22 Continuous-Hlow heat processing N.J. Heppell, Oxford Brookes University 22.1 Introduction: definition of the process The continuous-flow heat process is a thermal heat-hold-cool process where the foodstuff to be treated is pumped in continuous flow through heat exchanger systems where it is heated to a desired temperature, held at that temperature for a pre-determined time, then cooled to around ambient temperature. After heat treatment, the product is then packaged in an appropriate manner. This process different from in-container processes, such as canning or retorting, in which the product is firstly packaged and sealed, and then heat treated. Different thermal processes can be applied, from thermisation and pasteuria tion through to full sterilisation depending on the temperature and holding time employed, and packaging method selected. Terminology is difficult; for pas- teurisation, the process is often called High Temperature Short Time(HTST for sterilisation, it may be called Ultra High Temperature (UHT) or aseptic pro- cessing. The advantages of continuous-flow heat processes over in-container processes are the much faster rates of heat transfer that can be attained, which means that higher process temperatures can be applied, usually around 140 to 150oC, as opposed to a maximum of around 125oC for in-container processes In addition, the slow heating and cooling rates in in-container processes can incur considerable thermal damage to the product before the desired process has been attained. A comparison between the time-temperature profiles for the two processes is given in Fig. 22.1 Disadvantages of the process in comparison with in-container heat treatments include the fact that the process is less inherently safe than in-container process- ing as there are more points of potential contamination. Adequate sterilisation of the packaging material is essential; co ion may occur during the packag-
22 Continuous-flow heat processing N. J. Heppell, Oxford Brookes University 22.1 Introduction: definition of the process The continuous-flow heat process is a thermal heat-hold-cool process where the foodstuff to be treated is pumped in continuous flow through heat exchanger systems where it is heated to a desired temperature, held at that temperature for a pre-determined time, then cooled to around ambient temperature. After heat treatment, the product is then packaged in an appropriate manner. This process is different from in-container processes, such as canning or retorting, in which the product is firstly packaged and sealed, and then heat treated. Different thermal processes can be applied, from thermisation and pasteurisation through to full sterilisation depending on the temperature and holding time employed, and packaging method selected. Terminology is difficult; for pasteurisation, the process is often called High Temperature Short Time (HTST); for sterilisation, it may be called Ultra High Temperature (UHT) or aseptic processing. The advantages of continuous-flow heat processes over in-container processes are the much faster rates of heat transfer that can be attained, which means that higher process temperatures can be applied, usually around 140 to 150°C, as opposed to a maximum of around 125°C for in-container processes. In addition, the slow heating and cooling rates in in-container processes can incur considerable thermal damage to the product before the desired process has been attained. A comparison between the time-temperature profiles for the two processes is given in Fig. 22.1. Disadvantages of the process in comparison with in-container heat treatments include the fact that the process is less inherently safe than in-container processing as there are more points of potential contamination. Adequate sterilisation of the packaging material is essential; contamination may occur during the packag-
Continuous-flow heat processing 463 140 UHT 8 n-container Fig. 22.1 Temperature profiles of in-container and continuous-flow heat (UHT) ing operation and all parts of process equipment after the holding section must be sterilised before processing commences. In addition, the product must be capable of being pumped and it is therefore limited to liquids: however, the product may be purely liquid or liquid with comminuted solids or fibres(e.g milks,creams, fruit juices, tomato puree) or a liquid which contains substantial solid particulates, such as soups, stews and cook-in sauces 22.2 Principles of thermal processing 22.2.1 Thermal degradation kinetics Both the thermal death of microorganisms and thermal degradation of biochemi- cal components of food have been found to obey first order chemical reaction kinetics(with a few exceptions). At constant temperature, therefore, the reaction rate is given by k N and _dC =kC [22.1] where N is number of organisms, C is concentration of biochemical, k is the reaction rate constant and t is time In thermal sterilisation technology, equation 22.1 can be rewritten as N C 10%and上=10 [22.2] where n and c are the number of microorganisms and concentration of bio- chemical respectively at time t, No and Co are the initial number and concentra- tion and d is the decimal reduction time. The decimal reduction time is the time
Continuous-flow heat processing 463 ing operation and all parts of process equipment after the holding section must be sterilised before processing commences. In addition, the product must be capable of being pumped and it is therefore limited to liquids: however, the product may be purely liquid or liquid with comminuted solids or fibres (e.g. milks, creams, fruit juices, tomato purée) or a liquid which contains substantial solid particulates, such as soups, stews and cook-in sauces. 22.2 Principles of thermal processing 22.2.1 Thermal degradation kinetics Both the thermal death of microorganisms and thermal degradation of biochemical components of food have been found to obey first order chemical reaction kinetics (with a few exceptions). At constant temperature, therefore, the reaction rate is given by [22.1] where N is number of organisms, C is concentration of biochemical, k is the reaction rate constant and t is time. In thermal sterilisation technology, equation 22.1 can be rewritten as: [22.2] where N and C are the number of microorganisms and concentration of biochemical respectively at time t, N0 and C0 are the initial number and concentration and D is the decimal reduction time. The decimal reduction time is the time N N and C C t D t D 0 0 = = 10 10 - - -= -= dN dt k.N and dC dt k.C In-container sterilisation UHT sterilisation 140 0 0 Time (min) 30 Product temperature (∞C) Fig. 22.1 Temperature profiles of in-container and continuous-flow heat (UHT) processes
