Freeze-Drying Curves Start at the Eutectic Point

Freeze-Drying Curves Start at the Eutectic Point

Banana has a eutectic point of -55.5°C and strawberry -22.6°C. These two numbers decide whether a freeze-drying curve can be completed at all. Once the product temperature crosses the eutectic point during the sublimation stage, the freeze-dried skeleton collapses immediately and no amount of later shelf temperature adjustment rescues it.

This article covers three things: how the eutectic point is measured, how it determines the cold trap temperature, and what the sublimation data of a real production line looks like.

What the Eutectic Point Governs

The eutectic point is the temperature at which the last drop of liquid in the product is completely frozen. Above it, ice crystals and concentrated liquid coexist; below it, the whole mass is solid.

Freeze drying only works while the product stays frozen the whole time, with ice sublimating directly into water vapour. If liquid is still present it boils under vacuum, the porous structure collapses and the finished product becomes a hard, shrunken wafer.

The eutectic point therefore sets two design targets:

1. The pre-freezing temperature must be below the eutectic point, with margin.
2. The cold trap temperature must be 10–15°C below the eutectic point to guarantee enough water vapour partial pressure difference to drive sublimation.

How to Measure It: Resistance Method or DSC

Two methods, depending on what equipment the customer has.

Resistance method. A small sample sits between two electrodes inside a freezer. As the temperature falls, the resistance jumps sharply the moment the last drop of liquid freezes, and the inflection point is the eutectic temperature. Cheap and fast, and an ordinary laboratory can do it.

DSC (differential scanning calorimetry). A few milligrams of sample are scanned through the freezing range and the exothermic peak corresponds to the crystallisation event. More precise, and it also gives the glass transition data needed for secondary drying.

We recommend measuring a sample of the actual batch rather than copying textbook values. Moisture content, sugar content and fibre all shift the eutectic point. Strawberries from different farms can differ by 2–3°C.

Real Product Data (from Our Project Files)

The cold trap rule is the same on every vacuum freeze dryer we build: the cold trap runs 10–15°C below the eutectic point of the product. A -40°C cold trap is enough for strawberry, while banana must have a -65°C cascade system. There is no universal cold trap temperature.

Real Sublimation Stage: a 1 t Shiitake Line

In our July batch analysis we reviewed a 1000 kg per batch shiitake freeze-drying line (CVD-10000, 100 m² of shelf area). The physical calculation follows:

• Moisture content about 90%, so each cycle removes 855 kg of water
• Heat of sublimation: 855 × 2830 = 2,419,700 kJ
• Continuous sublimation rate: 42.75 kg/h (calculated on 20 h of primary drying)
• Continuous cold trap load: 33.6 kW at -40°C
• Full cycle: 16–24 h

The refrigeration system has a nominal capacity of 95.4 kW (Hanbell RC2-230B × 2), covering both the 34.1 kW peak of the pre-freezing stage and the 33.6 kW continuous cold trap load with plenty of margin. On the vacuum side two Leybold vane plus Roots sets at 66 kW pull the 31.7 m³ chamber down to 60 Pa in about 26 minutes.

Compare a 175 m² strawberry line: 1764 kg of water removed per cycle, a continuous sublimation rate of 88.2 kg/h and a cold trap load of 69.3 kW at -40°C. The scale changes and the cold trap load changes with it, but the logic of the curve is the same.

Do Not Skip the Refrigeration Iron Rule

The cold trap is the heart of a freeze dryer. The water vapour sublimated from the product is condensed there. The vacuum pump only handles non-condensable gas. If the cold trap is undersized or the refrigeration fails, water vapour pours straight into the pump, the oil emulsifies, and the machine is out of action within minutes.

That is why a freeze dryer quotation always includes a refrigeration system. It is not an option; it is the component that makes sublimation thermodynamically possible.

Three-Stage Curve Design

A production-grade freeze-drying curve has three stages:

1. Pre-freezing. The shelf goes from ambient down below the eutectic point at 1–3°C/h. Fruit loading density is about 7 kg/m² and meat 8–10 kg/m². Fast freezing gives small ice crystals and a fine structure; slow freezing removes more water per cycle. It is a process trade-off.
2. Primary drying. The product is held below the eutectic point while the cold trap continuously carries away the sublimated vapour. All the sublimation rates above happen in this stage. Pressure set-point control keeps the partial pressure gradient stable.
3. Secondary drying. The shelf temperature is raised to desorb bound water, bringing the finished moisture content down to 3% or less.

FAQ

Q: Can one cold trap temperature suit all products?
A: No. The cold trap must be 10–15°C below the eutectic point of the product. Strawberry (-22.6°C) is fine with -40°C, while banana (-55.5°C) needs -65°C cascade refrigeration.

Q: Is DSC mandatory for measuring the eutectic point?
A: DSC is more accurate, but the resistance method is faster and cheaper and is sufficient for most food products. What matters is measuring the actual batch rather than copying textbook values.

Q: How long is primary drying on a 1 t shiitake line?
A: About 16–24 h per cycle, removing 855 kg of water at a continuous sublimation rate of 42.75 kg/h, with the cold trap carrying 33.6 kW continuously at -40°C.

Author: Yuanxian Machinery engineering team | Dongguan Yuanxian Food Machinery Co., Ltd.
More technical notes at www.vacuum-fresh.com. Related reading: water catcher design and the CVD series product page.