Thermal processing of food

Thermal processing of foods: Blanching, sterilization and Pasteurization

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There are two main temperature categories employed in thermal processing: Pasteurization and Sterilization. The basic purpose for
the thermal processing of foods is to reduce or destroy microbial activity,
reduce or destroy enzyme activity and to produce physical or chemical changes to make the food meet a certain quality standard. E.g. gelatinization of starch and denaturation of proteins to produce edible food. There are a number of types of heat processing employed by the food industry.

  1. Blanching

The primary purpose of blanching is to destroy enzyme activity in fruit and vegetables. It is not intended as a sole method of preservation, but as a pretreatment prior to freezing, drying and canning. Other functions of blanching include:

• Reducing surface microbial contamination

• Softening vegetable tissues to facilitate filling into containers

• Removing air from intercellular spaces prior to canning

Blanching is carried out at up to 100°C using hot water or steam at or near atmospheric pressure. Some use of fluidized bed blanchers, utilizing a mixture of air and steam, has been reported. Advantages include faster, more uniform heating, good mixing of the product, reduction in effluent, shorter processing time and hence reduced loss of soluble and heat sensitive components.

Blanching is carried out at up to 100°C using hot water or steam at or near atmospheric pressure.
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There is also some use of microwaves for blanching. Advantages include rapid heating and less loss of water soluble components. Disadvantages include high capital costs and   potential difficulties in uniformity of heating.

Steam Blanchers

This is the preferred method for foods with large cut surface areas as lower leaching losses. Normally food material carried on a mesh belt or rotatory cylinder through a steam atmosphere, residence time controlled by speed of the conveyor or rotation. Often poor uniformity of heating in the multiple layers of food, so attaining the required time-temperature at the center results in overheating of outside layers.

Individual Quick Blanching (IQB) involves a first stage in which a single layer of the food is heated to sufficient temperature to inactivate enzymes and a second stage in which a deep bed of the product is held for sufficient time to allow the temperature at the center of each piece to increase to that needed for inactivation.

The reduced heating time (e.g. for 10 mm diced carrot, 25 s heating and 50 s holding compared with 3 minutes conventional blanching) results in higher energy efficiencies For small products  eggs, peas, sliced or diced carrots), mass of produce blanched per kg steam increases from 0.5kg for conventional steam
blanchers to 6-7kg for IQB. 

Hot Water Blanchers

Includes various designs which hold the food in hot water (70 to 100°C) for a specified time, then moves it to a dewatering/cooling section. In blanchers of this type the food enters a slowly rotating drum, partially submerged in the hot water. It is carried along by internal flights, residence time being controlled by the speed of rotation.

Pipe blanchers consist of insulated tubes through which hot water is circulated. Food is metered into the stream, residence time being controlled by the length of the pipe and velocity of the water.

The blancher-cooker has three sections, a preheating stage, a blanching stage, and a cooling stage. As the food remains on a single belt throughout the process, it is less likely to be physically damaged. With the heat recovery incorporated in the system, 16 to 20 kg of product can be blanched for every kg of steam, compared with 0.25 to 0.5kg per kg stream in the conventional hot water blanchers.

2. Pasteurization

Pasteurization is a relatively mild heat treatment in which food is heated to above 100°C. It is widely used throughout the food industry and is frequently employed as a CCP in various HACCP plans. As a unit operation in food processing it can be used to destroy enzymes and relatively heat sensitive micro-organisms (e.g. non spore forming bacteria, yeast and moulds). In this regard is it used to extend shelf life by several days e.g. milk or months e.g. bottled fruit.

Pasteurization is a relatively mild heat treatment in which food is heated to above 100°C.
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The severity of treatment and resulting extension of shelf life is determined mostly by pH of the food. In low acid foods (pH above 4.5), the main purpose is destruction of pathogenic bacteria, while below pH 4.5 the destruction of spoilage microorganisms or enzyme deactivation is usually more important. The extent of heat treatment required is determined bythe D value (Decimal reduction time or time to reduce numbers by a factor of 10 or 90% of the initial load) of most heat resistant enzyme or micro-organism which may be present.

In terms of checking the effectiveness of the process, alkaline phosphatase is a
naturally occurring enzyme in raw milk with a similar D value to heat-resistant
pathogens and so is routinely used as an indicator of adequate pasteurization.
If phosphatase activity is found, it is assumed that pasteurization is inadequate.

