Freezing rate, quality

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 ... 

The general rule for all freeze drying processes applies also to food The method of freezing, the freezing rate and the final temperature of freezing largely determine the quality of... 

Reetz and Haase [5.2] used different freezing rates to freeze Zr02, and found slow freezing of this product to result in better technological qualities, e. g. free-flow and sinter ability than quick freezing. On the other hand, complex Zn- solutions can only be frozen quickly to arrive at a product homogeneous in chemical structure and grain size distribution. 

Canet, W., Espinosa, J. (1984). The effect of blanching and freezing rate on the texture of potatoes, carrots and peas, measured by mechanical tests. In P. Zeuthen, J. C. Cheftel, C. Eriksson, M. Jul, H. Leniger, P. Linko, G. Varela (Eds.), Thermal Processing and Quality of Foods (pp. 678-683). Elsevier Applied Science, London. 

All substances conisdered in this section can only be dried in the presence of CPAs, if their natural qualities are to be protected. Greiff [3.24] studied the stability of purified influenza virus of strain PR 8 in physiological saline with calcium lactobionate and human serum albumin (each 1% in the solution). The freezing rate was 1 °C/min down to -30 °C. During the freeze-drying, the product temperature was raised from -30 to 0 °C in 12-16 h and the product was dried at this temperature. After 24 h, the first 145 vials were removed and additional vials after intervals of 24 h each. The residual moisture content was 3.0, 2.0, 1.5,1.0 and 0.5%.The stability of the freeze-dried virus (expressed in days during which the titer of the infectivity decreased by a factor of 10) was most unfavorable at 0.4% and 3.2% RM (4 and 7 days, respectively, at +10 °C) and best at 1.7% RM (145 days or more than 1000 days at -10 °C). 

Freezing method and freezing rate, storage conditions and temperature abuse, as well as thawing are factors influencing the quality of frozen foods (Chambers et al.,... 

High frozen food quality due to high freezing rate fine-grain crystal structure sharp reduction of the surface mass transfer easy incorporation of antioxidants, flavors, aromas, and micronutrients and improved nutritional value and sensory properties. 

Because water is an essential component of the liposomal bUayer structure, freezing can promote damage due to dehydration. Use of cryoprotectants and control of the freezing rate that can minimize the formation of large ice ciystals are of utmost importance. To achieve stability, the quality of phospholipids and the production process must be reproducible. 

The surface of concrete may be subjected to cycles of wetting and drying during normal service life. This process is a durability concern as harmful ions such as chlorides or sulfates can easily penetrate with the adsorbed water and there is the potential for saturation prior to freezing. Measuring the rate of water penetration into a dry concrete surface is thus important to the quality control of concrete mixes - hopefully yielding a durable product. 

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