Dairy Process Engineering

Module 4. Mechanization in manufacture of indigenous dairy products

Lesson 22
ICE-CREAM MAKING EQUIPMENT

22.1 Introduction

Ice cream is a frozen dairy product. In ice cream making refrigeration system and air incorporation system is required. The type of freezer play an important role in freezing of ice cream mix and subsequently on the quality of the product. There are batch and continuous type of ice cream freezers available in the market which are described here.

22.2 Ice Cream Freezer

The function of the freezer are

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

(b) to incorporate a predetermined amount of air uniformly into the mix to get proper overrun, n.

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.

Following factors influencing the freezing time

Mechanical factors

1) Type and make of freezer

2) Condition of the freezer wall and blades

3) Speed of the dasher

4) Temperature and flow rate of the refrigerant

5) Overrun desired and

6) Feed rate

Characteristics of the mix influencing the freezing time are

(1)Composition of mix

(2) Freezing point of mix and method of processing the mix

Freezers are of two basic types: the batch and continuous freezers.

22.2.1 Batch Ice-Cream Freezer

Tub freezer

It has a vertical can, surrounded by Ice & Salt mixture. The can is rotated with mix filled to ½ and rotated with handle and bevel gear. The dasher is stationary, while the can will be rotating when handle is rotated. The salt keeps the ice melting point low. This type of batch freezer is now not in common use. However, some of the traditional Indian dairy products like Kulfi is being frozen by this kind of a device.

Horizontal batch freezer

· It has refrigerated drum or cylinder which is stationary, with direct expansion of refrigerant in the coils surrounding the cylinder. The cylinder itself can be of SS or Brass with Nickel coating, to take advantage of good thermal conductivity of the Copper alloy, at the same time to prevent it from coming in contact with the product.

· The cylinder has a front openable door , which in its inner side has bush arrangement to hold a horizontal shaft. The door even when closed, the cylinder has an opening at the top to pour ice cream mix, and another opening at bottom to draw frozen ice cream.

· Dasher which is a combination of Beater and Scrapper has the following functions:

a) To scrape the frozen film from cylinder wall, so that ice crystals do not grow beyond 40 μ

b) Beat the mix to incorporate air to the maximum extent possible

c) Mix any fruit & flavouring material uniformly

d) Eject the finished ice cream rapidly

e) It is important to have the dasher in proper alignment and the blades must be sharp.

f) The beater and scrapper rotate in opposite direction to avoid slug type movement of the entire semifrozen mix.

g) The Dasher rpm is usually 150 to 250.

· The sequence of operations involves:

i) Fill the mixture one third to half the volume of the cylinder

ii) Start freezing the mix , with switching ON both the refrigeration system and beater

iii) Time to time check the stage of freezing and over run by taking the sample and weighting, as well as keeping a watch on the Ammeter reading of the Dasher motor. The power consumed will increase with the progress of freezing

iv) After required freezing is achieved ( approx. half the moisture to be in frozen condition), continue beating to incorporate air further till required over run is achieved ( which is maximum 100% for batch freezer).

v) The freezing is generally over in 5 to 7 minutes, and over run takes little more time, if the composition is suitable.

22.2.2 Continuous Ice Cream Freezer

Advantages

1) Large scale production capacity

2) uniform and smoother ice cream quality

3) More efficient controls

4) Shorter aging time is possible

5) Less tendency towards sandiness

Disadvantages

1) High initial cost

2) Not suitable for small scale production

Construction

1) The volume of the cylinder is comparatively reduced, by having more solid dasher and mix passing as a thinner layer, as compared to batch process
2) The freezing chamber is supplied with mix from tank by two pumps in some designs, where the first pump is a metering pump, designed to feed mix at a controlled rate. The outlet of this pump leads to a second pump through a pipe, which has provision to allow controlled quantity of air. The second pump being larger in capacity (approx. three times), it handles both incoming air and the mix. The second pump, then, leads the mixture of air and ice cream mix, in to the freezing chamber at about 5 to 6 kg/ cm2.
3) Other designs single large pump with air entry on suction like along with mix.
4) Both horizontal and vertical models are available.
5) In a continuous freezer, about 30 second time is taken for partial freezing the mix.
6) The ice crystal size is reduced to 45-55 μm and the air cell wall thickness to 100-150 μm.
7) For cooling, evaporating coolants in the temperature range of -20 to -30 °C are used in the cooling jacket.

