The food products covered now include soups, sauces, fruit juices, and other beverages, in addition to milk and milk products. Pasteurization, sterilization, and aseptic processing are all discussed, with emphasis on the underlying principles and problems of heat treatment of more viscous fluids, where streamline flow conditions are likely to prevail, and of products containing particles. Pasteurization and heat treatments designed to further extend the shelf life of pasteurized products are also discussed, and the pasteurization and sterilization processes are compared to highlight similarities and differences. Throughout, factors influencing the safety and quality of heated foods are emphasized. This book contains over illustrations and 50 tables, as well as extensive cross-referencing and a comprehensive reference section.
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Note the values above must be recorded at the same temperature as v, the velocity of flow. A value of is not particularly high and higher values are recommended. The higher the value, the less difference there will be between the average particle velocity and the highest particle velocity. Also there will be a more uniform particle velocity distribution.
This approach is fairly crude and can be improved by determining the minimum residence time by injecting a suitable tracer into the flow. This can be challenging to do in practice Burton, Following this determination the length of the holding tube can be increased or the temperature raised, which is usually easier to do, if required.
A free calculator for calculating holding tube length is available on the DFST website. Determination of Fo values has been described previously, particularly in relation to canning. While F0 calculation is less relevant in UHT processing since the target organisms are thermophilic sporeformers that are more heat resistant than Cl. As mentioned earlier there may also be a regulatory requirement to do this in some countries. Fo is calculated by determining the lethality L at appropriate points from the time vs lethality curve and integrating the time and lethality values using numerical integration to obtain the area under the curve.
L is calculated using equation 2. Equation 2. The minimum F0 value recommended for UHT treatment of good quality milk has been reported as 5 — 6 Bylund, This criterion was introduced by Horak and Kessler around reviewed by Burton, Kessler and Horak found that a heat treatment of The spores had a Z value of This well-known criterion is used to gauge the adverse effects of heat on milk.
While it predicts the destruction of thiamine, it is used as a general measure of the cumulative detrimental effects of heat. Equation 4. Importance of monitoring temperature during UHT processing It is the cumulative heat treatment that milk and other products receive during processing that determines the overall effectiveness of a heat treatment process.
Lethal effects and chemical changes occur during any pre-processing or conditioning period used, come-up or heat-up time to the holding temperature, during holding and during cooling. Recent work by Tran et al. Using furosine, hydroxymethylfurfural and lactulose formation as Temperature Time Integrators TTIs in UHT-treated milk The main chemical changes occurring upon UHT-processing of milk are protein denaturation, the Maillard reaction and lactose isomerization.
While a range of compounds have been studied as potential TTIs to allow the discrimination of pasteurised, UHT-treated and sterilised milk and temperature-abused UHT-treated milk, furosine, hydroxymethylfurfural and lactulose have received particular attention. The Maillard reaction is a chemical reaction between amino acids and reducing sugars that gives foods e. It is the reaction of reducing sugars with lysine residues in milk proteins that is mainly responsible for the reduction in nutritional value of proteins in severely heated milk products.
The presence of high concentrations suggests that severe heating has occurred. There are concerns that changes in the concentration of this compound during storage may complicate its use as a TTI. It is formed by a process called epimerization. Epimerization of lactose in milk is a heat-induced process that causes a molecular rearrangement of lactose into a compound with the same molecular weight. Lactulose is more soluble than lactose and significantly sweetener.
The rate of formation is dependent on pH and the time and temperature of the heat process. More lactulose is formed as the pH increases. It has been known for several decades that the concentration of lactulose in milk can be used as an indicator of severe heat treatment.
As discussed previously, Andrews and Morant found that the flavour acceptability of UHT milk over a wide range of heat treatment processes was closely correlated with the concentration of lactulose The IDF IDF, has proposed that the lactulose content of UHT-treated milk can be used to differentiate it from sterilised milk.
There have been many studies of the levels of lactulose in market milk. The partial results of a Belgium study are shown in Table 2. These results indicate the differences between direct and indirect-UHT treatment and sterilisation. As expected the data for high pasteurization and thermisation show only low concentrations of lactulose. While some workers have reported a small increase in lactulose concentration during storage of UHT milk, this increase is normally small and in many cases it is insignificant.
The formation of furosine is known to be significantly dependant on protein concentration. Claeys et al. The furosine content of UHT-milk has been widely studied particularly in Europe. Table 2. Concentration of lactulose and fursosine in market milk in Belgium Parameter.
Continuous thermal processing of foods : pasteurization and UHT sterilization
Note the values above must be recorded at the same temperature as v, the velocity of flow. A value of is not particularly high and higher values are recommended. The higher the value, the less difference there will be between the average particle velocity and the highest particle velocity. Also there will be a more uniform particle velocity distribution. This approach is fairly crude and can be improved by determining the minimum residence time by injecting a suitable tracer into the flow. This can be challenging to do in practice Burton, Following this determination the length of the holding tube can be increased or the temperature raised, which is usually easier to do, if required.