Fish Microbiology and Quality Assurance

Unit 2 - Growth and survival of microorganisms in food

2.2.2 Relative humidity (RH)
Relative humidity of the food storage environment refers to percentage of moisture present in the atmosphere. RH and water activity are closely related and RH is measure of aw of the gas phase of atmosphere.

RH = 100 x a_w

Effect of RH on food being stored
RH of storage environment influences aw of foods and growth of microorganisms on food surface. Foods of low aw stored in high RH environment leads to transfer of water from environment to the food, increasing aw of food until equilibrium is reached. Condensation of moisture on food surface results in localized regions of high aw on surface and subsurface. This favors growth of microorganisms which were viable but unable to grow due to low a_w, increases a_w of immediate environment due to metabolic activity of microorganisms which produce water as end product of respiration. Thus microorganisms grow and causes spoilage of food which was considered safe microbiologically due to the growth of microorganisms requiring high a_w.

Foods of high a_w when stored in low RH environment lose moisture, become flaccid and unfit for consumption due to loss of quality.

RH and spoilage of food

• RH and temperature of environment are related, and as the temperature increases RH decreases.
• Foods that are susceptible for spoilage by yeasts, molds and certain bacteria should be stored under low RH conditions.
• Improperly wrapped animal meat kept in refrigerator (high RH) undergone surface spoilage.
• Selection of suitable RH condition for storage is necessary to avoid surface microbial growth and maintain desirable qualities of food as food may lose/take up moisture under improper RH condition and lose its quality.