Module 2. Food freezing

Lesson 17


17.1 Introduction

Ice cream freezers, apart from freezing a portion of water of ice cream mix, also incorporates air in the mix to obtain a swell in volume or ‘overrun’. To get different freezing and good overrun in the ice cream it is very essential that the optimum quantity of the mix should be taken in the freezer for freezing. This quantity varies from freezer to freezer. The function of the freezer are:

(i) To freeze a portion of the water of the mix to get a smooth product,

(ii) To incorporate a predetermined amount of air uniformly into the mix to get proper overrun,

(iii) To mix fruits and flavouring into the mix while being frozen.

Fast freezing is essential for a smooth product because ice crystals that are formed quickly are smaller than those formed slowly. Therefore, it is desirable to freeze and draw from the freezer in as short a time as possible. Failure to provide adequate refrigeration during freezing or hardening, results in formation of large ice-crystals in ice cream. Also, since freezing continues after the ice cream is placed in the hardening rooms; the ice crystals formed during the hardening period are larger because they form more slowly than in the freezer. For this reason, it is desirable to freeze the ice cream as stiff as possible and yet have it liquid enough to draw out of the freezer.

Factors influencing the freezing time are: the mechanical factors and factors inherent in the mix itself. Mechanical factors are

(i) Type and make of freezer

(ii) Condition of the freezer wall and blades

(iii) Speed of the dasher

(iv) Temperature and flow rate of the refrigerant

(v) Overrun desired and

(vi) Rate of drawing the ice cream.

Characteristics of the mix influencing the freezing time are:

(i) Composition of mix

(ii) Freezing point of mix and method of processing the mix, and

(iii) Kind and amount of flavours added.

There are two types of ice cream freezers:

(I) Batch ice cream freezers and

(II) Continuous ice cream freezers.

(I) Batch Ice Cream Freezer

17.2 Construction

In the batch freezer a definite quantity of the mix is frozen at a time. It consists of a tubular chamber fitted with a rotating dasher (Fig-17(A). The chamber is fitted with a mix tank and hopper for adding fruits and flavours. The chamber is surrounded by a refrigerated jacket in which ammonia or other suitable refrigerant is evaporated to provide the cooling effect.

The freezing chamber is made of a liner, usually of nickel silver or stainless steel, pressed inside a steel or copper tube which forms the inside wall of the cooling jacket. If the jacket is cooled by brine, it is constructed of copper with narrow passageways to increase turbulence and heat transfer. The outside is insulated with cork and covered with an airtight metal housing. If the cylinder is to be cooled by direct expansion of a refrigerant, the outer jacket is usually built of steel and properly insulated.

The functions of the dasher are

(i) To scrape the frozen film from the cylinder wall, carry it to the centre, and circulate it from one end of the freezer to the other so that rapid and uniform cooling takes place,

(ii) To beat the mix and hold air into it,

(iii) To eject the frozen mix rapidly when the batch is finished.

The dasher consists of an outer frame carrying either two or more sets of scraper blades, which turn in one direction at a speed of 60-70 rpm. (Fig-17(B))The central part consists of beater having a series of longitudinal rods or paddles which rotate in opposite direction. The dasher is mounted on a shaft furnished with rotary seals, so that it can be completely taken apart for cleaning. It is important to have the dasher in proper alignment and the blades must be sharp.

17.3 Operation

A batch of mix is dropped into the chamber, the refrigeration is turned on and dasher is started. The temperature of the refrigerant is very important and should be from -24oC to -29oC in order to get a rapid formation of ice crystals. The mix is cooled down to a temperature of about -5oC in 6-10 minutes at which the refrigeration is turned off by means of a quick shut off valve and dasher allowed to rotate for 1-2 min to allow the mix to partially congeal. The mix will not absorb much air until a temperature of about -5oC is reached. At this point, it will rapidly absorb air of 100-120% of its original volume. The ice cream is then drawn out of the chamber and into cans or packages which are placed in a room at temperature between -24 to -30oC for hardening.

A slide or pivot valve on the bottom portion of the front door allows the ice cream to be drawn into containers or bulk cans. The dasher is designed to propel the product toward the discharge port. Subsequent batches are made in the same manner. The size of the batch freezers varies from 18-40 liters of 100% overrun. If greater than 100% overrun is desired, the mix charge must be reduced enough to prevent overflow of ice cream during whipping.

The beaters promote whipping, but when freezing at temperatures below -5oC , air is less readily incorporated, and some may even be expelled. Ice cream made with batch freezers has both larger ice crystals and bigger air cells than ice cream made with the same mix on continuous freezers. Overrun control to close tolerance is difficult with batch freezers, and it may vary by 80-100 from the beginning of drawing the batch to completely emptying the barrel. This occurs because whipping continues all during the drawing time.
Last modified: Friday, 12 October 2012, 6:49 AM