FAT RICH DAIRY PRODUCTS

Module 3. Processing of cream

Lesson 9
COOLING OF CREAM

9.1 Introduction

The primary purpose of cooling of cream is to minimize the growth of microorganisms and bring about the desired fat crystallization required in butter making. With the advent of more efficient heat exchangers, plate cooling of cream is common. Relatively higher viscosity of cream as compared to milk necessitates particular attention in the design of plate heat exchanger.

9.2 Methods of Cooling Pasteurized Cream

9.2.1 Farm-scale cooling

Cooling of cream can be accomplished by using tank type batch cooler or surface cooler, when the scale of operation is small as in case of farm cream production. For industrial purposes, surface coolers can be used but plate coolers and tubular coolers are commonly used. For small quantities of cream, use of ice water makes it possible to achieve rapid cooling to a sufficiently low temperature.

It is economical to provide a simple cream cooling tank, preferably insulated, in which the cream can is immersed in cool water. The ideal cream cooling tank is large enough to hold, sufficient body of water to avoid rapid warming up. It is sufficiently insulated to hold the temperature within a few degrees for eight to twelve hours. It is deep enough to allow the water to cover the cans at least as far up as the cream will reach when the cans are full.

9.2.2 Regenerative heater coolers

Under commercial conditions, cream is cooled as part of HTST pasteurization process, where regenerative cooling is followed by final cooling by means of chilled water or other cooling media. The regenerative heater coolers operating on the plate-based or tube-plate based principles of reciprocating heat exchanges between the hot outgoing and the cold incoming cream. They are in reality preheaters and pre-coolers, since they neither heat nor cool to the desired final temperature.

When using a combination of intra-tube heater and surface cooler, the hot cream flows up through the inside of the pipe unit and the cold cream flows down over the outside of the pipe unit. The incoming cold cream is thus heated by the hot cream coming from the pasteurizer, and the out flowing hot cream is cooled by the cold, raw cream flowing to the pasteurizer. Under average conditions the temperature of the cold cream is thus raised by about 12-15°C by the hot cream, and the temperature of the hot cream is lowered approximately 12-15°C by the cold cream, thus accomplishing a considerable saving of heat energy.

9.2.3 Surface cooler

Under average plant operating conditions, the surface cooler appears highly suitable for the cooling of flash-pasteurized cream. Surface cooler comprises two corrugated/undulated SS plates providing horizontal passages for cooling water or liquid refrigerant between the plates. The cream allowed to flow from top reservoir over the outside of the plates gets cooled. In order to avoid inconvenient height for operation and cleaning, the surface cooler is usually confined to two sections, each arranged with independent in-take and out-flow for the refrigerant. The upper section is cooled with water and lower section with brine or direct expansion ammonia.

The surface cooler has the added advantages of aerating the cream. This is especially desirable where steam jet pasteurization is practiced for the purpose of volatilizing objectionable flavors, thus facilitating escape of the volatilized products from the cream that is flowing in a thin film over the outside of the surface cooler. The surface cooler also permits some evaporation of moisture, which is desirable in the case of cream that has suffered dilution with steam condensate, as the result of steam injection pasteurization.

One of the drawbacks of the surface cooler is that, it “breaks” the closed system of the cream flow. Unless the cooler can be installed at an elevation high enough to permit the cooled cream to flow into the cream vat by gravity, an additional pump is necessary. In addition, the higher its elevation, the less accessible are its upper portions for proper cleaning. The cabinet surface cooler eliminates the drawbacks of excessive height. This type of surface cooler consists of multiples of cooling tube section, arranged on the same level, instead of one on top of the other as represented by the old line surface cooler. The cabinet cooler has distinct advantages over the open surface cooler.

9.2.4 Internal tube cooler

The internal tube cooler consists of the two-tube principles, one inside of the other. The cream flows through the inner tube, while the refrigerant flows in counter current between the inner and outer tubes. The usual unit consists of one or two sections for water and one section for brine or direct expansion ammonia.

This type of cooler has the advantage of preserving the “unbroken,” closed system of cream flow, discharging the cream into the vats and up to any reasonable elevation without the need of a pump at the discharge end of the cooler. The internal tube cooler does not subject the cream to aeration. If aeration is desired it should be provided before the cream reaches this cooler.

In the case of sour, neutralized cream, the internal tube cooler usually does not permit of cooling to a temperature much below 15.5°C without danger of the formation of a cream plug and of clogging. This tendency is especially prevalent when such cream has received severe prior treatment, such as, for instance, in connection with prolonged vacuum for removal of obnoxious flavors and odors. It is to refrain from attempting to cool such cream to churning temperature (below 15.5°C) in this cooler, and to finish the cooling to the final low temperature in the coil vat.

Forcing the cream through several standard sections of internal tube cooler, subjects it to considerable pressure, which further aggravates the tendency of cream in which the emulsion coefficient has already been lowered to near the” breaking” point, to churn either in the cooler or in the vat, causing delay and extra labor, and often costly loss of fat.