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Ageing
1. The mix is then aged for about at least four hours and usually overnight.
2. This allows time for the fat to cool by down and crystallize, and for the proteins and polysaccharides to fully hydrate.
3. Aging provides for the following functions:
• improves whipping qualities of mix and body and texture of ice-cream. It does so by;
• providing time for fat crystallization, so the fat can partially coalesce;
• allowing time for full protein and a resulting slight viscosity increase;
• allowing time for membrane rearrangement and protein/emulsifier interaction, as emulsifiers displace proteins from the fat
globule surface, which allows for a reduction in stabilization of the fat globules and enhanced partial coalescence.
4. Aging is performed in all insulated or refrigerated storage tanks, silos, etc.
5. Mix temperature should have be maintained as low as possible without freezing, at or below 5 °C.
6. An aging time of overnight is likely to have give best results under average plant conditions.
7. A "green" or unaged mix is usually quickly detected by at the freezer.
E x e r c i s e S e v e n
. Read the article below and make up questions.
Freezing
Following mix processing, the mix is drawn into a flavor tank where any liquid flavors, fruit purees, or colors are added. The
mix then enters the dynamic freezing process which both freezes a portion of the water and whips air into the frozen mix. The
"barrel" freezer is a scraped-surface, tubular heat exchanger, which is jacketed with a boiling refrigerant such as ammonia or freon.
Mix is pumped through this freezer and is drawn off the other end in a matter of 30 seconds, (or 10 to 15 minutes in the case of batch
freezers) with about 50 % of its water frozen. There are rotating blades inside the barrel that keep the ice scraped off the surface of the
freezer and also dashers inside the machine which help to whip the mix and incorporate air.
Ice cream contains a considerable quantity of air, up to half of its volume. This gives the product its characteristic lightness.
Without air, ice cream would be similar to a frozen ice cube. The air content is termed its overrun, which can be calculated
mathematically.
As the ice cream is drawn with about half of its water frozen, particulate matter such as fruits, nuts, candy, cookies, or whatever
you like, is added to the semi-frozen slurry which has a consistency similar to soft-serve ice cream. In fact, almost the only thing
which differentiates hard frozen ice cream from soft-serve, is the fact that soft serve is drawn into cones at this point in the process
rather than into packages for subsequent hardening.
E x e r c i s e E i g h t
. Read the article below and answer the questions.
1. When is the ice cream packaged and where is it placed?
2. What is the limit of the shelf life of the ice cream?
3. What can help one understand the freezing process?
4. What does hardening involve?
5. What freezing techniques are used in ice cream production?
6. What is the rate of heat transfer affected by?
7. What factors affect hardening?
Hardening
After the particulates have been added, the ice cream is packaged and is placed into a blast freezer at –30 to –40 °C where most
of the remainder of the water is frozen. Below about –25 °C, ice cream is stable for indefinite periods without danger of ice crystal
growth; however, above this temperature, ice crystal growth is possible and the rate of crystal growth is dependant upon the
temperature of storage. This limits the shelf life of the ice cream.
A primer on the theoretical aspects of freezing will help you to fully understand the freezing and recrystallization process.
Hardening involves static (still, quiescent) freezing of the packaged products in blast freezers. Freezing rate must still be rapid,
so freezing techniques involve low temperature (–40 °C) with either enhanced convection (freezing tunnels with forced air fans) or
enhanced conduction (plate freezers).
The rate of heat transfer in a freezing process is affected by the temperature difference, the surface area exposed and the heat
transfer coefficient. Thus, the factors affecting hardening are those affecting this rate of heat transfer:
• Temperature of blast freezer – the colder the temperature, the faster the hardening, the smoother the product.
• Rapid circulation of air-increases convective heat transfer.
• Temperature of ice cream when placed in the hardening freezer – the colder the ice cream at draw, the faster the hardening –
must get through packaging operations fast.
• Size of container – exposure of maximum surface area to cold air, especially important to consider shrink wrapped bundles –
they become a much larger mass to freeze. Bundling should be done after hardening.
• Composition of ice cream – related to freezing point depression and the temperature required to ensure a significantly high
ice phase volume.
• Method of stacking containers or bundles to allow air circulation. Circulation should not be impeded – there should be no
'dead air' spaces (e.g., round vs. square packages).
• Care of evaporator – freedom from frost – acts as insulator.
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