FACTORS AFFECTING THEIR GROWTH

The information gathered can be utilized not only to prevent the growth of harmful microorganisms during cooking, processing, storage and service but also to optimize beneficial microbial fermentations.

There are two factors:

  • Intrinsic factors
  • Extrinsic factors

In order to understand the principles of food spoilage, processing and preservation, the food service professionals must understand the factors that affect the growth of microbes.

INTRINSIC FACTORS:

  • Hydrogen ion concentration
  • Water activity
  • Oxidation reduction potential

Nutrient content:

  • The ability of microorganisms to utilize different types of simple sugar varies.
  • Simple sugar is an important tool in their identification for e.g. Lactobacillus grows in milk.
  • Accessory substances such as vitamins needed for the activities of microorganisms also come from the food they grow on. For e.g. vitamin B are obtained from meat and ascorbic acid from fruit.

Inhibitory substances:

    • Inhibitory substances are either present originally, added purposively of unintentionally or else developed through microbial growth or processing methods.
    • Some naturally occurring antimicrobial substances Prevent microbial growth. such as:

    I.Lysozyme in egg white,

    II.Eugenol in clove,

    III.Allicin in garlic

    IV.Thymol in oregano and

    V.Benzoic acid in cranberries

Biological structure:

  • Some foods have natural covering such as

    • Outer shell of eggs
    • Scales of fish,
    • Layers of lard over meat
    • Covering of nut
    • Peels of fruits and vegetables
    On the other hand, presence of oxygen promotes the growth of micro-organisms.
  • In modified gas atmosphere, the concentration of carbon dioxide is increased, whereas that of oxygen is lowered.
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EXTRINSIC FACTORS:

Storage temperature & Relative humidity: 

  • The rate of microbial growth is affected to a large extent by the storage temperature.
  • Different microorganisms have a minimum and a maximum temperature at which they can grow.
  • Based on the optimum temperature required for their growth.

    I.Thermophiles (grow best at 55-650C

    II.Mesophiles (need 30 – 400C for their optimal growth)

    III.Psychrotopic (grow at refrigerating temperature of 20-300C )Based on the optimum temperature required for their growth. Microorganisms can be divided into three groups:

  • Preservation methods such as heating, cooling, drying, removal of air and use of preservation control the growth of microbes in food.
  • Nevertheless, killing the spores is much more difficult than destroying the vegetative bacterial cells.
  • Therefore, all processing times for canned foods are computed by using both the time and the temperature required to kill bacterial spores.
  • Relative humidity of the storage environment also plays an important role in the growth of micro-organisms.
  • It also affects a within the foods. Dehydrated foods such as milk powder are highly hygroscopic and can absorb considerable moisture from the surrounding environment when relative humidity is high.

Presence and concentration of gases:

  • The most frequently used gas for control of microbial growth is carbon dioxide (CO2) which is effective against many micro-organisms including aerobes.
  • On the other hand, presence of oxygen promotes the growth of micro-organisms.
  • In modified gas atmosphere, the concentration of carbon dioxide is increased, whereas that of oxygen is lowered.
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