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Explore this issueApril/May 2014
Crystallization is important in determining the characteristics and quality of many foods. Good control over this process can establish factors such as whether a particular margarine is spreadable, how oily a product looks and feels and, with the human tongue able to detect crystals down to 55 microns, whether foods, such as ice cream feel gritty or smooth in the mouth. The production and control of these crystallized foods can be revolutionized by scraped surface heat exchanger (SSHE) technology.
How It Works
As a mixture is cooled in a SSHE the machine continually scrapes tiny, microscopic crystal seeds from the heat transfer wall. Without this scraping action the product would crystallize on the cooling surface and insulate subsequent product from the heat exchanger, either leading to blockages in the machine or an ineffective process where a thin layer of solidified product prevents cooling and warm product just flows through the heat exchanger. If using a tubular heat exchanger, viscosity increases as the temperature drops and results in less turbulence within the machine. This, in turn, reduces the heat transfer efficiency and requires high-pressure pumping systems for operation. In comparison, the agitation provided by the SSHE maintains turbulence and heat transfer efficiency as the product thickens, ensuring uniform cooling and crystallization throughout the process.
A SSHE provides a constant rate of heat transfer because the film at the product wall is continually being scraped away by its blades. The seeds scraped from the surface of a SSHE get warmed from the stream of product in the machine and, rather than getting a few seeds growing into large crystals, millions of small crystals are produced. This collection of very fine crystals results in a smooth product with excellent mouth feel.
If larger crystals are required, a portion of the crystallized discharge from the machine can be fed back into the inlet of the SSHE where the cooling process will grow these crystals further. This technique is useful, for example, if forming ice and separating it out in something like freeze concentration. If a product is particularly difficult to solidify, the formation of crystals can be encouraged by adding a certain amount of preformed crystals into the melt at the SSHE inlet. Once the process is running and crystallization achieved as required, the pre-formed solution added to the mix can come directly from the outlet of the SSHE.
As well as the agitation within the SSHE, which prevents the formation of solid non-plastic product, the cooling rate is also an important factor. Many mixtures contain multiple ingredients which set at different temperatures. If cooling is too slow, these ingredients will set in sequence which can cause separation with liquid weeping from the mixture. Rapid cooling, however, forces all ingredients to crystallize at approximately the same time. Combined with the agitation from the SSHE, this cooling forces the complete mixture to set with small crystal structure.
To enable rapid crystallization for high-capacity production, ammonia, carbon dioxide, or other refrigerants are often used on the jackets of the heat exchangers. Control of the refrigerant side of the SSHE is important for continuous and consistent control of the product discharge temperature and to maximize process capacity. The refrigeration temperature can be adjusted to handle different flow rates for flexible production.
Along with the control of temperature and time for the crystallization process, the constituents of a recipe influence the resulting characteristics of a particular product. In more recent times, awareness of health concerns relating to the use of trans-fatty acids and the amount of saturated fatty acids in food has led to a drive to remove or dramatically reduce these in product recipes. Such changes in requirements have led to enhanced use of palm oil fat and its fractions in recipes, but these oils have slower crystallization speeds than previously used alternatives. To handle this slower rate, the mixture is super-cooled to temperatures below the temperature required to form a solid. Although below the temperature for solidification, the crystals form slowly and the product remains as a liquid for a short time. This gives a choice of secondary processing to achieve the desired structure of the final product.