Heating is a means of food preservation and is also important in improving organoleptic and nutritional quality of food. Heat transfer involves movement of energy from a substance with high temperature to another that has a lower temperature. It can be via one of three methods or in combination
- Conduction: heat transfer from one particle to another e.g. through solid foods such as meats and plant foods
- Convection: heat transfer by fluid flow e.g. juices
- Radiation: heat transfer by thermal radiation e.g. heat generated from hot coils in stove during baking
Combination heat treatment methods include a) convection-conduction e.g. heating of liquids that have a high starch content which gelatinize and solidifies during heating b) conduction-convection e.g. liquefication of solid foods during heating e.g. when liquids are released from meats during heating.
Heat treatment may vary in severity. Categories of treatment may include sterilization, commercial sterilization, pasteurization and blanching.
- Sterilization: severe heat treatment used to destroy all spores and vegetative (multiplying) microbial cells. This is typically used in the medical and pharmaceutical industry.
- Commercial sterilization: heat treatment to destroy pathogens and spoilage organisms that may grow under normal temperature and storage conditions e.g. during food canning.
- Pasteurization: mild heat treatment to reduce pathogens and spoilage organisms; however the food is not sterile and can spoil easily and quickly if not held under controlled temperature environment
- Blanching: very mild heat treatment to destroy enzymes. Very little heat penetration. It may involve conveying material through a steam tunnel or immersing it in water for a few seconds
Commercial sterilization may be via canning or aseptic packaging. During canning, food is filled in cans, sealed and then cooked; while aseptic processing and packaging involves sterilizing food and packaging separately, and then bringing them together. Commercial sterilization equipment involves the use of retort vessels (i.e. still, agitating, and hydrostatic). Pasteurization processes may be batch, High Temperature Short Time (HTST) and Ultra High temperature (UHT).
One big question that food scientist must figure out is, what is the time and temperature that will be needed to ensure that thermal processed foods are safe? This is done using process schedule experiments where they study the rates of heat penetration during cooking, and the amount of target organism that is killed during cooking. The target organism used for canned foods is Clustridium butulinum since it is the most heat resistant pathogen in canned foods. From the processing schedule experiments, they are able to develop a log curve showing the rate at which the microorganism of interest is killed over time. Two important values that can be obtained from these curves are the D and Z-values. D-value is the time it takes to reduce bacteria population by 90% of its original number. This is called a 1 log reduction. A high D-value indicates that more time will be required to sterilize the product. Z-value is the temperature that is required to have a 1 log reduction in the D-value. A high Z-value means that it will take a higher temperature to sterilize the product.
Factors affecting heat penetration of foods during cooking includes:
- Arrangement in packaging
- Elevation at which the processing is done
- pH of the food (low pH in foods helps to reduce bacteria resistance against heat. In other words, products with higher acidity can be sterilized at a lower temperature)
- Foods high in starch, protein and/lipids may require more heat treatment since these food components protect spores against heat damage
Observes the steps involved in the canning operation below
Freezing and refrigeration are different, and are defined by their temperature ranges
- Refrigeration: 40oF-45oF (4.5oC to 7oC)
- Freezing: 32oF to 0oF (0oC to -18oC)
Refrigeration and freezing reduce the rate of growth of microorganisms, but do not destroy them. They will grow when temperature conditions becomes ideal. Enzymatic and microbial growth continues at refrigeration temperatures, leading to breakdown of texture, color and flavor. Microorganisms grow rapidly at temperatures above 50oF (10oC). To reduce rate of spoilage during refrigeration, the following considerations should be made were applicable:
- Cool and clean food materials before refrigeration storage
- Seal package properly
- Keep temperature low
- Organize packages to allow air flow
- Control humidity
- Control atmosphere e.g. nitrogen or carbon dioxide
- Reduce pressure (hypobaric control) to reduce availability of oxygen
Pure water freezes at 32oF (0oC), but when solutes (dissolved solids) are present, freezing temperature is lower. Freezing does not prevent undesirable changes to food product. Although changes may occur at a slower rate, the following changes are observed in freezing
- Separation of liquid components e.g. seeping in jellies
- Texture damage resulting in soggy fruits and vegetables
- Freezer burns
- Development of off flavors
Slow freezing normally leads to growth of ice crystals and rupturing of cell walls in fruits and vegetables. This cause them to become soggy and leak their juices. Rapid freezing retards the growth of ice crystals. Smaller crystals do not damage cell walls. This causes the texture and appearance of the products to be retained. Methods used for rapid freezing includes:
- Blast freeze
- Immersion freezing
- Fluidized bed freezing
- Follow these rules to improve efficiency of freezing food at home
- Freeze at 0oF or lower to facilitate rapid freezing
- Freeze immediately after packaging
- Do not overload
- Place packages in the coldest part of the freezer
- Leave space between packages for air circulation
- Avoid stacking/storing items on top of the freezer