Freezing by Evaporation of Product Water

The freezing of a product in the vacuum chamber by the evaporation of 15-25 % of its water would reduce the process time, the investment- and the operation cost, since the evaporation of this water could be done in a very short time, and no extra freezing installations are required. Oetjen [4.4, 4.5] points out, that this process has substantial disadvantages for the quality of the product and is only applicable for a limited number of products. Therefore this process is no longer used (see Section 2.1.5). The freezing processes for food are discussed in the Sections 2.1.1 and 2.1.3, and the possible freezing rates in Section 1.1.1. The decisive quantities for the freezing rastes are ... 

Endo-exo ratios of the micelle-catalysed reactions have been determined by adding 0.25 mmol of 5.1c and 0.5 mmol of 5.2 to a solution of 5 mmol of surfactant and 0.005 mmol of EDTA in 50 ml of water in carefully sealed 50 ml flasks. The solutions were stirred for 7 days at 26 C and subsequently freeze-dried. The SDS and CTAB containing reaction mixtures were stirred with 100 ml of ether. Filtration and evaporation of the ether afforded the crude product mixtures. Extraction of the Diels-Alder adducts from the freeze-dried reaction mixture containing C12E7 was performed by stirring with 50 ml of pentane. Cooling the solution to -18 C resulted in precipitation of the surfactant. Filtration and evaporation of the solvent afforded the adduct mixture. Endo-exo ratios... 

Concentration. The concentration of fmit juice requites removal of solvent (water) from the natural juice. This is commonly done by evaporation, but the derived juices may lose flavor components or undergo thermal degradation during evaporation. In freeze concentration, solvent is crystallized (frozen) in a relatively pure form to leave behind a solution with a solute concentration higher than the original mixture. Significant advantages in product taste have been observed in the appHcation of this process to concentration of certain fmit juices. 

Urine (2 1.) in a porcelain basin is evaporated to a syrup on the water bath. The flame is extinguished and the hot syrup is stirred with 500 c.c. of alcohol. After some time the clear extract is decanted and the residue is again warmed and once more digested in the same way with 500 c.c. of alcohol. If necessary, the combined extracts are filtered, most of the alcohol they contain is removed by distillation, and the aqueous-alcoholic residue, after transference to a small porcelain basin, is evaporated to dryness on the water bath. The dry residue is well cooled and is kept in an efficient freezing mixture while two volumes of colourless concentrated nitric acid are slowly added with thorough stirring. After the product has stood for twelve hours, the paste of urea nitrate is filtered dry at the pump, washed with a little ice-cold nitric acid (1 1), again filtered with suction till no more liquid drains off, and suspended in 100-150 c.c. of warm water. To this suspension barium carbonate is added... 

Clarification by removal of casein with such agents as calcium chloride, acetic acid, cooper sulfate, or rennin has often been employed to obtain a serum more suitable for refractometric measurements. Obviously the composition, and hence the refractive index, of such sera will depend on the method of preparation. Furthermore, some of the serum proteins may be precipitated with the casein by some of the agents used, particularly if the milk has been heated. Refractive index measurements of such sera are not generally considered as satisfactory as freezing point measurements for detection of added water (David and MacDonald 1953 Munchberg and Narbutas 1937 Schuler 1938 Tell-mann 1933 Vleeschauwer and Waeyenberge 1941). Menefee and Overman (1939) reported a close relation between total solids in evaporated and condensed products and the refractive index of serum prepared therefrom by the copper sulfate method. Of course, a different proportionality constant would hold for each type of product. 

Freeze-drying, called lyophilization by the pharmaceutical industry, is a drying technique by which a product is solifidied by freezing and the solvent that contains it (usually water) is evaporated by sublimation (a chemical phenomenon) upon heating. The transformation of the solid phase into the gas phase takes place with neither the water nor the solvent passing through the liquid state. 

Finally, the acetone acetal Is removed by acid hydrolysis. Because free sugars are difficult to Isolate It Is convenient to use an acidic resin known as Dowex . The resin (whose polymeric structure Is discussed In Chapter 52) can simply be filtered off at the end of the reaction and the solid product Isolated by lyophillzatlon—evaporation of water at low pressure below freezing point. The yield Is quantitative. 

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