冷冻干燥如何工作和冷冻干燥的食物健康？

What is freeze-drying?

冻干或冻干是基于产品中水的升华的一种脱水技术（ 1 ，，，， 2 ).

This means that the product’s water content transitions from a solid to a gaseous state — or from ice to vapor — without going through the liquid state ( 1 ).

Freeze-drying is considered a high quality dehydration method for a couple of reasons, including ( 1 ，，，， 2 ):

• it operates at low temperatures, which contributes to preserving a product’s nutritional value, taste, appearance, and heat-sensitive compounds
• 冻结抑制化学和微生物过程，这大大延迟了产品的恶化（从而延长其保质期）

How does it work?

Raw foods are comprised of about 80%–95% water, which can be divided into “free” and “bound” water. While free water freezes, bound water doesn’t ( 1 ).

在冷冻干燥过程中，必须去除所有自由水和一些结合的水，并且必须始终冻结自由水。实际上，这是冻干和真空干燥和最具挑战性的步骤之间的主要区别（ 1 ).

冻干过程可以分为三个步骤（ 1 ，，，， 3 ，，，， 4 ):

1. 冷冻：The product is often frozen under atmospheric pressure.
2. 初级干燥：这也称为适当的冷冻干燥，这是去除冷冻自由水的升华步骤。
3. 次要干燥：Also known as desorption drying, it’s the drying of products to their desired humidity by removing the remaining bound water.

冻干应在不融化水的受控条件下进行，看到液态水的存在可能会改变产品的成分，形状和物理特性（ 1 ).

这可能会导致产品的保质期大大降低（ 1 ).

Once frozen water is removed via sublimation, the remaining product develops a highly porous structure. However, adding water rehydrates the product again almost immediately ( 1 ).

Summary

Freeze-drying is a form of dehydration that removes a product’s water content by turning it from ice to vapor. The three-step process preserves a product’s nutritional value, taste, and appearance while extending shelf life.

冻干食品健康吗？

Freeze-dried foods are a healthy food choice. In fact, freeze-drying is one of the most common dehydration methods due to its numerous benefits.

冷冻干燥是保留有益植物化合物的活性（例如植物化学物质和营养）的最佳方法之一，同时保留颜色，风味和结构。这就是为什么它被广泛用于生产高价值食品的原因（ 3 ，，，， 5 ，，，， 6 ).

例如，研究表明，与其他干燥方法相比，冷冻干燥最有效地保留抗氧化剂，例如anthocyanins，，，，flavonoids, and ascorbic acid or vitamin C ( 3 ，，，， 7 ).

Antioxidants are beneficial compounds that help fight off the damaging effects of oxidative stress in your body. They’re also the compounds behind most fruit and vegetables’ health benefits ( 7 ，，，， 8 ).

However, while freeze-drying may sometimes even increase a fruit’s phytochemical concentration, the opposite may also be true, depending on the fruit ( 3 ).

Additionally, given that decreased water activity inhibits the growth of most bacteria, yeasts, and molds, by removing a product’s water content, freeze-drying helps prolong a food’s shelf life ( 2 ，，，， 4 ，，，， 5 ).

This is especially important for freshplant-based foods，这可能是全年可用的（ 3 ).

最后，去除产品的水含量会减少体积和重量，使其更容易处理，存储和运输（ 2 ，，，， 3 ，，，， 4 ).

Summary

Freeze-drying helps retain nutrients and phytochemicals in foods. Therefore, freeze-dried foods are a healthy food choice. It also prolongs a product’s shelf life and facilitates storage, handling, and transportation.

Foods that can be freeze-dried

冻干食品通常用于远足，露营，太空探索，紧急和生存应用以及军事口粮。由于重量轻，保质期长，便携性和易于准备，它们比常规食物更受欢迎（ 9 ).

While plant-based foods are the most popular freeze-dried products, a wide range of foods can be preserved using this method.

这是一些可以冷冻干燥的食物的清单（ 3 ，，，， 5 ，，，， 9 ):

• Fruits:strawberries，苹果，黑莓，香蕉，梨，橘子和水果泥
• Vegetables:几乎所有蔬菜，例如胡萝卜，芦笋，蘑菇，辣椒，南瓜和西红柿
• 肉：beef, fish, chicken, eggs, pork, turkey, and shrimp
• 谷物：大米，豆类，意大利面，藜麦和玉米粥
• Frozen meals:whole meals likePad Thai，炖菜，辣椒和小吃
• Beverages:milk, juices, coffee, and tea for instant drinks
• 香料：ginger, oregano, mint, basil, and garlic
• 甜味剂：糖粉的枫糖浆

Summary

可以通过冷冻干燥（包括水果，蔬菜，肉类，整餐，谷物，饮料，香料和甜味剂）来保存各种食物。

冻干的潜在缺点

Freeze-drying makes an excellent食物保存方法。然而,也有一些潜在的缺点s to consider.

First, while reducing a product’s water availability diminishes microbial growth, disease-causing microorganisms in raw foods can survive the drying process and remain present during storage. They can causefoodborne illnesswhen eaten ( 4 ).

Therefore, foods that need to be cooked before consumption must also be cooked before being freeze-dried.

Second, despite preserving foods’ antioxidant content, the high porosity of freeze-dried products allows easy access to oxygen, which may lead to higher levels of oxidation or degradation of bioactive compounds ( 2 ，，，， 7 ).

Summary

Freeze-drying doesn’t kill bacteria present in raw food. Thus, you must cook raw foods before freeze-drying to avoid foodborne illness. The high porosity of freeze-dried foods may lead to greater degradation of nutrients.

Can you freeze-dry foods at home?

冻干需要在特定温度和压力下工作的专业且通常昂贵的设备。

但是，如果您不想投资家庭冷冻干燥机器，则仍然可以通过冰箱方法在家中冷冻干燥的食物。

Here are the steps you need to follow for the freezer method. It doesn’t require special equipment, yet it does take the longest.

1. If necessary, wash, dry, and cook the food.
2. Cut it into 1-inch squares, or as small and even as you’d like. It is best to avoid large chunks of food.
3. Place the food into a single layer on a tray and place the tray in your freezer. It’ll take roughly one week to freeze-dry foods in the freezer properly.
4. Test your food by removing a piece from the freezer and letting it thaw. If it returns to its natural or standard color, the process is complete. Yet, if the food turns black or dark brown upon thawing, it hasn’t freeze-dried completely.
5. 将您的冷冻干燥食品存储在密封容器。

Summary

You can easily freeze-dry foods at home without purchasing specialized equipment by following the freezer method.

The bottom line

Freeze-drying is a way of dehydrating frozen food via a process that transforms ice into vapor, also known as sublimation.

它is a healthy food preservation method, as it retains most of the beneficial plant compounds and nutrients in foods, as well as their color, taste, and appearance.

However, it doesn’t kill harmful bacteria. Therefore, you mustcook raw foods在冷冻干燥之前。

如果您没有家用冷冻干燥机器，则可以在家中轻松冻结自己的食物。