FAT RICH DAIRY PRODUCTS

Module 3. Processing of cream

Lesson 8
PASTEURIZATION OF CREAM

8.1 Introduction

Heat treatment of cream is necessary to destroy organisms that may be pathogenic or cause spoilage and inactivate enzymes. Lactic acid produced by bacteria, causes souring and coagulation of cream. Protelytic enzymes may produce bitter peptides and also cause coagulation. Lipolytic enzymes will break down the lipids to produce free fatty acids which give a rancid flavor. Cream can be pasteurized by conventional means and the principles of heat treatment of milk can be applied to cream. Cream is more viscous and somewhat more susceptible to mechanical break down, and this should be borne in mind when choosing equipment for cream pasteurization. For example positive pumps would be preferred to centrifugal pumps. Whereas 72°C for 15 s might be a typical regime for pasteurizing milk, a higher temperature about 80°C would generally be used for pasteurizing cream. A higher temperature than 80°C may impair cream quality, possibly through activation of bacterial spores.

Pasteurization of cream refers to the process of heating every particle of cream to not less than 70°C for 20 min or 80°C/25 s.

8.2 Objective

The objectives of pasteurization of cream are:

* To destroy the pathogenic organisms in cream

* To destroy the undesirable micro organisms and inactivate the enzymes

* To complete the neutralization process in the manufacture of butter

* To eliminate some of the gaseous tainting substances

* To make possible the removal of some volatile off flavors by vacreation

8.3 Methods of Pasteurization

8.3.1 Batch pasteurization

It is a satisfactory and economical method for small scale operations, but is not practicable for larger scale operation. The cream is heated to 70°C for 20 minutes and then promptly cooled. An increase in temperature may reduce the time say, 25 s at 80°C. The vat pasteurizers in commercial use are of two general types, namely, jacketed vats with mechanical agitators and vats without jacket that are equipped with revolving or oscillating coil. In the jacketed vat the jacket contains the heating and cooling element. The hot water, cold water, or brine, respectively, is sprayed against the jacket side of the heating surface. The cream is agitated by a series of blades moving to and fro length wise or in the case of round tanks by a vertical rotating wing agitator, or by one or more impellers.

8.3.2 High temperature short time pasteurization

The HTST is done with the help of plate heat exchangers. The plate heat exchanger is made up of a series of thin metal plates that are held tightly together in a press. The plates are grooved (corrugated) either on one side or on both sides; in the latter case the grooved plates alternate with flat plates. The grooves serve as canals through which the cream flows. This pasteurizer is built for high temperature flash heating, or limited retarding or holding pasteurization, with regenerative and cooling units. The heating medium and the cold water and brine travel in counter current to the flow of cream. The depth of the grooves in the plates determines the thickness of the layer of cream between the plates. The grooves are shallow and even deep. They therefore, ensure a uniformly thin layer of cream. This fact together with the thinness of the metal plates and the high velocity of the heating medium ensures rapid and uniform heat exchange and makes possible the use of hot water at a temperature only a few degrees higher than the desired pasteurizing temperature of the cream. It also practically eliminates the tendency of scorching and of cooked flavor. The plate heat exchanger is flexible to alter the capacity. For increasing its capacity more sections of plates are added. Generally plate type pasteurizers are used with regenerative system. This is a continuous process and most suitable for large scale operation. The plate type pasteurizers may have some problem with some acidic/neutralized cream as this forms burnt-on-films more easily on the plate while it is most suitable for freshly separated sweet cream. In this system, the maximum heating temperature may be 95-100°C for 15 seconds. A temperature of 82.2 to 85°C / xxxxxx time is most common.

8.3.4 Vacuum pasteurization

The pasteurization of cream by this system, using the principle of reduced atmospheric pressure has advanced rapidly in recent years. In some countries it is replaces all other machines. Vacuum pasteurizers are available as vacreator or volatilizer.

8.3.4.1 Vacreator

Pasture feeding of animals can produce flavour taints through herbage derived substances dissolved in the fat. As most of the tainting substances are relatively volatile, a process was devised in New Zealand both to pasteurise the cream and to remove the volatiles through what is essentially a steam distillation process. The piece of equipment is known as a Vacreator, which was the trade name adopted for the Murray Vacuum Pasteuriser (present manufacturers and agents NDA Engineering Group, Auckland, New Zealand). The process is known as vacreation. Vacreation has been used in a number of countries, and not only it improves the flavour of creamery butter, but also extends the shelf-life significantly when compared with butter derived from plate-pasteurised cream. In the Vacreator, steam is intimately mixed with cream and the condensed vapour plus volatiles are removed by flash evaporation under vacuum. Figure 8.1 shows a diagram of a Vacreator consisting of five vessels. The typical pressure and temperature conditions pertaining to each vessel are shown on the diagram. Raw cream is preheated in a tubular heat exchanger by vapours exiting from vacuum vessels 3 and 4. The cream is mixed with steam and vapours. It exits from vessel 1 and passes into vessel 3, where the pressure is reduced slightly and the cream and vapours are separated. The cream is then mixed with steam and vapour, exits from vessel 2 and the mix is passed into vessel 4 for separation. The vapours from vessels 3 and 4 are combined and passed through the preheater, before passing to a water jet condenser which provides vacuum and condenses the remaining condensable vapours. A spring-loaded baffle valve applies a back pressure to vapours from vessel 3, so that the pressure difference required to transfer cream between vessels is maintained.

