The very first time I came to know about freeze-drying was in my under graduate years. One of my classmates did his final year project on freeze-drying of jackfruits and durian. At that time, I knew it as a drying process which takes a long long time. However, the fruits could be kept fresh without any use of preservatives. It was indeed an intriguing process for me.
Freeze-drying is one of the advanced food processing techniques which is used in Nutritional Immunology superfoods.
So what is freeze-drying? I will attempt to explain it in a simple manner that you can easily understand without a chemical engineering background.
Imagine your orange without water, what would it look like? In this bottle, you will find freeze-dried orange slice, green pea, carrot slices, purple cabbage. Does it look like the real fruits and vegetables itself?
Some may look at this picture and say, it just looks like dehydrated foods. Indeed, the water is taken out of the fruits and vegetables. However, in dehydrated or oven-dried foods, the nutrient content is often also diminished. In contrast, freeze-drying maintains at least 95% of the nutrients!!
During this step, the fruits and vegetables are rapidly frozen within a tightly controlled temperature range (-50 deg C to -80 deg C).
With rapid freezing, small ice crystals are formed. Hence, it will not pierce and damage the fibre matrix of the fruits and vegetables.
In our home refrigerator at around -5 deg C, the freezing is much slower resulting in large ice crystals. Hence, when we defrost the vegetables, they start to get mushy.
As the name suggests, water is taken out from the fruits and vegetables in this step. After freezing, water is in its solid form as ice. With the right pressure conditions, ice will sublime out as water vapour. It does not need to go through the liquid phase.
Even though freeze-drying sounds so simple as above described, it can be technically challenging to carry out due to the low temperatures and low pressure condition required. You can read more in the screenshots below on more details in each of the freeze-drying steps as well. 🙂
Despite the high energy requirement/cost to do freeze-drying, it brings on many benefits, especially in the area of quality.
i.e. Bananas have the most nutrients when they're picked ripe from the tree. However, bananas we get in our cities are often harvested green so that they will not spoil before they arrive in our supermarkets.
With freeze-drying, bananas can now be picked ripe from the tree and freeze-dried for maximum nutrients to us.
Other supply chain benefits:
Hope this article gives you a better understanding of freeze-drying as a food processing method. Do let me know if you have more questions on this topic. 🙂
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