UHT Treatment

Field of the invention

The invention relates to a method of UHT treatment of liquid foods.

Background prior art

Many food products require Ultra-high temperature processing (or UHT) to kill micro-organisms in the food. UHT is the (partial) sterilization of food by heating it for a short time, at a temperature that kills micro-organisms. The most common UHT product is milk, but the process is also used for fruit juices, cream, yoghurt, wine, soups, and stews.

The high temperature is necessary to kill micro-organism. However, the high temperature also influences the quality of the product. For instance, it affects the taste of the food product which is often referred to as 'cooked taste'. For example, UHT milk tastes different from fresh, pasteurized milk. The cooked taste is often not appreciated by consumers.

Currently, UHT plants operate at a constant flow-rate and constant temperature. These are optimized for each product for a specified production rate, e.g. 10 tons per hour. The flow rate determines the production rate and also determines the time a product is heated, because of the time the product is present in the holding tubes (see figure 1 ). If the production rate needs to change, e.g. from 12 tons per hour to 16 tons per hour, the flow rate has to increase. However, if the flow rate increases the time the product is heated is shortened because the residence time of the product in the holding tube is shorter. This jeopardizes the microbiologically safety as maybe not all micro-organisms are killed. If the production rate needs to go down, e.g. from 16 tons per hour to 12 tons per hour, the flow rate has to go down which also gives problems. At a lower flow rate the time the product is heated is increased because the residence time of

the product in the holding tube is longer and the product might lose quality because of the longer heat treatment. Therefore, to maintain a good microbiological safety and product quality the holding tubes are changed, they become larger or smaller. For this, the line needs to be stopped, a different holding tube needs to be inserted and the line needs to be restarted. This process causes loss of valuable time and loss of product.

It is therefore an object of the present invention to provide a simple method for changing the production rate in an UHT line wherein the tubes need not to be changed and still have an acceptable microbiological safety and product quality. Furthermore, an object of the present invention is to provide a method that is capable of variable flow-rates. A further object of the present invention is to minimize product degradation. Another object of the present invention is to provide a method that increases the efficiency of the UHT process. Additionally an object of the present invention is to provide a process that supplies good microbiological stable products.

Summary of the invention

It was surprisingly found that one or more of the objects mentioned above are attained when flow-rates are varied to change the production rate, but the holding tubes are kept constant, and the F value is kept constant by varying the temperature of the line. Surprisingly the quality of a product does not change if the F value is kept constant.

Detailed description of the invention

To indicate microbiological safety an F value also known as cumulative lethality is used. F is the time (t) a food product needs to be kept at a certain temperature (T) to have a desired inactivation of a certain micro-organism. F is calculated as the summation of the lethal rate (LR) according to the following formula:

F=J LR dt = J 10 ((T-T _R )/z) dt

LR is a measure of the lethal effect of a heat treatment and is the rate of microbial reduction at the used temperature (T) relative to a reference temperature (T _R ). z is the temperature change required to affect a 10-fold change in the rate of microbial destruction.

Dividing the F value by the D _R value yields the microbial log reduction: Log (Initial microbial population / Final microbial population) = F / D _R

D _R (decimal reduction time) is the time at a given temperature to destroy 90% of the microbial population (i.e. a ten fold reduction in microbial population).

For each micro-organism T _R , z and D _R are specified. As the environment the micro-organism is in may have an impact on the growth of the micro-organism the values T _R , z and D _R are usually specified for specific conditions, like for example the food product properties (e.g. pH value and fat content). These values can be found in common handbooks.

As a common safety standard for sterilized products the 12 log inactivation at T = 121.1 ⁰ C of the micro-organism proteolytic C. botulinum cook is taken. Given the specific T _R , z and D _R of this organism this requires that the food product is kept at 121.1 ⁰ C for 2.5 minutes. Thus, commercial sterility of a product is achieved by holding the product for at least 2.5 min at 121.1 ⁰ C.

For proteolytic C. botulinum cook the following values are known: T _R = 121.1 ⁰ C z = 10 °C

D _R = 0.2 min

The Cooked value is a quantitative measure of the effect of heat on quality factors, also referred to as C value. The calculation of the C value is similar to the calculation of the F value. Each quality aspect has different values, for example:

T _R = I OO ⁰ C z = 30 ⁰ C

Thus

C= J i0 ((T-100)/30) dt

The higher the C-value the more 'cooked' the product tastes.

The present invention is directed to a method of UHT treatment of liquid foods susceptible to thermal damage, wherein the flow-rate is variable, wherein the F value is kept constant between 2.5 minutes and 3.75 minutes by adjusting the temperature during operation. Preferably the F value is kept constant between 2.5 minutes and 3.25 minutes, more preferably between 2.5 minutes and 3 minutes and most preferably between 2.5 minutes and 2.75 minutes.

The present invention allows an UHT line wherein the flow rate can be varied without stopping the production line. The temperature is adjusted throughout the process to maintain a F value between 2.5 minutes and 3.75 minutes.

Preferably a feed-back mechanism for the temperature and an unit that monitors the F value is introduced in the UHT-line such that the UHT treatment proceeds automatically.

Liquid foods that are suitably sterilized by the method of the present invention are selected from the group comprising dairy products and non-dairy variant of milk, creme fraiche, whipped cream, cooked cream, desert, yoghurt, and soup, sauces, fruit, fruit juices, and jam.

Examples

Take an UHT line with a nominal capacity of 12 tons / hour and the following product data:

Product data:

The UHT line has been designed around the following data:

Design data: operating temperature set point: 131 ⁰ C; residence time 15.3 seconds.

Therefore at nominal capacity, the product is heated up to 131 ⁰ C and kept at this temperature for 15.3 seconds, which results in the desired minimum F value of 2.5 minutes. A measure for the quality loss is the cook value or, C (the lower the loss value, the better), which is C = 2.2.

Reducing the flow rate to 4 tons / hour at the same temperature will lead to the following: F = 7.5 min and C = 6.7. This results in a significant quality loss of

the product. Reducing the wall temperature to 126.2 ⁰ C will bring the values to F = 2.5 minutes and C = 4.8.

Increasing the flow rate to 16 tons / hour at the same temperature will lead to the following: F = 1.9 minutes and C = 1.7. This results in an unsafe product as the F value is below the minimum value. Increasing the wall temperature to 132.2 ⁰ C will bring the values to F = 2.5 minutes and C = 1.8.

Therefore the wall temperature needs to be adjusted (depending on the flow) to obtain a constant F (prevention of loss of safety) and a minimum C (prevention of overcooking).