 0.85- flour, jam, jellies etc.
  • 0.85 to 0.93- sweetened condensed milk
  • Below 0.60- chocolate, honey, biscuits and confectionery products.
• Each microbes has a maximum, optimum and minimum value of A_w needed for the growth.
• Lowest value of A_w that permits growth of some microbes includes:

Micro-organisms	Minimum Aw
Most bacteria	0.91
Most yeast	0.88
Most mold	0.80
Halophilic bacteria	0.75
Xerophilic fungi	0.65
Osmophilic yeast	0.60

• Therefore, food with A_w value lower than 0.91 (e.g. flour) are not spoiled by bacteria.
• However, Staphylococcus aureus which is halophilic can grow up to A_w value of 0.86.
• Most yeast grow up to A_w value of 0.88 and most molds up to 0.80.
• Therefore, relatively dry food like bread are spoiled by mold and yeast but not by bacteria.
• Food with very low value of A_w (e.g. honey) are microbiologically more stable because most microbes cannot grow at this A_w value.

iii. Oxidation-Reduction (O-R) potential of food:

• O-R potential of food determines types of microbes that can grow in it.
• So, it determines type of microbial spoilage of food.
• If food is in aerobic condition, its value of O-R potential is given +ve sign and if it is in reduced condition, value of O-R is given as -ve sign.
• O-R potential of food is determined by various factors like its components, concentration of O₂ in storage environment and state of food. For examples components like vitamin C (Ascorbic acid), reducing sugar and cysteine make the food anaerobic.
• Mos differ in their requirement of O-R potential.
• Molds require highly +ve value of O-R potential because they are obligate aerobes.
• Aerobic bacteria like Bacillus and most yeast also require +ve value of O-R potential.
• Obligate anaerobes like Clostridium grow only in -ve value of O-R potential.
• Facultative anaerobes can grow either in positive or in negative value of O-R potential.
• Therefore, food with highly positive value of O-R potential (e.g. plant juice = +300 to +400mV) are usually spoiled by mold and aerobic bacteria.
• Similarly, food with -ve value of O-R potential (e.g. fresh solid cut of meat -200mV) are spoiled by anaerobic bacteria like Clostridium.
• Ability of food to resist change in O-R potential is called poising capacity.
• O-R potential is changed by processing technique like heating, grinding etc. because this processes damage reducing sugar, cysteine, and ascorbic acid that makes food anaerobic.
• This is the reason why fresh solid cut of meat undergoes anaerobic spoilage by bacteria but ground meat undergoes aerobic spoilage by mold.

iv. Nutrient content:

• Mos utilize various nutrients and growth factor present in food for their growth.
• Therefore, food which are rich in nutrients or which contain all essential nutrients (e.g. meat, milk is spoiled faster by microbes).
• On the other hand, food which lack particular nutrients or growth factors or are poor in nutrients (e.g. flour, cereals etc) are microbiologically more stable with respect to nutrients requirement, molds have lowest requirement followed by yeast, gram -ve bacteria and gram +ve bacteria.
• All mos utilize glucose or other mono-saccharides as source of carbon and energy.
• Very few microbes can utilize complex polysaccharides such as starch, pectin etc.
• If monosaccharides are not available, therefore only very few species of mold and bacteria that can hydrolyse pectin, can grow in skin of fruits and cause its spoilage.
• Proteolytic bacteria like Clostridium, Bacillus etc. can utilize protein as source of carbon and energy, if carbohydrates are not available.
• Therefore, food which are deficient in carbohydrate and rich in protein (e.g. meat, egg etc) are mainly spoiled by proteolytic bacteria.
• Some organism also requires various vitamins and growth factors in food for their growth.
• Biotin present in egg is made unavailable by avidin.
• Therefore, microorganisms that require biotin cannot grow in egg.
• Similarly, microorganisms that require vitamin B complex usually cannot grow in fruits because vitamin B is deficient in fruits.

v. Antimicrobial constituents:

• Some food contains various antimicrobial components that inhibit growth of many micro-organism.
• Therefore, food which contain one or more of these anti-microbial agents (alcoholic beverages) are microbiologically more stable.
• In some food antimicrobial constituents are present naturally and in others.
• They are developed antimicrobial constituents include ethanol present in alcoholic beverages and propionic acid present in cheese that inhibit growth of mold.
• For e.g. natural, antimicrobial constituents present in some food include:
  • Eugenol in cloves
  • Allicin in garlic
  • Cinnamic aldehyde in cinnamon
  • Allylisothiocyanate in mustard
  • Lysozyme present in egg white
• Similarly, lysosome, lactoferrin, conglutinin and lactoperoxidase present in fresh milk are also anti-microbial.
• Furthermore, some plant products like tea and coffee contain caffein, caffeic acid etc which are antimicrobial.

vi. Biological structure

• Some foods have natural covering on their surface that prevent entry of spoilage organisms into inner tissue of food and prevent microbial spoilage.
• Some such natural covering of some food includes shell of egg, skin of fruits, testa of seed, hide of animal etc.
• Nowadays, fruits are covered by artificial covering to protect them from microbial spoilage.

2. Extrinsic parameters:

• Characteristic of storage environment that affects growth of micro-organism on food are called extrinsic parameters. Some such examples include:

i. Relative humidity (RH):

• RH of storage environment that affects moisture content (A_w value) of food.
• Therefore, RH of storage environment affects microbial growth on food and determine microbial spoilage.
• When relatively moist food is placed in dry environment, food loss water and its A_w value decreases.
• Similarly, when relative dry food is placed in humid atmosphere food absorbs moisture and its A_w value increase that facilitate growth of many spoilage organism.
• Suppose A_w value of food is 0.6. But when these food is stored in humid atmosphere, its A_w value gradually increase and food undergoes microbial spoilage.
• RH of storage atmosphere mainly affects surface od food. Therefore, food that undergo surface spoilage can be controlled by controlling RH of storage environment.

ii. Storage temperature:

• Storage temperature is an important extrinsic parameter that affects growth and activity of microbes in food.
• Very few species of bacteria can grow in Psychrophilic (e.g. Pseudomonas fluorescence) and thermophilic (e.g. Clostridium and Bacillus species) range and most bacteria grow best in mesophilic temperature.
• Similarly, except few exceptions most mold and yeast grow in mesophilic range.
• Since most microbial groups grow best in mesophilic range, food stored at room temperature is spoiled faster than the food stored in very high temperature or very low temperature.
• Some Psychrophilic organism like Pseudomonas fluorescence, Cladosporium and Aspergillus species cause spoilage of refrigerated food like egg and meat.
• Selection of appropriate storage temperature is very important to preserve food.
• Usually foods are preserved by storing them in cold environment because growth of most microbes is inhibited at lower temperature.
• However, quality of some food (e.g. banana) is damaged at very low temperature.
• Therefore, storage temperature must be selected depending on nature of food.

iii. Composition of gases in storage environment:

• Composition and concentration of gases in storage environment affects growth and activity of mos.
• Usually concentration of gases like CO₂, O₃, and N₂ is increased in storage environment of food.
• Increased concentration of CO₂ is used for the preservation of meat and fruits.
• CO₂ is competitive inhibitor of ethylene. Therefore, CO₂ prevents ripening and softening of fruit is prevented, spoilage of fruit by mold can be blocked.
• Increased concentration of CO₂ is also used for preservation of meat.
• CO₂ storage and cold storage are used in combination for preservation of meat. CO₂ dissolve in water of meat to form carbonic acid that inhibit growth of many spoilage organism. Solubility of CO₂ in water increases at lower temperature. By this technique, psychrophilic bacteria like Pseudomonas fluorescence are also inhibited because it is highly sensitive to acidic pH.
• Ozone is also anti-microbial and is placed in storage atmosphere of many food. However, O₃ is oxidizing agent and causes oxidative rancidity of lipid food. Therefore, it should not be used for preservation of lipid food.

Factor affecting growth of microorganisms in food