Vacuum Cooling & Time & the Weight You Lose
Computes cycle time
Pick the commodity and enter the field heat and load, and read the cycle time, the percent weight (water) loss — about 1% per 6 °C — the kilograms evaporated and the chamber pressure, plus whether the crop even suits vacuum cooling.
Vacuum-cool a load
Next: run the cycle to 1°C (≈ 22 min, chamber near 0.66 kPa). Pre-wet the load first so the ≈3.87% water loss comes from added spray, not the product's own moisture.
Vacuum cooling: water flash-evaporates as chamber pressure falls along the saturation curve; the latent heat cools the product. Energy balance: weight loss fraction = c_p·ΔT / L ≈ 3.95·ΔT / 2450 ≈ ~1% per ~6 °C. Suitability scales with surface-area-to-volume (leafy = excellent, dense fruit = poor). Sources: Thompson et al., UC ANR 21567; ASHRAE Refrigeration; Sun & Zheng 2006.
Vacuum cooling — key facts
- Weight loss
- ≈ 1% per ~6 °C drop
- Loss formula
- fraction = c_p·ΔT ÷ L ≈ 0.0016·ΔT
- Cooling
- flash-evaporation of surface water
- Cycle time
- ≈ 20–30 min for leafy crops
- Chamber pressure
- ≈ 0.66 kPa to reach 1 °C
- Best for
- leafy / high-surface-area crops
- Unsuitable for
- dense fruit & roots (low surface)
- Privacy
- Runs in your browser; nothing uploaded
Commodity suitability for vacuum cooling
Vacuum cooling suits high-surface-area produce. Leafy crops cool fast and evenly; dense fruit and roots cool slowly and lose surface moisture without cooling the centre.
| Commodity | Suitability | Cool rate (°C/min) | Target pulp (°C) | Pre-wet |
|---|---|---|---|---|
| Lettuce (crisphead) | excellent | 1.1 | 1 | yes |
| Leaf / romaine lettuce | excellent | 1.2 | 1 | yes |
| Spinach | excellent | 1.3 | 0 | yes |
| Baby-leaf greens | excellent | 1.3 | 1 | yes |
| Celery | good | 0.8 | 0 | yes |
| Cabbage | fair | 0.4 | 0 | no |
| Cauliflower | good | 0.7 | 0 | yes |
| Broccoli | good | 0.9 | 0 | yes |
| Sweet corn | fair | 0.5 | 0 | yes |
| Mushroom | good | 0.9 | 1 | no |
| Leek / green onion | good | 0.8 | 0 | yes |
| Parsley / herbs | excellent | 1.2 | 0 | yes |
| Asparagus | good | 0.8 | 2 | yes |
| Cut flowers (leafy) | good | 0.9 | 1 | no |
| Carrot (topped) | poor | 0.25 | 0 | no |
| Potato | poor | 0.2 | 7 | no |
| Tomato | poor | 0.25 | 10 | no |
| Apple | poor | 0.2 | 0 | no |
| Strawberry | fair | 0.5 | 0 | no |
Water saturation pressure (chamber target)
| Pulp temperature (°C) | Saturation / chamber pressure (kPa) |
|---|---|
| 30 | 4.25 |
| 25 | 3.17 |
| 20 | 2.34 |
| 15 | 1.71 |
| 10 | 1.23 |
| 5 | 0.87 |
| 2 | 0.71 |
| 0 | 0.61 |
Sources: Thompson et al. (2008) "Commercial Cooling of Fruits, Vegetables, and Flowers", UC ANR Pub 21567 (~1% weight loss per ~6 °C); ASHRAE Handbook—Refrigeration; Sun & Zheng (2006) "Vacuum cooling technology for the agri-food industry"; CRC steam tables. Values are representative planning figures.
The signature trade-off: speed for water
Vacuum cooling is the fastest way to pull field heat out of leafy produce — but it cools by boiling water off the product under reduced pressure, and that water is weight the produce never gets back. The physics is unavoidable: the latent heat needed to evaporate enough water to cool the product means roughly 1% of weight is lost for every 6 °C of temperature drop. Cool lettuce from a hot field to near freezing and you will shed about 4% of its mass as water.
That is the trade-off every other cooling calculator skips. This tool shows it directly: a vacuum chamber with a falling-pressure gauge, the pulp-temperature curve dropping to the target, and a weight-loss bar that grows with each degree removed. It also tells you whether the commodity even belongs in a vacuum cooler — and when to pre-wet so the lost water comes from a surface spray rather than the produce itself.
