Freeze drying, also known as lyophilization, is a dehydration process that works by freezing a substance and then reducing the surrounding pressure to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase.
This allows the water to be removed from the material being freeze dried without passing through the liquid phase. The result is a dehydrated product that retains its original shape and structure as well as most of its nutritional content.
Freeze drying was originally developed in the early 20th century to help preserve food and biological materials. Today it has a wide range of applications from preserving food and pharmaceuticals to creating instant coffee.
When it comes to dairy products, freeze drying can be used to create milk powder as well as instant yogurt, ice cream mixes, cheese powders and more. But how well does it work for milk in particular? Can milk be successfully freeze dried? Let’s take a closer look.
How Freeze Drying Works on Milk
To understand if milk can go through the freeze drying process, it helps to first understand exactly what happens during freeze drying:
1. The milk is frozen, usually to temperatures below -40°C.
2. The pressure is lowered, allowing the frozen water in the milk to sublimate from solid ice directly to water vapor.
3. This sublimation continues until almost all moisture is removed, leaving behind only about 1-4% residual moisture.
4. The dried milk maintains its original shape and physical structure.
So in theory, yes, milk can be freeze dried just like many other food products. The extremely low temperatures prevent damaging casein micelle aggregation while the sublimation removes almost all water content.
However, there are some challenges with freeze drying dairy products like milk:
Challenges of Freeze Drying Milk
– Fat separation: The high fat content of whole milk (3-4%) can lead to fat separation issues during the freeze drying process.
– Foaming: Dissolved gases and air bubbles create foaming and puffing.
– Protein denaturation: Casein and whey proteins are vulnerable to freeze damage.
– Lactose crystallization: Lactose sugar can crystallize, creating unwanted graininess.
– Oxidation: Milk fat oxidizes quickly when exposed to air.
Solutions
Fortunately, there are solutions for overcoming these challenges:
– Using skim or low fat milk prevents fat separation issues.
– Adding anti-foaming agents can prevent foaming.
– Fast freezing and optimal freeze drying temperatures preserve proteins.
– Annealing the frozen milk prevents lactose crystallization.
– Using antioxidants prevents oxidation of milk fat.
– Encapsulation protects the dried milk from air exposure.
So while freeze drying milk presents some hurdles, the process can certainly be engineered to work effectively.
Nutritional Comparison of Freeze Dried vs Regular Milk
One concern when freeze drying milk is whether it retains the same nutritional value as fresh milk. Let’s compare them:
| Nutrient | Regular Milk (whole) | Freeze Dried Milk (reconstituted) |
|---|---|---|
| Water | 87.8 g | 87.5 g |
| Protein | 3.15 g | 3.10 g |
| Fat | 3.25 g | 3.20 g |
| Carbs | 4.8 g | 4.7 g |
| Calcium | 276 mg | 270 mg |
| Vitamin B12 | 0.45 mcg | 0.40 mcg |
As you can see, freeze dried milk is very similar in nutritional content to regular fluid milk. The freeze drying process preserves most of the proteins, fat, carbohydrates, vitamins and minerals found in fresh milk.
The main difference is the lower water content. But when the dried milk powder is reconstituted with water, the nutritional profile closely matches that of milk before drying.
Taste and Texture of Reconstituted Freeze Dried Milk
In addition to nutritional content, the taste and texture of reconstituted milk is an important factor to consider with freeze drying:
Taste
– Natural sweetness is concentrated
– Can have a flat, cooked flavor if proteins are damaged
– Oxidized flavors if fats are exposed to air
Texture
– Smooth, creamy texture can be achieved
– Grainy or chalky if lactose crystallizes
– Watery if fats are lost
– Foamy if contains excess air
With optimal freeze drying methods, reconstituted milk can taste quite similar to fresh milk. But sub-par techniques can create off-flavors and textures.
Using fast freezing, annealing, encapsulation and gentle rehydration are keys to maximizing quality.
Types of Freeze Dried Milk
There are a few different types of milk that can be freeze dried:
Whole Milk
– Highest fat content (~4%)
– Richest taste
– Can have fat separation issues
2% Reduced Fat Milk
– Contains 2% fat
– Compromise of taste and issues
Skim (Non-Fat) Milk
– No fat content
– Avoid fat related problems
– Less rich taste
For optimal freeze drying, skim milk avoids fat issues. But milk powder made from 2% milk offers a better taste.
Cost of Freeze Dried vs Powdered Milk
Freeze dried milk is more expensive than traditional spray dried powdered milk. Here’s a comparison:
| Milk Type | Cost |
|---|---|
| Regular Milk | $3.50/gallon |
| Powdered Milk | $0.50/quart equivalent |
| Freeze Dried Milk | $3.00/quart equivalent |
The higher cost of freeze dried milk can be attributed to:
– High energy requirements of the freeze drying process
– Low batch yields compared to spray drying
– Precise processing conditions needed
– Costs of cold storage
For most uses, traditional powdered milk offers the best value. But freeze dried milk provides quality that more closely resembles fresh milk.
Storage and Shelf Life of Freeze Dried Milk
One of the major advantages of freeze dried milk is its long shelf life. Here is how its storage life compares:
| Milk Type | Unopened Shelf Life | Opened Refrigerated Life |
|---|---|---|
| Regular Milk | 2 weeks | 7 days |
| Powdered Milk | 1 year | 3 months |
| Freeze Dried Milk | 20-25 years | 6-12 months |
The low moisture content allows freeze dried milk to be shelf stable for decades. It lasts over 200 times longer than fresh milk. This makes it ideal for food storage and emergency preparedness.
Once opened, it still lasts 6-12 months when properly re-sealed and refrigerated.
Applications of Freeze Dried Milk
Some of the main applications for freeze dried milk powder include:
Food Storage & Emergency Preparedness
– Dairy source with 20+ year shelf life
– Great for food storage programs
Backpacking & Hiking
– Lightweight and compact
– Add to boiled water for instant milk
Baking & Cooking
– Add to recipes in place of fresh milk
– Doesn’t change volume or moisture
Infant Formula
– Retains nutrients lost in other drying methods
– Mix into bottles while traveling
Smoothies & Shakes
– Rehydrate for instant milk
– Use for higher protein shakes
The extremely long shelf life along with the convenience make freeze dried milk useful in many situations where fresh milk isn’t practical.
Conclusion
Freeze drying allows removing water from milk while preserving most of its nutritional content, texture and flavor. With optimal processing techniques, freeze dried milk can closely match fresh milk when reconstituted.
Though costs are higher, the much longer shelf life and convenience of storage and transport make it worthwhile for many applications. Freeze dried milk isn’t likely to replace fresh fluid milk anytime soon, but it certainly fills an important niche.
So in summary, yes milk can successfully be freeze dried with results far superior to traditional powdered milk in terms of quality and shelf life. While it still has some limitations, freeze drying enables milk to be stored for years and then rehydrated into a highly nutritious product.