FOOD ENGINEERING

Module 2. Food freezing

Lesson 18
STUDY OF CONTINUOUS ICE CREAM FREEZER

18.1 Continuous Ice Cream Freezer

The continuous ice cream freezer was developed in the early 1930’s. The principle of operation is very similar to that of a batch freezer, except that the freezing is done under pressure (which increases the heat transfer rate) and the air is forced in or drawn into the freezing chamber continuously at a metered rate. The expanded and congealed ice cream is then forced out of the freezer cylinder, ready for filling in packages and containers. In a continuous freezer (Fig.18.1), about 30 second time is taken for partial freezing the mix. The ice crystal size is reduced to 45-55 μm and the air voids are in the range of 100-150 μm diameter.

The continuous freezer consists of a stainless steel shaft carrying a set of blades (dasher) rotating at 150-200 rpm inside a chromium-plated nickel surface of the freezing cylinder (Fig.18.2). The blades made of spring steel continuously scrape off the thin film of ice which form on the inner wall or the tube and at the same time thoroughly mix the ice cream as it is formed. A special eccentrically placed beater operates inside the dasher frame and is rotated by re-action with the ice cream. The eccentric position of the beater gives a thorough mixing of the ice cream as it freezes and is moved from one end of the cylinder to the other. For cooling, evaporating coolants in the temperature range of -20 to -30^oC are used in the cooling jacket.

The mix is pumped into a cylinder, which is flooded with liquid refrigerant. The freezing process is very rapid and the layer of the frozen mix on the cylinder wall is continuously scraped off with a rotating blade equipped mutator inside the cylinder.

The required amount of air is supplied continuously whilst the ice cream is worked in the freezer, so that the air is worked into the mix at the same time. This gives the ice-cream the desired texture before it is fed from the freezer through a pipe to a forming or filling machine.

The advantages of the continuous or instant freezers are:

(i) Less stabilizer is needed because a larger amount of ice crystals can be formed and less viscosity is needed in the mix.

(ii) A shorter ageing time is possible because less viscosity is needed and incorporation of air is less.

(iii) Less flavoring material is needed because the smaller ice crystal melt more rapidly in the mouth and make the flavour slightly more pronounced.

(iv) Smoother ice cream is obtained.

(v) There is less tendency towards sandiness because rapid freezing favours small lactose crystals.

(vi) Ice cream consistency is uniform due to continuous production

18.2 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, which is provided by 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.

Ice cream freezer pumps are driven by various means, but all of these provide for varying pump speed. Usually the set of pumps for each cylinder is powered by one drive. Drives are of three types

(i) Electric motor powering a mechanical variable speed;

(ii) Frequency inverters with electronic speed control for standard electric motors. A gear reducer is always used between motor and pump;

(iii) Hydraulic pumping systems connected to hydraulic motors on the pumps. The hydraulic pumping units may be located within the freezer housing or remotely outside the production room.

18.3 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, watt meter 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. 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.

18.4 Fruit and Nut Feeder

Flavouring materials are added after the mix has been made. These may be added at the ageing or holding tanks, or in flavour tanks located just upstream of the ice cream freezer. Fruit juices, flavour extracts, colour and similar materials are added at these points. 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. Further, seeds in fruit or other gritty content harm the close-fitting pumps, the dasher bearing and seal, and dulls the scraper blades.

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.

1. Packing, Extrusion and Moulding

Ice cream is packed in cups, cones and containers (1 to 6 liters) in a rotary or in-line filling machine. These can be filled with various flavours, and the products may be decorated with nuts, fruits and chocolate. The packs are lidded before leaving the machine, after which they are passed through a hardening tunnel where final freezing down to -20^oC takes place. Before or after hardening the products can be manually or automatically packed in cartons or bundled.

Moulding of ice cream or water ices bars are made in special machines, also called stick novelty freezers, with pockets in which the ice cream or water ice is moulded. Ice cream is supplied direct from the continuous freezer at a temperature of approx. -3^oC . The filled moulds are conveyed stepwise through a brine solution having a temperature of -40^oC , which freezes the ice cream or water ice solution. Sticks are inserted before the moulds are completely frozen. The frozen products are removed from the moulds by passing them through a warm brine solution which melts the surfaces of the products and enables them to be removed automatically by an extractor unit. After extraction of bars (novelties) may be dipped in chocolate before being transferred to the wrapping machine. Since the products are fully frozen, they can be taken straight to the cold store after wrapping and cartoning, otherwise go for hardening it in a hardening tunnel before wrapping and cartoning.

2. Hardening & Cold Storage

The manufacture of ice cream is not complete until it has been thoroughly harden at a temperature of around -20^oC . For products produced in an extrusion line or a stick novelty freezer, the hardening operation is included in the process. Products packed immediately after freezing must however be transferred to a hardening tunnel, The faster the hardening, the better the texture. After hardening the products are transferred to the cold store where they are stored on shelves or pallet racks at a temperature of -25^oC . The storage life of ice cream depends on the type of product, the packaging, and maintenance of a constant low temperature. The storage period of ice cream at -18^oC ranges from 0 to 9 months in the ice cream cold storage.