Cooling deposited products

Most food processes incorporate some form of cooling. This is often assumed to be a less important part of the process, simply requiring some means of blowing a lot of cold air over the product. However, as has been shown in previous issues of Update, cooling is an important processing operation, vital to the achievement of a high quality finished product.

Previous articles have focused on the cooling of chocolate products and the effect of poor cooling on the appearance and perceived quality of the finished product. We now examine the cooling of deposited products where the product is retained in a mould during the cooling cycle. Such products include hard candy, toffee, chews, fondant gums and jellies.

Different products require different cooling times and cooling air temperatures. It is important to determine accurately the thermal properties of each product before attempting to design the cooling system. It is often necessary to perform tests in a laboratory test rig to determine a product's 'cooling factor'. Cooling is related to heat transfer theory and is therefore dependant upon the heat energy and heat transfer rate of the product. The rate at which a product will lose heat is, in part, a function of its thermal conductivity. Having determined the product's particular cooling factor, cooling times can be accurately predicted for differing cooling air and product temperatures.

A product's weight, size, shape (aspect ratio) and formulation will all have a significant effect on the rate at which it cools. However, as deposited products are cooled whilst still in their forming moulds, the design of the moulds and their material of construction will also have to be taken into account.

The material from which a mould is made will depend upon the product and the ejection method that is appropriate:

Metal mould and pin ejection for hard products i.e. hard candy, milk boils.
Metal or plastic mould with air ejection for soft, deformable products such as fondant creme and jelly.
Rubber mould and roll or finger ejection for toffee, chews and fondant.
Hard candy deposited into metal moulds at 140°C (284°F) typically requires a cooling time of five minutes, using circulating air at ambient temperature. During cooling, the metal mould absorbs heat and expands but the sweet cools and contracts helping ejection, the sweet being ejected typically at 35°C (95°F). It is not possible to reduce cooling times by simply using colder air. This would lead to condensation forming on the sweets, causing stickiness and ejection problems.

Toffees and chews deposited in rubber moulds at 110°C (230°F) will require at least 10 minutes cooling, using refrigerated air. The rubber moulds insulate the product making cooling more difficult. The product will need to be cooled below ambient temperature if ejection is to be achieved successfully without deforming the product.

Other product characteristics may affect total cooling time. Although jellies may lose their heat relatively quickly, sufficient time must be allowed for them to 'gel' before attempting ejection.

Cooling time may also be affected by the recipe. A low fat recipe can solidify quicker than a high fat one. 'New' products such as sugarfree hard candies have demanding requirements. Their high depositing temperature and high specific heat capacity mean longer cooling times than for sugar based products.

As with all food products, the specific nature of the product will affect the design of the process equipment. High quality end products can only be achieved if the physical characteristics of the product are accurately determined and allowed for in the process design.  The best designed cooler cannot make up for poor quality ingredients or for variations in the upstream manufacturing process but a poorly designed cooling system can lead to a low quality end product, poor in appearance or difficult to package.