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Food

Food irradiation

High hydrostatic pressure

Microwave heating

Pulsed electric field

Centrifugation

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Expression  

Quality Management in Fish Processing

Properties of foods

Heat transfer In Food

Water activity

Fruit and juice processing

Carbohydrate and intense sweeteners Used In soft driks

Ingredients used in soft drinks

Non-carbonated beverages

Processing and packaging of Soft drinks

 

Heat transfer In Food

Most unit operations in food processing involve the transfer of heat into or out of a food. There are three ways in which heat may be transferred: by radiation, by conduction and by convection. Radiation, is the transfer of heat by electromagnetic waves for example in an electric grill.Conduction is the movement of heat by direct transfer of molecular energy within solids (for example through metal containers or solid foods). Convection is the transfer of heat by groups of molecules that move as a result of differences in density (for example in heated air) or as a result of agitation (for example in stirred liquids). In the majority of applications all three types of heat transfer occur simultaneously but one type may be more important than others in particular applications.

 Energy balances:- An energy balance states that ‘the amount of heat or mechanical energy entering a process= the total energy leaving with the products and wastes+stored energy+energy lost to the surroundings’. If heat losses are minimised, energy losses to the surroundings may be ignored for approximate solutions to calculation of, for example, the quantity of steam, hot air or refrigerant required. For more accurate solutions, compensation should be made for heat losses.

Mechanisms of heat transfer

Steady-state heat transfer takes place when there is a constant temperature difference between two materials. The amount of heat entering a material equals the amount of heat leaving, and there is no change in temperature of the material.

This occurs for example when heat is transferred through the wall of a cold store if the store temperature and ambient temperature are constant, and in continuous processes once operating conditions have stabilised. However, in the majority of food-processing applications the temperature of the food and/or the heating or cooling medium are constantly changing, and unsteady-state heat transfer is more commonly found. Calculations of heat transfer under these conditions are extremely complicated but are simplified by making a number of assumptions and using prepared charts and computer models to give approximate solutions.

Steady-state conduction

The rate at which heat is transferred by conduction is determined by the temperature difference between the food and the heating or cooling medium, and the total resistance to heat transfer. The resistance to heat transfer is expressed as the conductance of a material, or more usefully as the reciprocal which is termed the thermal conductivity. Under steady-state conditions the rate of heat transfer is calculated using

rate of heat transfer= Thermal conductivity X Surface area X Temperature difference
                                                 Thickness of the material.

Although, for example, stainless steel conducts heat ten times less well than aluminium, the difference is small compared to the low thermal conductivity of foods (twenty to thirty times smaller than steel) and does not limit the rate of heat transfer. Stainless steel is much less reactive than other metals, particularly with acidic foods, and is therefore used in most food-processing equipment that comes into contact with foods. The thermal conductivity of foods is influenced by a number of factors concerned with the nature of the food (for example cell structure, the amount of air trapped between the cells, and the moisture content), and the temperature and pressure of the surroundings. A reduction in moisture content causes a substantial reduction in thermal conductivity. This has important implications in unit operations which involve conduction of heat through food to remove water (for example drying, frying and freeze drying). In freeze drying the reduction in atmospheric pressure also influences the thermal conductivity of the food. Ice has a higher thermal conductivity than water and this is important in determining the rate of freezing and thawing.

Unsteady-state conduction

During processing, the temperature at a given point within a food depends on the rate of heating or cooling and the position in the food. The temperature therefore changes continuously. The factors that influence the temperature change are:

  • the temperature of the heating medium
  • the thermal conductivity of the food
  • the specific heat of the food.

The basic equation for unsteady-state heat transfer in a single direction (x) is

                                             

where  change in temperature with time. where p density, c (J/kg/K) specific heat capacity and k thermal conductivity.

Convection

When a fluid changes temperature, the resulting changes in density establish natural convection currents. Examples include natural-circulation evaporators, air movement in chest freezers, and movement of liquids inside cans during sterilisation . Forced convection takes place when a stirrer or fan is used to agitate the fluid. This reduces the boundary film thickness to produce higher rates of heat transfer and a more rapid temperature redistribution. Consequently, forced convection is more commonly used than natural convection in food processing. Examples of forced convection include mixers, fluidised-bed driers, air blast freezers and liquids pumped through heat exchangers.

Sources of heat and methods of application to foods

The cost of energy for heating has become one of the major considerations in the selection of processing methods and ultimately in the cost of the processed food and the profitability of the operation. Different fuels have specific advantages and limitations in terms of cost, safety, risk of contamination of the food, flexibility of use, and capital and operating costs for heat transfer equipment. The following sources of energy are used in food processing:

• electricity
• gas (natural and liquid petroleum gas)
• liquid fuel oil.

Solid fuels (anthracite, coal, wood and charcoal) are only used to a small extent.

Direct heating methods

In direct methods the heat and products of combustion from the burning fuel come directly into contact with the food. There is an obvious risk of contamination of the food by odours or incompletely burned fuel and, for this reason, only gas and, to a lesser extent, liquid fuels are used. These direct methods should not be confused with ‘direct’ steam injection where the steam is produced in a separate location from the processing plant. Electricity is not a fuel in the same sense as the other types described above. It is generated by steam turbines heated by a primary fuel (for example coal or fuel oil) or by hydro-power or nuclear fission. However, electrical energy is also said to be used directly in dielectric heating or microwave heating.

