Process equipment and product quality Selection of heat exchangers

The selection of heat exchangers for continuous flow heat processes is dependent on the physical properties of the food to be processed, especially its viscosity, the presence of solid particulates or fibres and any tendency of the foodstuff to 'burn-on'.

Heating may be indirect, where the heating medium is kept separate from the product, or direct, where the heat transfer medium is mixed with the food product (almost exclusively steam). For indirect heat exchangers, the heating medium may be steam or pressurised hot water and cooling may be by mains water, refrigerated brine or liquid refrigerant as part of a refrigeration cycle. The process plant is based on corrugated plate, plain or corrugated tube or scraped-surface heat exchangers, a description of which, as well as applications and relative advantages and disadvantages, is given in many standard food engineering texts. Where possible, a heat recovery section (often called a regeneration section) is incorporated into the process to minimise its energy requirements. In this way, heat recovery of up to 80-90% of the total requirement may be achieved. For detailed process arrangements, see Lewis and Heppell (2000).

The rate of heating and cooling of the food product within the equipment is important, as can be seen later, and is maximised in this type of equipment in three ways:

1 By increased turbulence in the food, minimising the liquid-heating surface boundary layer and therefore increasing heat transfer. This is usually achieved by corrugating the heat transfer surface to form a convoluted channel configuration for the liquid to flow down.

2 By increasing the ratio of heating area to liquid hold-up in the equipment, i.e. by decreasing the size of the product channel.

3 By increasing the temperature difference between the food and the heating or cooling medium.

Of these, the first two can be easily accommodated, but the last often cannot be used as many food products are heat-sensitive and will increasingly form a deposit on the heating surface, increasing heat resistance and reducing heat transfer. Direct heating takes two forms:

1 Steam injection, sometimes called steam-into-product, where steam is injected directly into the food through a steam injector nozzle and the steam bubbles condense.

2 Steam infusion, sometimes called product-into-steam, where the product is pumped into a pressurised steam chamber, forming a liquid curtain onto which the steam condenses.

Direct heating is the most rapid heating method but suffers from the disadvantages that the process is noisy and the steam must be of a culinary grade, using only permitted boiler feed water additives. In addition, during heating the condensed steam dilutes the product by adding about 10% extra water and can be handled in one of two ways:

1 The recipe for the feed to the process can be made more concentrated, so that after cooling (using a conventional indirect heat exchanger) the final product is at the correct concentration.

2 An equivalent volume of water is removed from the final product to bring the concentration back to the original value, e.g. for milk, where adulteration is illegal.

The latter may be achieved using a 'flash-cooling' process, where the foodstuff passes from the holding tube through a restriction into a cyclone under vacuum. The sudden decrease in pressure causes the excess water to vaporise (or 'flash') and simultaneously cools the product. By altering the level of vacuum in the cyclone, the same concentration of solids as in the inlet can be achieved, in the outlet stream. One disadvantage of this process, however, is that the large temperature drop in flash cooling means that heat recovery is much lower than for indirect-heating systems and the process is more expensive to operate.

22.3.2 Effect of rate of heating on product quality

The rate of heating and cooling of the foodstuff as it passes through the process gives a measurable effect on the nutritional and organoleptic quality of the product. Direct-heating systems, both injection and infusion, give the fastest rate of heating and flash evaporation gives the fastest cooling rate; both are virtually instantaneous. For indirect heating systems, on the other hand, the rate of heating is controlled by several factors:

1 The temperature difference between the foodstuff and the heating medium. The difference is limited by the heat sensitivity of the foodstuff, especially its tendency to 'burn-on' to heated surfaces.

2 The area available for heat transfer, i.e. the area of contact between the foodstuff and the heating or cooling medium. One important factor is the presence of solid particulates in the foodstuff. The channel size through the equipment must be a minimum of three times the particulate size, which reduces the ratio of heat transfer area to liquid volume in the process, severely reducing the rate of heat transfer.

3 The heat transfer coefficients either side of the heat exchanger wall. These are controlled by both the turbulence in the foodstuff or heating/cooling medium, and their thermal conductivities. In addition, the physical state of the heating medium (whether liquid or condensing steam) is important.

4 The heat recovery section. The greater the heat recovery, the cheaper the system is to operate but the larger physical size of process plant means the rate of heating is much slower.

The time-temperature profile of a continuous-flow heat process can be determined from physical measurements taken on the process. The temperature points are determined by sensing the temperature at key points in the process, e.g. at the inlet and outlet points of each heat exchanger and any holding sections. The residence time in each section is determined by measuring the volume of process plant between these temperature points and calculating the time as:

Direct heating system

Direct heating system

Fig. 22.2 Time-temperature profiles for direct heating, indirect heating, with low regeneration and indirect heating with high regeneration processes.

Fig. 22.2 Time-temperature profiles for direct heating, indirect heating, with low regeneration and indirect heating with high regeneration processes.

From this, a time-temperature profile similar to that obtained for heat penetration into a container can be constructed and used in the same way to calculate F0 values, chemical changes etc. Typical time-temperature profiles for direct and indirect-heating processes are given in Fig. 22.2. For a further explanation of this area, the reader is directed to work by Reuter (1982) and Kessler and Horak (1981a, 1981b), who collected time-temperature profiles for a wide variety of milk UHT plants and calculated sterilisation and biochemical changes expected.

22.3.3 Effect of residence time distribution on product quality

Not all elements of the fluid food will move through the equipment at the same rate; generally, the fluid near the wall will move more slowly and that in the centre of the flow channel will move more quickly than the average flowrate. This has implications for the sterilisation of the product in that, in the holding tube, the residence time at the process temperature is lower than expected and therefore either the product will not be sterile or the holding time must be increased to compensate. This must necessarily mean that all slower-moving foodstuff will be over-processed with concomitant thermal degradation. The greater the spread of residence times, the worse the loss of thermosensitive nutrients.

The most important factor in the residence time distribution is the flow regime in the liquid; either mean residence time =

volume of section

volumetric flowrate of product through it

• Streamline flow (for slow flowrates or high viscosity fluids) where the fastest element of fluid has a velocity twice that of the average velocity.

• Turbulent flow (for higher velocities or low viscosity fluids) where the fastest element of fluid is about 1.3 times faster than the average velocity.

For further information relating to prediction and measurement of residence time distribution and its effect on sterilisation and quality, see Lewis and Heppell (2000).

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