Accelerated freeze drying follows essentially the same process as freeze drying, where products are flash frozen and the moisture is sublimated from the food through application of radiant heat in a vacuum. Accelerated freeze drying, however, uses metal plates to accelerate the heating process, reducing processing time and cost. Freeze dried food can be stored and shipped at normal temperatures and, as almost all water is removed during the process, the product is significantly lighter and resistant to microbes. Although this process is marginally faster than traditional freeze drying, most of the associated costs persist.
The biggest limitation of accelerated freeze drying is its cost in comparison to alternative methods of preservation (like canning or conventional freezing). Accelerated freeze drying involves several steps, each requiring space, specialised equipment and labour. Typically, freeze-drying plants are very large. They require loading areas (where raw ingredients come in), an industrial kitchen (where foods are made ready to be freeze dried), large freezing and drying chambers and space for packaging. All of these costs are passed on to the consumer.
Damage to Food Cells During Freezing
During the initial flash freezing process, food can be damaged. While flash freezing is typically less damaging than conventional freezing, it still causes many of the cells within the food to rupture as the water within them expands. Furthermore, as water leaves the cell, many previously diluted chemicals become more concentrated, which can further damage the cells.
Only Certain Foods Are Appropriate
While accelerated freeze drying works well for foods with a high surface area to volume ratio (such as small pieces of fruit, thinly sliced meat or ground coffee), the process is ineffective when applied to foods with a low surface area to volume ratio (such as large pieces of meat or whole fruits and vegetables).
The Process Must Be Carefully Controlled
Numerous variables come into play when undertaking accelerated freeze drying. The size of the food, the cooking time and temperature of the raw product, the rate of freezing, the final temperature after freezing, the residual moisture level after processing and the method for rehydration all factor into how products are processed and stored (i.e. vacuum sealed or packed with nitrogen). For this reason, there are very high quality control standards at accelerated freeze drying plants and sophisticated systems of monitoring foodstuffs throughout the process. This makes the process more time consuming and labour-intensive.