Microorganisms such as bacteria, yeasts, molds, and viruses are widely encountered in air, water, soil, living organisms, and unprocessed food items, and cause off-flavors and odors, slime production, changes in the texture and appearances, and the eventual spoilage of foods. Holding perishable foods at warm temperatures is the primary cause of spoilage, and the prevention of food spoilage and the premature degradation of quality due to microorganisms is the largest application area of refrigeration. The first step in controlling microorganisms is to understand what they are and the factors that affect their transmission, growth, and destruction.
Of the various kinds of microorganisms, bacteria are the prime cause for the spoilage of foods, especially, moist foods. Dry and acidic foods create an undesirable environment for the growth of bacteria, but not for the growth of yeasts and molds. Molds are also encountered on moist surfaces, cheese, and spoiled foods. Specific viruses are encountered in certain animals and humans. Poor sanitation practices such as keeping processed foods in the same area as the uncooked ones and being careless about hand washing can cause the contamination of food products.
When contamination occurs, the microorganisms start to adapt to the new environmental conditions. This initial slow or no-growth period is called the lag phase and the shelf life of a food item is directly proportional to the length of this phase (Figure 1).
FIGURE 1: MICROBIAL GROWTH CURVE
The adaptation period is followed by an exponential growth period during which the population of microorganisms can double two or more times every hour under favorable conditions unless drastic sanitation measures are taken. The depletion of nutrients and the accumulation of toxins slow down the growth and start the death period. The rate of growth of microorganisms in a food item depends on the characteristics of the food itself such as the chemical structure, pH level, presence of inhibitors and competing microorganisms, and water activity as well as the environmental conditions such as the temperature and relative humidity of the environment and the air motion (Figure 2).
Microorganisms need food to grow and multiply, and their nutritional needs are readily provided by the carbohydrates, proteins, minerals, and vitamins in a food. Different types of microorganisms have different nutritional needs, and the types of nutrients in a food determine the types of microorganisms that may dwell on them. Microorganism growth in a food item is governed by the combined effects of the characteristics of the food and the environmental factors. We cannot do much about the characteristics of the food, but we certainly can alter the environmental conditions to more desirable levels through heating, cooling, ventilating, humidification, dehumidification, and control of the oxygen levels. The growth rate of microorganisms in food is a strong function of temperature, and temperature control is the single most effective mechanism for controlling the growth rate. Microorganisms grow best at “warm” temperatures, usually between 20 and 60°C. The growth rate declines at high temperatures, and death occurs at still higher temperatures, usually above 70°C for most microorganisms. Cooling is an effective and practical way of reducing the growth rate of microorganisms and thus extending the shelf life of perishable foods.
Microorganisms in food products can be controlled by
- preventing contamination by following strict sanitation practices,
- inhibiting growth by altering the environmental conditions, and
- destroying the organisms by heat treatment or chemicals.The storage life of foods can be extended by several months by freezing and storing them at subfreezing temperatures, usually, between -18 and -35°C, depending on the particular food (Figure 3).
Figure 3: Temperature for refrigerated and frozen storage of food.
It was determined that the rate of freezing has a major effect on the size of ice crystals and the quality, texture, and nutritional and sensory properties of many foods. During slow freezing, ice crystals can grow to a large size, where as during fast freezing a large number of ice crystals start forming at once and are much smaller in size.
A crust forms rapidly on the outer layer of the product and seals in the juices, aromatics, and flavoring agents.
Quality and Safety
Quality and safety are not the same. High quality food tastes, smells and looks good. A safe food will not make you sick. People need to be careful because an unsafe food may look and smell fine, but contain microbes that cause illness. For example, a huge pot of hot chili in the refrigerator will not cool quickly and may support bacterial growth that will make someone sick. On the other hand, chicken with freezer burn is safe, yet poor in quality. Refrigerated foods should be cold with packages sealed and clean. Frozen foods should be frozen solid.
Magnetic Field Assisted Freezing
The quality of frozen food is related to its freezing rate. In the conventional methods of freezing, freezing rate is slow with low degree of super cooling, the speed of ice nucleation is slower than the speed of ice growing, thus resulting in the formation of big ice crystals in the frozen food and their distribution in the extracellular region. But the focus now-a–days is on enhancing the freezing rate. Freezing with induced magnetic field is one of them. In this system, magnetic field energy generating equipment generates weak energy evenly in the freezing equipment so as to produce good tasting food with original flavor and without any drip loss.
Food products, especially, the perishable fruits and vegetables contain a large amount of water. The water contained is constituted of bound water tied to protein or other molecules and free water without being tied. With the application of magnetic field (Figure 4), vibration is supplied to water molecules which prevents them from clumping together and keep them under super cooling condition.
Figure 4: Application of magnetic field
By doing so, fast freezing generates small and uniform ice crystals, with their distribution mainly in the intracellular region. As a result, the microstructure of food is protected, preventing the destruction of cells in the food material and producing highly fresh and high quality frozen product.
Figure 5: Comparison of freezing with and without magnetic system (Cells Alive system; CAS)
Foods That Don’t Freeze Well
- Because of flavor changes
- Garlic (uncooked)
- Onion (raw; better cooked or as ingredient)
- Spices – clove, sage (flavor is stronger or bitter)
- Because of texture changes
- Cake icing – soft or boiled (butter cream freezes well)
- Cream sauces
- Custard or cream filling
- Egg whites (cooked) & meringue
- Fried foods (homemade)
- Gelatin
- Mayonnaise or salad dressing
- Lettuce
- Pasta (cooked, unsauced)
- Potatoes
AUTHORS CREDIT & PHOTOGRAPH
Maninder Kaur
Department of Processing and Food Engineering
Punjab Horticultural Post Harvest Technology Center
Punjab Agricultural University, Ludhiana