Refrigeration Unit

1) Cooling is by Direct expansion using R502 refrigerant (Cattabriga)

2) Semihermatic compressor, with oil sum heating element is provided.

3) Condenser is water cooled, along with a safety valve in case of high pressure being developed.

4) Cooling water is regulated

5) Liquid refrigerant line with provided with Ceramic filter, sight glass, solenoid valve

6) The expansion valve is Thermostatic type

7) Two hours after oil heater is switched ON, the compressor is to be switched on. This removes the refrigerant in the oil.

8) Controls include switches for Beater, compressor. When the beater motor current consumption exceeds a limit, the controls activate a solenoid, which allows hot gas from compressor immediately below the expansion valve. This control can be operated manually also. This limiting value can be adjusted by a Rheostat.

Fig. 22.2 Continuous ice cream freezer

22.3 Pumps

Pumps of ice-cream freezers are usually of the rotary type with the capability to pump against pressure of 7-14 kg/cm²(690-1380 kPa) with reasonable volumetric efficiency. There are two general pumping arrangements, both designed as a part of the overrun system. The first employ a pump (or a pair of pumps or compound pump) to pump or meter the mix into the freezing cylinder, plus a hold-back valve at the ice cream discharge port. The hold-back valve may be spring loaded with manual adjustment, it may have an air operator with adjustable air pressure supplying the operating power. The hold-back valve permits imposing a pressure on the cylinder during freezing which compresses the air admitted with the mix for overrun. Cylinder pressure of 3.5-4.0 atmospheres keeps the volume of air in the freezing cylinder sufficiently small so that it does not significantly lower the internal heat transfer out from and through the mix. That pressure is sufficient for proper air dispersion and small air cell size. Higher pressures may be imposed on the cylinder, but in most cases, the improvement of heat transfer and air cell size is not great enough to offset the disadvantages of increased pumping cost.

22.4 Controls and Automation

All continuous ice cream freezers have control for operation which include on-off switches for pump and dasher motors, and for air compressor motors (when these are part of the freezer), for solenoid valves on hot gas defrost lines, air lines and refrigerant supply lines, speed regulation of pumps, refrigeration supply and back pressure, pressure gauges for the refrigeration system and cylinder or air pressure and dasher motor ammeter, wattmeter or motor load indicator. In addition more sophisticated machines may have a viscosity meter and controller, and a programmable controller or micro-processor to operate and control most functions of the ice cream freezer.

The modern ice cream freezer consists of a micro-processor programmed to control all the function of operation including overrun, viscosity of product, cylinder pressure, all operating steps such as start up, routine or emergency shutdown, resumption of operation after an automatic shutdown when the reason for shutdown has been corrected. Preparation for cleaning and the valve and pump by pass is required for automatic cleaning. The micro-processor shows the time of day, mix flow rate, percentage of overrun, product rate, hours of operation, accumulated production in that time interval, the program step in operation, and various warnings. In case of an impending freeze-up, the warning is displayed and corrective action is taken. If a freeze-up should occur, the micro processor automatically causes defrosting of the cylinder and operation to be resumed when conditions are satisfactory. The display can be in one or more of several common languages.

The micro-processor programmed operation assures that all functions are performed in the proper sequences, and under the conditions envisioned by the designer of the freezer. This is especially beneficial to the ice-cream maker in preventing damage to the freezer in emergency situations, thus avoiding the incidental unplanned down time in production.

22.5 Adding Ingredients and Flavours

Flavouring materials are added after the mix has been made. Pieces of fruit and purees should not be added to the mix prior to freezing in continous freezers, as they tend to settle out in the tank with subsequent poor distribution in the frozen ice cream. The ingredient feeders often referred to as fruit feeders have a hopper for the ingredient, an auger or other means for metering or proportioning the fruit, a rotator or plunger for inserting the ingredient.