Vacuum Freeze Dryer Application Technology Analysis

Introduction

Vacuum freeze drying — also called lyophilization — removes water from food by turning ice directly into vapor. No liquid phase. The material stays frozen throughout primary drying. This preserves cell structure, nutrients, color, and flavor better than any hot-air drying method.

A freeze-dried strawberry keeps its shape. It weighs 90% less than fresh. Bite into it and it crunches, then dissolves. Rehydrate it and it looks and tastes close to the original. Hot-air drying cannot do this. Spray drying cannot either.

The global freeze-dried food market has grown steadily. Food processors in Southeast Asia, Europe, and North America are expanding freeze drying lines. The main drivers: consumer demand for clean-label snacks, pet food with real meat content, and ingredients that ship cheaply and store at room temperature.

This article covers how vacuum freeze drying works, what process parameters matter for different foods, and where the CVD series fits.

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Core Technology

Sublimation Principle

Freeze drying has three stages:

1. Freezing — The product must be frozen below its eutectic point (the temperature where everything solidifies). If you start vacuum before reaching this point, the product boils under vacuum and collapses.

2. Primary drying (sublimation) — Vacuum is pulled to 10-100 Pa. Heat is applied through the shelves. Ice crystals sublimate — solid to gas, skipping liquid. The vapor travels to the condenser where it refreezes as frost.

3. Secondary drying (desorption) — After visible ice is gone, the remaining bound water is driven off by raising shelf temperature to 40-80°C. Final moisture content: 2-5%.

The driving force is the vapor pressure difference between the product and the condenser. Higher product temperature = higher vapor pressure = faster drying. But you cannot exceed the eutectic point of the product, or it melts and collapses.

Refrigeration System

All CVD freeze dryers use a dedicated refrigeration system with the following design:

• Condenser (cold trap) temperature: -40°C to -50°C during operation. Some models go to -65°C for products with very low eutectic points.
• Compressors: BITZER or Hanbell (Hanbell RC2 series commonly used). For larger units, two compressors run in parallel.
• Refrigerant: R404A. The system includes an oil separator, air-cooled or evaporative condenser, expansion valves (Danfoss), and a liquid-vapor separator.
• Cooling method: Direct expansion or indirect (circulating glycol/ethanol mixture).

The refrigeration system serves two purposes simultaneously: freezing the product on the shelves and maintaining the condenser at low temperature. Most CVD machines use a shared system with separate circuits.

Vacuum System

The vacuum system maintains 10-100 Pa during sublimation. Components:

• Primary pump: Leybold rotary vane pump (旋片泵) or Busch screw pump
• Booster pump: Roots pump (罗茨泵) — kicks in automatically when vacuum reaches ~1,000 Pa
• Valves: Pneumatic butterfly valve between chamber and condenser
• Vacuum gauge: Pirani or capacitive gauge, displayed on the PLC touch screen

The chamber and condenser are made of 304 stainless steel. All joints use vacuum sealing. Leak rate must stay below 0.02 Pa·m³/s for reliable operation.

Heating System

Contact heating through hollow aluminum shelves. A heated water-glycol mixture (60-120°C) circulates through the shelves. The flow rate and temperature are PLC-controlled. Shelf temperature range: -55°C to +120°C. During primary drying, shelves are typically set 5-10°C above the product’s eutectic point.

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Application Cases

Freeze-Dried Fruits — Strawberries and Mangoes

Strawberries are the most common freeze-dried fruit in the snack market. Whole strawberries, sliced, or diced.

• Eutectic point of strawberry: -22.6°C
• Pre-freezing temperature: -30°C to -35°C (hold 2-3 hours)
• Shelf temperature during primary drying: -10°C to -5°C (ramp up slowly)
• Condenser temperature: -45°C
• Vacuum: 20-50 Pa
• Primary drying time: 14-18 hours
• Secondary drying: 40°C shelf, 2-4 hours
• Final moisture: ≤4%
• Yield per batch (CVD-1000, 10m²): ~1,000 kg fresh → ~100 kg dried

Mangoes have higher sugar content. Sugar depresses the eutectic point and slows sublimation. Pre-treatment is often needed — some processors dip slices in calcium chloride solution to firm the cell wall.

• Eutectic point of mango (varies by variety): approximately -24°C to -28°C
• Pre-freezing: -35°C
• Shelf temperature: -8°C to -2°C
• Total cycle: 20-26 hours
• Loading density: 8-12 kg/m²

Mango slices come out porous and sweet. The main issue is stickiness — high fructose absorbs moisture fast during packaging. Nitrogen flushing is mandatory.

Vegetables — Mushrooms

Mushrooms (shiitake, button, oyster) are excellent candidates for freeze drying. They have high moisture content (85-92%) and a spongy structure that survives freeze drying well.

• Eutectic point of mushroom (straw mushroom): -33°C
• Pre-freezing: -35°C to -40°C
• Shelf temperature: -5°C to 0°C (mushrooms tolerate slightly higher temperatures than fruits)
• Condenser temperature: -45°C
• Vacuum: 30-60 Pa
• Primary drying: 12-16 hours (mushrooms dry faster than fruits because of porous structure)
• Secondary drying: 50°C, 3-4 hours
• Final moisture: ≤5%
• Rehydration ratio: 1:6 to 1:8 (one kg dried yields 6-8 kg after rehydration)

Real project data — a 50m² CVD-5000 system processing shiitake mushrooms handles ~500 kg fresh per batch. The customer runs one cycle per day.

Pet Food

Freeze-dried pet food is the fastest-growing segment in freeze drying. Raw meat, organs, and bone ground and formed into nuggets or patties, then freeze-dried. The process preserves proteins and enzymes without cooking.

