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Getting the Most From a Crystallization Process
by Tony Mathis
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.
If the mixture is allowed to stand in static conditions, crystals will continue to grow in a rapid solidification phase and a strong, solid mass will result. If, however, the cooled mixture is moved to a secondary processing unit which agitates the product and adds shear to the mixture, the crystals will be kept small and be prevented from growing together into a solid mass, creating a more fluid result. A pin rotor machine can be used for this purpose and the amount of agitation applied during solidification can be varied to get the right crystal size, texture, and final product characteristics required.
Fats and Sugars
SSHEs are widely used to crystallize products containing fats or sugars which react differently in the process. Fondants, for example, crystallize very quickly. Bakery sandwich or cookie creams have firm, poor flowing properties and require a machine which can handle the associated high pressure and high motor shaft torque. The production of low trans- and saturated-fat bakery filling creams, however, also requires care within the process such as distributing gas in the mixture. These creams tend to get too soft if gas is distributed into the mixture at the end of the process with a mixing unit.
Having the right configuration of SSHE means that the gas can be added at the crystallization stage to achieve ideal homogenization while protecting the structure of the crystallized oils and creating the desired consistency of the filling cream for its application. Shortenings and margarines require rapid cooling to avoid separation of the mixture and, as the crystals form, temperature and agitation can influence the resulting product characteristics. SSHEs can be used across all these applications and more but it is important to completely understand all aspects of the process to get the highest product qualities.
A continuous crystallization process in a SSHE not only maximizes production capacity but also minimizes operator intervention and facilitates repeatable, consistent quality results. The quality of the crystallization process largely depends upon the time taken to lower the temperature to the point crystallization occurs, as well as the amount of agitation of the crystals during formation. Faster cooling and more vigorous agitation generally result in the desired smaller crystal sizes within the mixture, giving smooth end product results.
Mathis is a process technical manager for SPX Flow Technology. His 30-year career has primarily focused on sanitary SSHE heat transfer applications. Reach him at email@example.com.