Heat Processing

Heating is a common way of processing food. Applying heat to food offers several benefits. These may include killing harmful organisms; improving organoleptic properties; destroying anti-nutrients, and improving digestibility. Methods of heating may involve the use of dry heat, steam, or water. During these processes, heat may be transferred by conduction, convection, radiation or a combination of the three. Conduction is heat transfer from one particle to another, e.g. through solid foods such as a steak or whole potatoes. Convection is heat transfer by fluid flow, e.g. beverages. Radiation is heat transfer by electromagnetic radiation, e.g. heat generated from hot coils in stove during baking. Sometimes the type of heating may change from one form to the other during processing. For example:

1. Convection to conduction – may occur during heating of liquids that have a high starch content. During cooking, the starch gelatinized and turns to a highly viscous semi-solid
2. Conduction-convection – may occur during heating of meats. The heating starts out purely by conduction but due to the seeping of meat juices into the container, convection heating also comes into play.

Heat treatment may increase in severity. Different levels of heating according to severity include blanching, pasteurization, commercial sterilization, and sterilization.

1. Sterilization: Severe heat treatment used to destroy all spores and vegetative microbial cells. This is typically used in the microbiology lab to prepare materials for microbiological evaluation. It is not commonly used in food processing since organoleptic properties would be lost
2. Commercial sterilization: Heat treatment to destroy all pathogens, spoilage organisms and some spores. The surviving spores are incapable of growing at the normal storage temperature at which the food is held.
3. Pasteurization: Mild heat treatment to destroy pathogens. However, spoilage organisms and spores remain.
4. Blanching: Very mild heat treatment to denature enzymes. It may involve conveying material through a steam tunnel or immersing it in water for a few seconds

Commercial sterilization is generally by canning or aseptic packaging. During canning, food is filled in cans, sealed and then cooked; while aseptic processing and packaging involves sterilizing the food and packaging separately, and then bringing them together. Commercial sterilization canning equipment involves the use of high-temperature vessels called retorts. Pasteurization processes may be by batch, high temperature short time (HTST), or ultra high temperature (UHT) cooking.

Determination of Safe Processing Conditions

One big question that food scientists must figure out is, what is the time and temperature that will be needed to ensure that thermal processed foods are safe? This is generally done using inoculated pack studies where they inoculate a target organism in the food to be processed, and then process the food to determine the effect of heat in reducing the number of microorganisms to a safe level. The primary target organism used for studies relating to canned foods is Clostridium botulinum because of its high heat resistance. Essential data derived from these studies include the D, Z and F_o values.

1. D-value: The time it takes at a specific temperature to reduce organisms in a population by 90%. This is called a 1 log reduction where the population reduces each time by a factor of 10. Therefore, the larger the D-value the longer it takes to reduce the bacteria population by 90%. Hence, bigger D-value = greater heat resistance.
2. Z-value: The temperature change needed to produce a 1 log reduction in the D-value. A bigger z-value means that it requires a greater temperature change to reduce the D-value by a factor of 10. In other words, the bigger the z-value, the greater the heat resistance of the organism.
3. F-value: The number of minutes at a set temperature required to destroy a specific number of organisms having a given z-value. Hence, the F-value measures the capacity of the heat treatment to produce sterilization.
4. F_o-value (or sterilization value): A reference F-value indicating the number of minutes at 121^_oC (250^_oF) required to destroy a specific number of organisms whose z-value is 10^oC (18^oF). An F_o value of 6 means that it takes 6 minutes to destroy the population of organisms. Hence the greater the F_o value, the greater the heat resistance of the organism.

Factors Affecting Heat Penetration

Food manufacturers must be aware that the effectiveness of their heating process may be affected by one or more factors. These factors are summarized below.

1. Composition – Foods high in starch, protein and/lipids may require more heat treatment since these food components protect microorganism against heat damage
2. Texture – Thicker or denser foods slows down heat penetration
3. Arrangement in packaging – Foods arranged in containers against the grain of the direction of heat flow will take longer to heat up
4. Elevation at which the processing is done – Water will boil faster and at lower temperature at high elevation due to reduced atmospheric pressure
5. pH of the food – Heat is more effective in destroying bacteria when pH is low. Therefore products with higher acidity can be sterilized at a lower temperature

Reference: Potter, N. N. & Hotchkiss, J. H. (1998). Food Science, 5th edition. New York, NY: Springer Science+Business Media LLC.