Key parameters differ from produce:

• Meat-based formulations have eutectic points around -15°C to -25°C (depends on salt and fat content)
• Pre-freezing: -30°C to -40°C (fat content requires deeper freezing)
• Shelf temperature: -5°C to +5°C (very slow ramp — fat melts if too hot)
• Condenser temperature: -45°C to -50°C
• Vacuum: 15-40 Pa
• Primary drying: 18-24 hours
• Secondary drying: 50-60°C, 4-6 hours
• Final moisture: ≤5%

The challenge with pet food is the fat content. Fat does not freeze. Under vacuum, liquid fat can migrate to the surface and oxidize. The trick is to keep the product temperature below the fat melting point during primary drying. For chicken-based formulations, this means staying below 10°C shelf temperature until most free water is gone.

CVD-1000 units (10m²) are popular with mid-size pet food producers. A single machine produces ~100 kg dried output per batch. Most run 2 batches per day.

Functional Foods and Ingredients

Freeze drying is used for probiotics, enzyme preparations, herbal extracts, and nutritional powders. These products are heat-sensitive and require precise temperature control.

• Probiotic powders: freeze at -40°C, dry at condenser -50°C, shelf max +30°C
• Herbal extracts (ginseng, cordyceps): eutectic point -25°C to -30°C
• Tea powder (matcha, instant tea): common project — 50m² CVD-5000 used for tea powder production
• Papain enzyme: projects delivered for enzyme drying at controlled low temperatures

For functional foods, freeze drying preserves bioactive compounds that hot drying destroys. The trade-off is longer cycle times — 24-30 hours for some probiotic products — and lower loading density (6-8 kg/m²).

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Product Reference — CVD Series

The CVD series covers the range from lab/R&D to full production:

Model	Drying Area	Capacity (fresh)	Shelves	Shelf Temp Range	Power
CVD-1.0W	1 m²	10 kg	8	-55°C ~ 120°C	6 kW
CVD-2.0W	2 m²	20 kg	12	-55°C ~ 120°C	9 kW
CVD-5.0E	5 m²	50 kg	12	-55°C ~ 120°C	16 kW
CVD-10E	10 m²	100 kg	18	-55°C ~ 120°C	28 kW
CVD-20E	20 m²	200 kg	39	-55°C ~ 120°C	56 kW
CVD-30E	30 m²	300 kg	55	-55°C ~ 120°C	70 kW
CVD-5000	50 m²	500 kg	—	-55°C ~ 120°C	—

All models use 304 stainless steel for the chamber and condenser. The control system is PLC + touch screen with temperature curve recording and USB data export.

Standard refrigeration: BITZER or Hanbell compressors running R404A. Vacuum: Leybold rotary vane pumps. Heating: water-glycol circulation through hollow aluminum shelves.

For full technical data sheets, visit the product page at www.vacuum-fresh.com.

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FAQ

Q1: What is the difference between a vacuum freeze dryer and a food vacuum cooler?

A food vacuum cooler uses evaporative cooling under vacuum to rapidly drop the temperature of vegetables or cooked food. A vacuum freeze dryer, on the other hand, uses refrigeration to freeze the product first, then sublimates the ice under vacuum. They are completely different machines serving different purposes — the CVD freeze dryer series is for dehydration, not cooling.

Q2: How long does one freeze drying cycle take?

For most fruits and vegetables, 16-24 hours total. Pet food with high fat content takes 22-28 hours. Functional foods and probiotics can take up to 30 hours. The actual time depends on product thickness, moisture content, and eutectic point.

Q3: What vacuum level is needed for freeze drying?

10-100 Pa (0.1-1.0 mbar). Most products run at 20-50 Pa. The vacuum must be low enough for ice to sublimate at the product temperature. Higher vacuum (lower pressure) speeds drying but costs more in pumping equipment.

Q4: Can I freeze dry different products in the same machine?

Yes, but not in the same batch unless their eutectic points and drying curves are similar. Each product needs its own freeze drying recipe — pre-freezing temperature, shelf temperature ramp, and drying time. The CVD series stores multiple recipes on the PLC. You switch recipes between batches.

Q5: What is the shelf life of freeze-dried food?

Properly packaged (nitrogen flushed, moisture barrier) freeze-dried food lasts 15-25 years at room temperature for low-fat products. Pet food with high fat content has a shorter shelf life — 5-10 years — because fats oxidize slowly even in low-oxygen packaging.

Q6: Does the refrigeration system run continuously during the entire cycle?

Yes. The condenser must stay at -40°C to -50°C throughout primary drying to trap the vapor. During secondary drying (desorption), when vapor load drops, the condenser temperature rises slightly but the refrigeration system still runs. The BITZER/Hanbell compressors in CVD machines are designed for continuous operation.

Q7: How much fresh product fits per square meter of shelf area?

Standard loading: 8-12 kg/m² for fruits and vegetables. Pet food: 6-10 kg/m². Functional powders: 6-8 kg/m². Thicker loading slows drying and may cause incomplete sublimation.

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中文总结

真空冻干技术是食品工业中保留营养成分和原有形态最有效的脱水方式。核心原理是在真空条件下将冻结产品中的冰直接升华，跳过液态阶段。CVD系列冻干机涵盖1m²至50m²加热面积，采用汉钟/BITZER压缩机组（R404A制冷剂）提供-45℃冷凝温度，配合莱宝真空泵维持20-50Pa工作真空。草莓（共晶点-22.6℃）、香菇（共晶点-33℃）、宠物食品及功能性食品是主要应用方向。单批次干燥周期16-28小时，装载密度6-12kg/m²。304不锈钢箱体、PLC触摸屏控制。设备均配备完整制冷系统，不可与真空预冷设备混淆。更多技术参数请访问源鲜官网 www.vacuum-fresh.com。