Factors Affecting Time of Thermal Process

Factors Affecting Time of Thermal Process
The thermal process time necessary for preservation of a chosen food depends on the rate of heat penetrating into a food.

Since every part of the food in a container or can must receive the same adequate heat treatment to prevent its spoilage, the process time work depended on the part which receives heat most slowly.

The food part near the center of the container usually receives heat most slowly. Heat penetration in food is mainly due to conduction as well as convection, which may operate simultaneously or successively.

Conduction occurs in solid particles of foods and is slow while heat transfer by convection occurs in liquids.

Some foods such as sugar syrup, thick soup and tomato juice undergo changes in their consistency during heating.

A variety of factors influence the time required for thermal processing for food. These include:

1. The material of the container – glass having a slower rate compared to that of a metal.
   
2. The size and shape of the container – smaller and slim cylindrical cans attain the desired temperature faster
   
3. Initial temperature of the food - a higher initial temperature provides the lethal range for the microorganisms for a longer time and its average temperature during heating is higher than the food in the can with a lower initial temperature. A higher temperature is necessary for foods which heat slowly, e.g. meat, pumpkin etc.
   
4. Retort temperature a higher retort temperature heats the food rapidly.
   
5. Consistency of the food – nature, sizes and shapes pieces decide the rate of heat penetration.
   
   The pieces of food may retain the identity as in peas, plums, beets, asparagus and whole grain corn with smaller pieces requiring less time.
   
   Pieces may become mushy or change into a viscous mass as in the case of pumpkin, sweet potatoes, squash and cream-style corn, which get heated slowly as conduction is the main mechanism of heat transfer.
   
   Pieces may form layers as in the case of spinach and asparagus. In addition, the consistency of the foods depends on the different types of sauce starch salt or sugar that may be present.
   
   For example tomato sauce on baked beans slows down heat penetration more compared to plain sauce. Starch concentrations up to 6% decreases heat penetration, but at higher concentration, has no additional effect.
   
   Sodium chloride does not influence heat penetration, as its concentration is usually low. On the other hand increasing of sugar decreases heat penetration. However, this effect is counteracted by the decrease in viscosity of sugar solutions with increasing temperature.
   
6. Rotation and agitation of the container during heat processing hasten heat penetration, particularly if the food is a fluid, but may cause undesirable physical damage to some solid foods.

Cooling of the canned foods also is influenced by the same factors as the heating process. Rapid, artificial cooling is usually adopted to effect a greater control on the cooling rate, because slow cooling may cause overcooking of the food and also allow the growth of thermophilic organisms.
Factors Affecting Time of Thermal Process

Softdrinks Processing – Dechlorination of Water

Softdrinks Processing – Dechlorination of Water
Because of the oxidation reactions that are taking place during the sterilization period some of the chlorine will be consumed but the amount of chlorine dosed will be enough to ensure that there will always be at least a 5 ppm residual at the end of the sterilization period.

The residual free chlorine and the majority of the organic molecules will be removed from the sterilized water prior to use by absorption on activated carbon.

The activated carbon will be housed in a pressure filter which is similar in construction and arrangement to the sand filter.

The activated carbon will be supported in the filter on a bed of graded gravel and the volume for carbon will be calculated to a give a contact time of five minutes with the water.

This contact time will be long enough to remove all the free chlorine from the water.

Activated carbon will also remove other organic molecules that remain in the water after the coagulation reaction or are formed as part o the sterilization reactions.

A five minute contact time may be enough to remove not only the free chlorine but also a majority of the taste and odor forming organic molecules.

Some residual organic compounds may however, remain in the water because the contact time with activated carbon necessary for their total removal will be longer than five minutes.

In general, the organic residual is expected to be below the detection threshold with only a short contact time.
Softdrinks Processing – Dechlorination of Water