UHT
|
UHT
|
|
|
The sterilization process is defined as a UHT (Ultra High Temperature) process if the product is heat-treated in a continuous flow at a temperature of not-less-than 135°C for a very short time, aseptically packaged in sterile containers, and has undergone minimal chemical, physical, and Organoleptic changes in relation to the severity of the heat treatment required for sterilization.
In other words, the product should have been subjected to a heat treatment having a sufficiently high lethal effect - so that, after incubation at 30°C ± 1°C for 5 days - no spoilage occurs and the changes in flavor, odor, color, and nutritional value are minimized. In addition to ensuring the destruction of micro-organisms, the heat treatment of milk also results in a number of other reactions and changes.
The main changes are:
Choosing the type of process and temperature-time combination best suited to the handling of a range of products of widely varying initial quality and composition, should be based on the bacteriological and physio-chemical changes referred to above. The question then arises how the effect of the heat treatment, in particular, can be measured and assessed. Understanding the various parameters will afford an insight into the manner in which a sterilization process should best be effected. During the course of time, the following terms have gained general acceptance in connection with food sterilization processes.
D-value or decimal reduction time The D-value, which denotes the decimal reduction time, is the time required at a specific temperature and under specified conditions to reduce a microbial population by one decimal. The decimal reduction time is dependent on the temperature, the type of micro-organism and the composition of the medium containing the micro-organism.
Z-value The Z-value is the increase or decrease in temperature required to reduce or increase the decimal reduction time by one decimal. It is a measure of the change in death-rate with a change in temperature.
Sterilizing effect or lethality The sterilizing effect, which is also called lethality or death-rate, indicates the effect of a heat treatment, expressed as the number of decimal reductions in the number of micro-organisms.
initial plate count ----------------------- = log number of spores in sterilized product. sterilizing effect
F-value or process value The F-value indicates the effect of a heat treatment, which is governed by the product heating temperature and the time during which the product is held at this temperature (product holding time). An exponential function expresses the effect's dependence on the temperature. The time and temperature factors govern the ultimate effect, it being understood that this effect is directly proportional to the time; triplication of the time at the relevant temperature triplicates the effect. The effect of the heating temperature, however, is considerably greater than that of time. The effect of the heat treatment can be expressed by the following equation:
As the effect of the heat treatment differs from micro-organism to micro-organism, and from quality attribute to quality attribute, it stands to reason that there are a great many different F-values, each having its own values for reference temperature To and Z-value.
What does the F-value represent? The F-value can be interpreted as the time required to obtain a given reduction in the number of micro-organisms at a temperature T = To. Given the effect of a heat treatment at a temperature To and at different treatment times, the F-value gives an idea of the effect of the heat treatment under review. Consequently, it follows that the value for To is a relatively random choice and that the F-value depends on this choice for To. An old-established F-value, commonly used in the canning industry, is the Fo-value, which is based on 12 decimal reductions for Clostridium botulicum. The reference temperature To was 250°F (121.1°C) and the Z-value was 18°F (10°C). The time was expressed in minutes, where 1 minute at 250°F signified a lethality of 12 for Clostridium botulicum. These parameters were subsequently adjusted to suit UHT processing applications, the reference temperature To being set at 138.9°C, the Z-value remaining unchanged at 18°F (10°C) and the time being expressed in seconds. The calculated Fo-value is identical for both methods, it being understood that the former is expressed in a given number of minutes at 121.1°C and the latter is expressed in a corresponding number of seconds at 138.9°C. Consequently, both methods give an identical lethality for Clostridium botulicum.
Fm-, Fe-, Fc- and Fsh-values Recent studies show that chemical and biochemical changes occurring in milk processed in UHT systems can also be expressed as functions of thermal destruction of micro-organisms. The various F-values developed and defined by Professor Reuter have minimum and maximum limits (expressed in seconds). The Fm-value was defined as the value for thermal destruction of micro-organisms, based on the destruction of the most heat resistant spore former occurring in milk, Bacillus Stearothermophilus.
Fm>15
The Fe-value was defined as the value for enzyme inactivation, based on a 90% inactivation of proteases from Pseudomonas in milk. The requisite Fe-value greatly depends on the enzyme content of the raw milk supplied. Raw milk having a high enzyme content (caused, for instance, by prolonged storage in refrigerated farm tanks) requires a more severe heat treatment at a higher Fe-value.
Fe>2.8
The Fc-value is the value for heat treatment-induced chemical change, based on the loss of vitamin B1 in whole milk.
Fc<70
The Fsh-value is the value for the formation of HMF (hydroxymethylfurfural). This is an intermediate product of the Maillard browning reaction. The HMF content depends on the severity of the heat treatment.
Fsh<33
B*- and C*-values The German researcher Horak has conducted an extensive study on the lethal effect of heat treatments on a mixed population of micro-organisms present in good-quality German milk and the effect of these heat treatments on product quality.
B*-value On the basis of this study, he suggests a B*-value for bacteriological effect, where the minimum B*-value required for a UHT process is 1.0. He bases this on the satisfactory destruction of the thermophilic spores present in a mixed population of micro-organisms occurring in milk and recommends that the thermophilic death time line (for B* = 1) be used with the thermal death value of 9, which will ensure a reduction of 109 in the number of spores. He found a Z-value of 10.4°C and a holding time t* of 10.1 seconds at a reference temperature T*o = 135°C.
However, if it is assumed that B* = 1, this reference temperature cannot be used in the equation, for otherwise, it would be found that B* = 10. Consequently, a reference temperature of 135 + 10.4 x log 10.1 = 145.4°C should be used in the equation.
B*>1
C*-value In his study, Horak used the following attributes to assess the quality of milk which had been subjected to UHT treatment:
Horak introduces the C*-value for chemical effect, where good-quality UHT milk should satisfy the following condition: C* < 1. The C*-value reflects the degree in which the quality of the milk has suffered from the heat treatment, i.e. the lower the C*-value, the better the quality. To this end, he bases this on a max. 3% loss of thiamine (vitamin B1), using a holding time t* of 30.5 seconds at a reference temperature T*o of 135°C and a Z-value of 31.4°C. Here, too, the T*o-value cannot be used as a reference temperature in the equation, for it would be found that C* = 30.5, where C* should be 1. So the reference temperature T*o used in the equation is equal to:
135 + 31.4 x log 30.5 = 181.6°C.
Consequently, the equation for the value for chemical effect, suggested by Horak, and the limits for T (°C) and t (sec) recommended by him, will be: (The diagram shows the temperature/time combinations for UHT processing and the limiting lines for the UHT region specified by Horak. The green area is the area where commonly in-container processing takes place.)
C*<1
|
Last modified: Monday, January 17, 2005 14:51:40 1999-2005 Copy right © ASEP-TECH USA LLC. ALL RIGHTS RESERVED. |
Inactivation of enzymes
