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Evaporated milk
Inhalt
In the past people used to pour hot and boiled milk into their coffee to soften the taste. Cold milk would have cooled the coffee down too much. Therefore manufacturers started to produce more concentrated milk, so it can be added to the coffee immediately. So evaporated milk has a thicker structure than ordinary milk and a higher fat content.
Evaporated milk is available in the varieties whole, semi skimmed and skimmed.
Production evaporated milk
There are two production processes: production with sterilisation of wrapped milk and the streaming sterilized production. The production of evaporated milk which is sterilized inside the wrapping will be described, followed by a short description of the other production process.In wrapping sterilized evaporated milk
Raw material
No sour or other metabolism products are allowed to be present in significant quantities in the raw milk which is used. The bacterial count must be low, in order for the milk to be sterilized easily. The milk may also not contain colostrum, as this liquid decreases the heat stability of the product.The fat and protein content of the raw milk is measured by means of infrared measurements.
Standardisation
According to the Food and Drugs Act, the evaporated milk has to have specific fat and dry matter content:- whole evaporated milk: at least 7,5% fat and at least 25% dry matter
- semi skimmed evaporated milk: between 4 and 4,5% fat and at least 24% dry matter
- skimmed evaporated milk: less than 1% fat and at least 20% dry matter
Due to these demands, the milk has to be standardized initially.
While during the evaporation process only water is removed from the product, the milk already needs to have the desired proportion between the fat content and the fat free dry matter content. Standardisation takes place by adding separated milk or sweet buttermilk or whey to the whole raw milk, in order to obtain the desired ratio.
Preheating
After the standardisation, the milk is preheated very intensely for 1 to 3 minutes at 100 to 120°C in a plate or tube heat exchanger to:- kill pathogenic micro-organisms and a large part of the heat resistant micro-organisms and endospores;
- inactivate enzymes;
- increase the heat stability of the evaporated milk;
- decrease concentrating of the evaporated milk during storage;
- increase the temperature of the milk for the first step of the concentration installation.
The heat stability is increased, due to the precipitation of the heat unstable serum protein onto the casein protein. Now the causeinmicels are coated with the serum protein, they will not coagulate during sterilisation. Also the amount of dissolved calcium and phosphate decreases due to the preheating, which has a positive effect on the heat stability.
After the preheating the milk is whiter, because the serum proteins have denatured during the heating and therefore scatter the light more.
After the preheating the milk is cooled down to approximately 70°C.
Concentrating
The milk is concentrated to a dry matter content of approximately 31% by means of a vacuum falling film evaporator or a circulation evaporator. Concentrating the milk too much gives a lower output and lowers the heat stability of the concentrated milk. By using vacuum the boiling point of the milk is lowered (65 to 70°C), so the chance for developing a boiling taste is minimal and burning the milk is prevented.Homogenizing
To prevent creaming of the evaporated milk during storage for a longer period of time at a higher temperature, the concentrated milk is subsequently homogenized by means of a two-stage high-pressure homogeniser at a temperature of 60 to 65 °C and at a pressure between 125 to 250 bar.The viscosity slightly increases due to the homogenizing and the heat stability decreases. The milk may therefore not be homogenized too intensely.
The homogenizing increases the whitening strength of the evaporated milk in the coffee.
Cooling
After the homogenizing, the concentrated milk is cooled to approximately 8 °C by means of a heat exchanger. Then the dry matter content is checked and can be standardized again with water, if necessary. However, one must then take into account that by adding the 10% solution of stabilisation salt, the milk is being thinned again.Afterwards, the whole is temporarily stored in isolated tanks, until the right amount of stabilisation salt is determined.
Stabilizing
To ensure that the concentrated, homogenized evaporated milk will not coagulate during the sterilizing, a stabilisation salt (usually Na2HPO4) is added. The stabilisation salt is alkaline and therefore raises the pH of the evaporated milk. The salt also binds a part of the present Ca2+, causing the casein protein to get a more negative, deflecting charge. Both changes increase the heat stability.As each lot of milk can differ, small amounts are generally first used for a number of test sterilisations, to determine the right amount of stabilisation salt.
One can also choose to add a vitamin to the evaporated milk.
Packaging
After adding the stabilisation salt, the evaporated milk is put in glass bottles or tinned cans. These cans have an internal varnish, to prevent iron or tin from eventually ending up in the product.Before these cans are seamed, a steamflow is applied in the headspace, to restrict the overpressure during the sterilisation.
Sterilizing
The cans and bottles are then, while rotating, sterilized in a batch autoclave or in a continuous steriliser for 15 to 20 minutes at 110 to 120 °C. When the desired sterilizing effect is obtained, the products are cooled to storage temperature.Due to Maillard reactions (caramelization) the evaporated milk gets a more brown colour. This colour can be influenced by using a different temperature and time combination. Sterilizing also increases the viscosity of the evaporated milk.
Labeling
The wrappings are labeled and packed in transport units. If the evaporated milk is stored at a temperature between 0 and 15 °C, it can be preserved for a long period of time.Streaming sterilised evaporated milk.
Until the homogenizing, the production process of streaming sterilized evaporated milk is the same. As this evaporated milk is sterilized UHT, the homogenizing does not have to be as intense. The homogenizing is only applied to prevent creaming during the temporary storage. The evaporated milk is then cooled to 5 to 8°C and stored temporarily. The homogenizing can even be skipped, if the entire process is completed without storage in between. This latter method is also preferred, because temporary cold storage increases the chance of concentration afterwards.The evaporated milk is sterilized UHT for 4 seconds to 8 minutes at 122 to 140 °C by means of a tube heat exchanger. In general, it is not necessary to add stabilizing salt to the evaporated milk.
After the sterilizing, the milk is aseptically homogenized to reduce possibly formed protein aggregates. When the mass is cooled, various wrappings can be filled aseptically with the evaporated milk. The evaporated milk can then be stored for 6 to 9 months at room temperature.
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