Food Preservation and Classification

• Food preservation uses physical or chemical agents to prevent microbial spoilage, keeping food fit for future use
• The aim of food preservation is to extend the storage life of food

Food Classification

• Foods are classified into three categories based on their perishability

Perishable Foods

• Perishable foods deteriorate quickly after harvesting.
• Examples include tomatoes, mangoes, papayas, peaches, plums, and other juicy fruits.
• Meat, fish, and poultry are also perishable due to their high moisture content.

Semi-Perishable Foods

• Semi-perishable foods have less moisture content.
• Examples: beetroots, carrots, peas, green beans, pumpkins, and apples.
• Eggs, pasteurized milk, highly smoked fish, and pickled vegetables are semi-perishable.

Non-Perishable Foods

• Non-perishable foods have very low moisture content.
• Examples: mature food grain cereals, pulses, and nuts.
• These foods are not easily susceptible to spoilage by microorganisms and enzymes.

Principles of Food Preservation

• Food preservation employs two general principles: inhibition and killing.

Inhibition Principle

• Food preservation is achieved by inhibiting the growth and multiplication of microorganisms.
• This can be achieved by:
  • Reducing water activity through drying or salting
  • Reducing pH through fermentation
  • Using preservatives such as sodium benzoate
  • Using low temperatures such as chilling or freezing
  • Smoking to create a drying and preservative effect
• Inhibition methods do not necessarily destroy organisms, and spoilage can occur if the inhibiting influence is removed

Food Preservation by Lowering pH

• Lowering pH preserves food products by preventing the growth of spoilage and pathogenic bacteria.
• This can be achieved by adding acids or through fermentation
• Fermentation: Breakdown of carbohydrates under anaerobic conditions into alcohol or lactic acid and carbon dioxide

Food Preservation by Lowering Water Activity

• Lowering water activity can be achieved by:
  • Adding high content of salt such as sodium chloride, and sometimes nitrates and nitrites
  • Adding high content of sugar
• Drying through sun/air drying, electrical drying, or freeze-drying

Salting Procedure

• The salting procedure can be performed in four ways:
  • Dry cure: meat or fish is rubbed with salt
  • Pickling: products are immersed in a pickle of brine, usually containing about 15% salt
  • Injection cure: concentrated salt solution is injected into muscle tissues
  • Direct salt addition method

Food Preservation by Low Temperatures

• Low temperatures arrest microbial growth and multiplication using two methods:
  • Chilling: keeping food at temperatures between 0–15°C; common chilling temperatures range between 4–5°C
  • Freezing: keeping food at temperatures between -1°C and -35°C

Effect of Low Temperatures

• Low temperatures retard chemical reactions and the actions of food enzymes.
• Low temperatures slow down or stop the growth and activity of microorganisms in food.

Food Preservation by Preservatives

• A preservative is any substance capable of inhibiting, retarding, or arresting the growth of microorganisms, their enzymes, or their genetic mechanisms
• Chemical preservatives are generally added after the foods are processed

Types of Preservatives

• Preservatives are divided into two classes:
  • Class one: Common salt, sugar, dextrose, glucose (syrup), vinegar, or acetic acid
  • Class two: Benzoic acid, sodium benzoate, methyl and propyl p, nitrites, chlorine, carbon dioxide

Preservation of Food by Drying

• Drying is one of the oldest methods of preserving foods
• All cereal grains are preserved by drying
• Sun-drying remains the greatest food preservation action
• Milk is preserved through drying in the form of milk powder
• Meats are also preserved through drying

Killing Principle

• Spoilage microorganisms are destroyed (killed) and the food is protected against contamination by being enclosed in an airtight container

Methods Employed to Achieve the Killing Principle

• Heat treatment through pasteurization or sterilization.
• Irradiation with either ionizing or electromagnetic radiation, e.g., Gamma rays, cobalt 60 radioactive particles.
• Use of gases: Ethylene oxide or ozone destroys both vegetative cells and spores

Radiation

• Radiation kills microorganisms by destruction of DNA and creates toxic reactive compounds in a medium and in microbial cells

Pasteurization

• Pasteurization involves heat treatment, mainly of milk, at specific temperatures and times.
• It destroys all pathogenic microorganisms without affecting the nutritive value of the food
• Pasteurization commonly used in milk processing.

Methods of Pasteurization

• Low temperature long time (LTLT): 63°C for 30 minutes
• High temperature short time (HTST): 72°C for 15 seconds
• Flash method: 80°C for 1-2 seconds

Sterilization

• Sterilization uses physical or chemical means to destroy all microorganisms, including spores, present in the food

Methods to Achieve Sterilization

• Heating at high temperature, e.g., 100°C - 140°C
• Irradiation kills bacteria, spores, and insects and also inactivates enzymes

Combination Strategies

• A combination of inhibition and killing principles and various methods is used in food preservation, depending on the food type
• Example: pasteurization and chilling of milk, preservatives, and low-temperature storage in fruit juices

Decimal Reduction Time (D-Value)

• The time required at any temperature to destroy 90% of the spores or vegetative cells of a given organism
• The higher the temperature, the faster the rate of destruction and the shorter it takes to kill 90% of the cells
• For example, the D-value of Clostridium sporogenes in a given food at 120°C is 1 minute, at 115°C is 4 minutes, and at 110°C is 10 minutes