Pasteurization is normally used for the destruction of all disease causing organisms (e.g. pasteurization of milk) or the destruction or reduction in the number of spoilage organisms in certain foods e.g. vinegar.


organisms that can survive exposure to relatively high temperatures but do not
necessarily grow at these temperatures e.g. Streptococcus and Lactobacillus.

organisms that not only survive relatively high temperatures but require high temperatures for their growth.

Batch (holding) Method

In this method every particle (e.g. milk) must be heated to at least 63°C and held for at least 30 minutes, however this is not used commercially these days

High-Temperature-Short-Time (HTST)

In this method the heating of every particle of milk to at least 72°C and holding for at least 15 seconds. Carried out as a continuous process. Ultra Heat Treatment (UHT) a sterilization treatment, can also be performed using higher temperatures and shorter times e.g. 1 s at 135°C

Typical Equipment employed for this method includes:

• Plate heat exchanger (PHE)

• Holding tube – sized to ensure the correct treatment time is achieved

• Holding tanks – for storage of the raw and pasteurized milk

• Balance tank – to assist in maintaining full flow, and to take returned milk if temperature not achieved

• Control and monitoring system – to record temperature and to divert flow back to the balance tank if correct temperature is not achieved.

Pasteurization of packaged foods

Some liquid foods (e.g. beer and fruit juices) are pasteurized after filling into
containers. Hot water is normally used if the food is packaged into glass, to reduce the risk of breakage due to thermal shock. Maximum temperature between the container and the liquid are 20°C for heating and 10°C for cooling. Metal and plastic containers may be pasteurized using steam-air mixtures or hot water.

Pasteurisers may be batch or continuous. A simple batch type may be a water bath in which crates of the food are heated to a pre-set temperature, and then cooled by draining and adding cold water. A continuous version may convey containers through a hot water batch followed by a cold water bath.

Steam tunnels may also be used with the advantage of faster heating, resulting in shorter residence time and smaller equipment. Temperatures in the heating zones may be controlled depending on the amount of air present. Acid products such as fruit or acidified vegetables like beetroot can be pasteurized in a retort.


Unlike pasteurized products where the survival of heat resistant microorganisms is accepted, the aim of sterilization is the destruction of all bacteria including their spores. Heat treatment of such products must be severe enough to inactivate/kill the most heat resistant bacterial microorganisms, which are the spores of Bacillus and Clostridium.

Food products filled in sealed containers are exposed to temperatures above 100°C in pressure cookers. Temperatures above 100°C, usually ranging from 110-121°C depending on the type of product, must be reached inside the product. Products are kept for a defined period  of time at temperature levels required for the sterilization depending on type of product and size of container.

If spores are not completely inactivated, vegetative microorganisms will grow from the spores as soon as conditions are favorable again. Favorable conditions will exist when the heat treatment is completed and the products are stored under ambient temperatures. The surviving microorganisms can either spoil preserved food or produce toxins which cause food poisoning. Amongst the two groups of spore producing microorganisms Clostridium is more heat resistant than Bacillus.

Temperatures of 110°C will kill most Bacillus spores within a short time. In the case of Clostridium temperatures of up to 121°C are needed to kill the spores within a relatively short time. These sterilization temperatures are needed for short-term inactivation (within a few seconds) of spores of Bacillus or Clostridium. These spores can also be killed at slightly lower temperatures, but longer heat treatment periods must be applied.

From the microbial point of view, it would be ideal to employ very intensive heat treatment which would eliminate the risk of any surviving microorganisms. However, most food products cannot be submitted to such intensive heat stress without suffering degradation of their sensory quality or loss of nutritional value (destruction of vitamins and protein components). In order to comply with above aspects, a compromise has to be reached in order to keep the heat sterilization intensive enough for the microbiological safety of the products and as moderate as possible for product quality reasons.

Two typical forms of sterilized product are:

• In package sterilized, in which product is packed into containers and the container of product is then sterilized e.g. canning, some bottled products, retort pouches

• UHT or Aseptically processed products in which the product and the package is sterilized separately then the package is filled with the sterile product and sealed under specific conditions Long life milk, tetrapack or combibloc fruit juices and soups etc.

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