How to use it in five steps
- 1Pick the commodity
The tool sets the target pulp temperature, cooling rate and vacuum-cooling suitability.
- 2Enter the field heat
Measure and enter the incoming pulp temperature.
- 3Enter the load mass
Enter the mass of produce in the chamber.
- 4Read time and weight loss
Read the cycle time, percent weight loss, kilograms of water lost and chamber pressure.
- 5Pre-wet and decide
Pre-wet leafy loads to limit loss, or switch methods if the commodity is unsuitable.
Frequently Asked Questions
How much weight does produce lose in vacuum cooling?+
Vacuum cooling removes heat by flash-evaporating water, so the product loses water as it cools — roughly 1% of weight for every 5 to 6 °C of temperature drop. From an energy balance, the loss fraction equals the specific heat times the temperature drop divided by the latent heat of vaporisation (about 3.95 × ΔT ÷ 2450), which works out to about 1% per 6 °C. Cooling lettuce from 25 °C to 1 °C therefore costs roughly 4% of its weight.
How does vacuum cooling actually cool the produce?+
Lowering the chamber pressure lowers the temperature at which water boils. As the pressure falls along the water saturation curve, water flash-evaporates from the produce surface, and the latent heat carried away by that vapour cools the product. To cool to a given pulp temperature the chamber must reach roughly the saturation pressure of that temperature — about 0.66 kPa to reach 1 °C.
Which crops are suitable for vacuum cooling?+
Vacuum cooling suits produce with a high surface-area-to-volume ratio: leafy vegetables such as lettuce, spinach, baby-leaf greens and herbs cool fast and evenly. Celery, broccoli, cauliflower, mushrooms and leeks are good candidates. Dense, low-surface-area items — carrots, potatoes, apples, tomatoes — cool slowly and unevenly and are unsuitable; use forced-air or hydrocooling for those.
How long does a vacuum-cooling cycle take?+
For leafy crops, a vacuum-cooling cycle typically takes about 20 to 30 minutes to reach near 0 to 1 °C — one of the fastest precooling methods available. The tool estimates the cycle from the temperature drop and the commodity's cooling rate (around 1.1 to 1.3 °C per minute for leafy crops, much slower for dense items).
What is pre-wetting and why does it matter?+
Pre-wetting is spraying water onto the produce before or during the vacuum cycle. Because vacuum cooling removes about 1% of weight per 6 °C as water, pre-wetting supplies that water from a surface spray instead of from the product's own tissue — limiting wilting and weight loss. It is recommended for most leafy crops; the tool flags when it applies.
Why can't dense produce be vacuum-cooled?+
Vacuum cooling removes heat from the surface as water evaporates, so it works best when there is lots of surface relative to volume. A dense fruit or root has little surface per unit mass and high internal resistance to heat flow, so the surface cools while the centre stays warm — uneven, slow and inefficient. The energy that would cool it instead just strips surface moisture.
What chamber pressure does vacuum cooling reach?+
Very low — around 0.6 to 0.9 kPa absolute (roughly 5 to 7 torr), close to the saturation pressure of water at the target pulp temperature. To boil water at 1 °C the chamber must fall to about 0.66 kPa; at 5 °C about 0.87 kPa. The tool shows the target chamber pressure for your chosen pulp temperature.
How do I calculate the kilograms of water lost?+
Multiply the load mass by the fractional weight loss: kg lost = mass × (specific heat × temperature drop ÷ latent heat) = mass × about 0.0016 × ΔT. For a 1000 kg load cooled 24 °C, that is roughly 1000 × 0.039 ≈ 39 kg of water evaporated.
Is vacuum cooling faster than forced-air cooling?+
Yes, for leafy crops. Vacuum cooling has one of the highest effective cooling rates because it removes heat from the entire wetted surface at once, cooling lettuce in 20 to 30 minutes versus an hour or more for forced-air. The trade-off is the weight loss and the limitation to high-surface-area commodities.
Is 4% weight loss acceptable for vacuum-cooled lettuce?+
It is on the high side but typical for a large temperature drop. About 1% loss per 6 °C is unavoidable physics, so cooling from 25 °C to 1 °C (a 24 °C drop) loses about 4%. Pre-wetting offsets most of it by supplying surface water; without pre-wetting, a 4% loss shows as visible wilting, so most operations pre-wet leafy loads.
Is anything uploaded?+
No. The calculation runs entirely in your browser using the vacuum-cooling energy balance, the water saturation-pressure table and the built-in commodity suitability data. Nothing you enter is sent anywhere.