Indirect methods

Indirect heating methods employ a heat exchanger to separate the food from the products of combustion. At its simplest an indirect system consists of burning fuel beneath a metal plate and heating by energy radiated from the plate. The most common type of indirect-heating system used in food processing is steam or hot water generated by a heat exchanger (a boiler) located away from the processing area. A second heat exchanger transfers the heat from the steam to the food under controlled conditions or the steam is injected into the food. A variation on this system involves an additional heat exchanger which transfers heat from the steam to air in order to dry foods or to heat them under dry conditions. Indirect electrical heating uses resistance heaters or infrared heaters. Resistance heaters are nickel-chromium wires contained in solid plates or coils, which are attached to the walls of vessels, in flexible jackets which wrap around vessels, or in immersion heaters which are submerged in the food. These types of heater are used for localised or intermittent heating.

Effect of heat on micro-organisms

The preservative effect of heat processing is due to the denaturation of proteins, which destroys enzyme activity and enzyme-controlled metabolism in micro-organisms. The rate of destruction is a first-order reaction; that is when food is heated to a temperature that is high enough to destroy contaminating micro-organisms, the same percentage die in a given time interval regardless of the numbers present initially. This is known as the logarithmic order of death and is described by a death rate curve. The time needed to destroy 90% of the micro-organisms (to reduce their numbers by a factor of 10) is referred to as the decimal reduction time or D value. D values differ for different microbial species, and a higher D value indicates greater heat resistance.

There are two important implications arising from the decimal reduction time: first, the higher the number of micro-organisms present in a raw material, the longer it takes to reduce the numbers to a specified level. In commercial operation the number of microorganisms varies in each batch of raw material, but it is difficult to recalculate process times for each batch of food. A specific temperature–time combination is therefore used to process every batch of a particular product, and adequate preparation procedures are used to ensure that the raw material has a satisfactory and uniform microbiological quality. Second, because microbial destruction takes place logarithmically, it is theoretically possible to destroy all cells only after heating for an infinite time. Processing therefore aims to reduce the number of surviving micro-organisms by a pre-determined amount. This gives rise to the concept of commercial sterility. The destruction of micro-organisms is temperature dependent; cells die more rapidly at higher temperatures. By collating D values at different temperatures, a thermal death time (TDT) curve is constructed. The slope of the TDT curve is termed the z value and is defined as the number of degrees Celsius required to bring about a ten-fold change in decimal reduction time. The D value and z value are used to characterise the heat resistance of a micro-organism and its temperature dependence respectively.

There are a large number of factors which determine the heat resistance of microorganisms,but general statements of the effect of a given variable on heat resistance are not always possible. The following factors are known to be important.

1. Type of micro-organism. Different species and strains show wide variation in their heat resistance. Spores are much more heat resistant than vegetative cells.

2. Incubation conditions during cell growth or spore formation. These include:

    (a) temperature (spores produced at higher temperatures are more resistant than those produced at lower temperatures)
    (b) age of the culture (the stage of growth of vegetative cells affects their heat resistance)
    (c) culture medium used (for example mineral salts and fatty acids influence the heat resistance of spores).

3. Conditions during heat treatment. The important conditions are:

    (a) pH of the food (pathogenic and spoilage bacteria are more heat resistant near to neutrality; yeasts and fungi are able to tolerate more acidic conditions but are less heat resistant than bacterial spores)
    (b) water activity of the food influences the heat resistance of vegetative cells; in addition moist heat is more effective than dry heat for spore destruction
    (c) composition of the food (proteins, fats and high concentration of sucrose increase the heat resistance of micro-organisms; the low concentration of sodium chloride used in most foods does not have a significant effect; the physical state of the food, particularly the presence of colloids, affects the heat resistance of vegetative cells)
    (d) the growth media and incubation conditions, used to assess recovery of microorganisms in heat resistance studies, affect the number of survivors observed.

Most enzymes have D and z values within a similar range to micro-organisms, and are therefore inactivated during normal heat processing. However, some enzymes are very heat resistant. These are particularly important in acidic foods, where they may not be completely denaturated by the relatively short heat treatments and lower temperatures required for microbial destruction. A knowledge of the heat resistance of the enzymes and/or micro-organisms found in a specific food is used to calculate the heating conditions needed for their destruction. In practice the most heat resistant enzyme or micro-organism likely to be present in a given food is used as a basis for calculating process conditions. It is assumed that other less heat-resistant species are also destroyed.

Effect of heat on nutritional and sensory characteristics

The destruction of many vitamins, aroma compounds and pigments by heat follows a similar first-order reaction to microbial destruction. In general both values are higher than those of micro-organisms and enzymes. As a result, nutritional and sensory properties are better retained by the use of higher temperatures and shorter times during heat processing. It is therefore possible to select particular time-temperature combinations from a TDT curve (all of which achieve the same degree of enzyme or microbial destruction), to optimise a process for nutrient retention or preservation of desirable sensory qualities. This concept forms the basis of individual quick blanching , high-temperature short-time (HTST) pasteurisation, ultrahigh-temperature sterilisation and HTST extrusion.