Fig. 8.1 Schematic diagram of a Vacreator ®

The cream from vacuum vessel 4 passes into an internal cream pump and is pumped to vessel 1, where it meets fresh incoming steam. The cream separated in vessel 1 is mixed with fresh steam again before passing into vessel 2. The cream exiting from vessel 2 passes into vessel 5 which acts purely as a flash cooler, with the vacuum removing water vapour and the associated latent heat. The cream exits at approximately 55-60°C, and the vapours are removed in the condenser. The somewhat complicated system is called 'weaving flow', and is essentially counter-current with the cleanest cream meeting the cleanest steam.

The somewhat complex nature of flows has been found to be necessary because of the tendency of cream to foam under vacuum, so that the separation of the liquid and vapours becomes difficult. The liquid and 'vapours are separated via a cyclonic centrifugal action with the cream being fed tangentially into each vacuum vessel at a slight down-wards angle. The vapours are removed through a centrally mounted pipe. If foaming is severe, liquid gets carried over with the vapour stream resulting in product loss. The flow of steam assists in providing the cream with sufficient kinetic energy to flow through the system, but loss of energy occurs in the separation process which necessitates the use of the internal cream pump to push the cream to the final two stages. The flow from vessel to vessel is also controlled through the pressure differential between the vessels, but the high operating temperatures (and thus low vacuums) mean that transfer of cream by pressure differences is limited to two vessels in series. The vacuum levels, temperatures and flows of cream and steam thus require very careful control to ensure that excessive product loss through foaming or flooding does not occur. The modern Vacreator is now equipped with microprocessor control to assist in achieving optimum temperature conditions during operation.

The amount of taint removed is proportional to the quantity of steam used. In the spring or during prolonged periods of wet weather, feed growth and the proportion of green feed in the diet results in increased levels of taints, and consequently high steam flows during vacreation are required, typically 0.25-0.3 kg steam/kg cream. During drier parts of the season, less green feeds are consumed so that less steam is required, typically 0.18 kg steam/kg cream, to remove the lower levels of taint. Taints resulting from poor quality cream may also be removed by vacreation, but high steam flows are required.

The major disadvantage of vacreation is its energy usage through the relatively large quantities of steam required, as the design of the Vacreator is such that vessel flooding will prevent operation at steam flows less than 0.15 kg steam/kg cream. Some heat is recovered through the preheater. Also available are thermorecompressors, which will generate low pressure steam from waste heat recovered from the vapours. The low pressure steam can be fed to the raw steam entering the Vacreator, but the cost of thermorecompressors warrants their use only with large, high-throughput units. The high energy usage of the Vacreator has led to some companies investigating flash-pasteurisation which incorporates a limited vacuum treatment. Such a process is acceptable for treating cream with a low taint level, but is generally unsuitable for cream with a high taint level if the cream is to be used for producing butter. Further experimental work in New Zealand has indicated that steam: cream ratios can be reduced if the proportion of steam entering vessels 1 and 2 is more carefully controlled. Control is by a valve that limits the quantity of steam passing to vessel 2, which is at lower pressure. With such control, the differential pressure between the two vessels can be maintained, and flooding of vessel 1 is eliminated even at steam flows as low as 0.09 kg steam/ kg cream

Steam quality is of utmost importance in Vacreator treatment. It must be of culinary standard and filtered because of the intimate contact with the cream. This limits the use of certain chemicals for the treatment of boiler feed water.

Steam is injected into cream during vacreation at a velocity of approximately 140 m s-1(500 km h-1), and this violent treatment causes disruption of fat globules with an increase in the proportion of fat present as small globules (<2 µm). Vacreation will also increase the number of large fat globules (> 10 µm) due to agglomeration resulting from foaming or flash-boiling. The increase in the number of small fat globules can lead to higher losses of fat in buttermilk. The introduction of low velocity steam diffusers alleviates this, and is possible when the steam split into vessels 1 and 2 is carefully controlled.

Certain models of Vacreator have the water ejector condenser replaced by a plate heat exchanger surface condenser and liquid-ring mechanical vacuum pump. Cream throughput of these models has been increased from maximum of 10,000 kg h-1 to 18,000 kg h-1. The four primary vacuum vessels lie in a line with the pre-heater lying horizontally just underneath and to the rear of these. The flash vacuum vessel and condenser are situated behind the pre-heater.

8.3.4.2 Volatilizer

It consists of a steam injection flash pasteurizer and high vacuum retort (vacuum pan). Steam enters the pasteurizer (Stainless steel) at the top through direct steam jets. Cream is uniformly heated here to 85°C by direct steam. This heated cream is discharged from the pasteurizer through float controlled valve to a high vacuum chamber (which is made of glass-coated steel) at the top by vacuum discharge tubes. Its release into high vacuum pan causes it to form a fine mist which impinges against the sides of the pan and flows down over the glass coated surface in a thin film. The escaping water vapours and volatilized products are voided through the condenser, and the high vacuum (27-20 inches) lowers the temperature of the cream to about 38°C. The treated cream discharges from the vacuum pan at the bottom.