464 The nutrition handbook for food processors taken to reduce the number of microorganisms or concentration of biochemical by a factor of 10 (i.e. to a value 1/10th of that initially ) This decimal reduction time is constant, 1. e. for any time interval D, there will be a reduction to one-tenth When quantifying the effect of temperature change on the D value, there are two major models used: the traditional ' Canning(constant-z) model and the Arrhenius model. The former is D,=106 22.3] where D, and D2 are the decimal reduction times at 0, and e2 respectively. The z value is the temperature change required to change the decimal reduction time by a factor of 10 The Arrhenius equation is: k=Ae-Ea/Rek [224 where A is a constant, the frequency factor, Ea is the activation energy, R is the gas constant and ek is the absolute temperature These two models are actually mutually exclusive but will agree within experi- mental error over a relatively short temperature range, which is usually the case for death of microorganisms. For larger temperature ranges, which is more usual for biochemical degradations, the Arrhenius relationship has a better theoretical basis and is generally used Research work which reports thermal death of different strains of micro- organisms or degradation of biochemical components usually gives D and z values at a defined reference temperature(often 121 1C)or the activation energy Ea and frequency factor, A. There are tables of data given in Holdsworth(1992, 1997)and Karmas and Harris(1988)for microorganisms and biochemical components 22.2. 2 Effect of change in temperature An examination of the kinetics for microbial death and for degradation of bi chemical components shows that the z values for the former are in the region of 10C, while for the latter they are around 30oC. This difference is the basis of UHT processing: by increasing the temperature of a foodstuff, the microbial death rate increases much faster than biochemical degradation and, for equal levels of sterilisation, higher temperatures will give a better nutritional and organoleptic quality food than lower temperatures. One major disadvantage of this is that some enzymes may survive, especially heat-resistant proteases and lipases. It is impor- tant, therefore, that as high a process temperature be attained as is feasible and this is usually in the region of 137 C to 147C. 22.3 Process equipment and product quality 22.3.1 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
464 The nutrition handbook for food processors taken to reduce the number of microorganisms or concentration of biochemical by a factor of 10 (i.e. to a value 1/10th of that initially). This decimal reduction time is constant, i.e. for any time interval D, there will be a reduction to one-tenth. When quantifying the effect of temperature change on the D value, there are two major models used: the traditional ‘Canning’ (constant-z) model and the Arrhenius model. The former is: [22.3] where D1 and D2 are the decimal reduction times at q1 and q2 respectively. The z value is the temperature change required to change the decimal reduction time by a factor of 10. The Arrhenius equation is: [22.4] where A is a constant, the frequency factor, Ea is the activation energy, R is the gas constant and qk is the absolute temperature. These two models are actually mutually exclusive but will agree within experimental error over a relatively short temperature range, which is usually the case for death of microorganisms. For larger temperature ranges, which is more usual for biochemical degradations, the Arrhenius relationship has a better theoretical basis and is generally used. Research work which reports thermal death of different strains of microorganisms or degradation of biochemical components usually gives D and z values at a defined reference temperature (often 121.1°C) or the activation energy Ea and frequency factor, A. There are tables of data given in Holdsworth (1992, 1997) and Karmas and Harris (1988) for microorganisms and biochemical components. 22.2.2 Effect of change in temperature An examination of the kinetics for microbial death and for degradation of biochemical components shows that the z values for the former are in the region of 10°C, while for the latter they are around 30°C. This difference is the basis of UHT processing: by increasing the temperature of a foodstuff, the microbial death rate increases much faster than biochemical degradation and, for equal levels of sterilisation, higher temperatures will give a better nutritional and organoleptic quality food than lower temperatures. One major disadvantage of this is that some enzymes may survive, especially heat-resistant proteases and lipases. It is important, therefore, that as high a process temperature be attained as is feasible and this is usually in the region of 137°C to 147°C. 22.3 Process equipment and product quality 22.3.1 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, k A.e E R a k = - q D D 1 z 2 10 2 1 = ( ) q q-
Continuous-flow heat processing 465 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, refrig- erated 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 advan- tages 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 way y 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 con- figuration 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 Steam injection, sometimes called steam-into-product, where steam is injected directly into the food through a steam injector nozzle and the steam 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 dis- dvantages 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%o extra water and can be handled in one of two ways: e process can be made more concen after cooling(using a conventional indirect heat exchanger )the final product is at the correct concentration
Continuous-flow heat processing 465 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 con- figuration 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
466 The nutrition handbook for food processors 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 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 tem- perature 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 heatin 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 to heated surfaces 2 The area available for heat transfer. i.e. the area of contact between the food- stuff and the heating or cooling medium. One important factor is the pre- ence 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 deter mined from physical measurements taken on the process. The temperature points are determine 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 resi- dence time in each section is determined by measuring the volume of process plant between these temperature points and calculating the time as:
466 The nutrition handbook for food processors 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: