After the product is frozen, its vapor pressure drops radically. In fact, this will occur after only the surface is frozen. The vapor pressure we are concerned with is that prior to freezing. It becomes obvious, that the colder the product is before it enters the freezing zone, the lower will be the evaporation losses. Precooling in a moist environment is imperative to minimize evaporation losses.
The fact that vapor pressure of virtually all freezing media is practically zero tends to wipe out an important difference among methods in this respect. What is left, then, is exposure time, which must certainly be the most important factor in evaporation losses. This highlights the importance of fluidization in air-blast freezing so that all surfaces are equally exposed to freezing temperatures and quickly surface frozen. This might make a case for using cryogenics for surface freezing and finishing the job in air blast.
Whether or not a more costly freezing system should be used to avoid evaporation losses depends on the value of the product and the amount of loss. If a 10 percent evaporation loss is experienced on a 10cent-a-pound product, up to I cent a pound can be spent on a system to avoid the loss. A similar amount can be spent to avoid only a I percent loss on a dollar-a-pound product. Figure 6 shows the amounts that can be spent to avoid evaporation losses in products of various values. Only extremely low evaporation losses can be tolerated in high-value products. At the other end of the scale, substantial losses can be tolerated on low value products before a more costly freezing system can be justified, as long as quality is not impaired.
