Vacuum Cooling: How Lowering the Pressure Chills Lettuce, Bread and Cooked Food

Vacuum cooling is a way of chilling food quickly by lowering the pressure around it until some of the water in the food starts to boil. Boiling takes energy, and in a sealed chamber with almost no air that energy comes mostly from the heat already in the food. As water evaporates from its surface and from inside, the food gets colder. The idea sounds paradoxical, but the physics is simple and the effect is fast. A carton of field-warm lettuce can reach near-freezing temperatures in well under an hour, compared with many hours in a cold room.

The technique is most closely linked with the fresh vegetable trade, especially iceberg and other lettuces grown in California. It changed how that industry harvests, packs and ships its crops from the late 1940s onward. It is also used for other leafy vegetables, some flowers and herbs, and increasingly for bakery products and cooked foods, where fast cooling helps quality and food safety.

This article explains the physics, the equipment, the history, which products suit the method and which do not, how it compares with other cooling methods, its costs, its drawbacks, and some common misunderstandings.

Why cooling fresh produce matters

Fruits and vegetables are alive after harvest. They keep respiring, which means they burn their own sugars, release heat and slowly deteriorate. They also lose water through their surfaces, which makes them wilt and shrivel. Both processes speed up sharply as temperature rises. The heat a crop carries in from the field is called "field heat". Removing it soon after harvest is one of the most important steps in keeping produce fresh.

The US Department of Agriculture's handbook The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks, known as Agriculture Handbook 66, makes this point in its chapter on precooling. The chapter's author is James F. Thompson of the University of California, Davis. He advises harvesting in the cool morning where possible, shading produce, and cutting the delay between picking and cooling as much as possible. Fruits and vegetables transpire and respire at high rates at field temperatures. The handbook also warns that highway trailers do not move enough air to cool produce and "should never be depended on for initial cooling". Rapid precooling has to happen before shipping.

Vacuum cooling is one of six main ways of doing this initial cooling. The others are forced-air cooling, hydrocooling, package icing, room cooling and transport cooling. Each suits different products.

The physics: boiling at low pressure

Boiling point depends on pressure

Water boils when the pressure of its vapour equals the pressure of its surroundings. At sea level, atmospheric pressure is about 101.3 kilopascals (760 millimetres of mercury), and water boils at 100 °C. Lower the surrounding pressure and water boils at a lower temperature. This is why water boils below 100 °C on a high mountain.

The table below gives standard values for the saturation vapour pressure of water: the pressure at which water boils at each temperature.

These numbers explain the operating range of a vacuum cooler. Agriculture Handbook 66 describes produce being placed in a steel vessel while vacuum pumps reduce the pressure from 760 mm Hg to 4.6 mm Hg. Water in warm produce begins to boil at 20 to 30 mm Hg, depending on its temperature. As the pressure keeps falling, the boiling continues at ever lower temperatures. At the end of the cycle the pressure is 4.6 mm Hg, where water boils at 0 °C (32 °F). If the product is held at this pressure long enough, it will cool to 0 °C.

The moment when the falling pressure first reaches the boiling pressure for the product's temperature is often called the "flash point". Before it, the pumps are mostly removing air and the product barely cools. After it, rapid evaporation begins and the temperature drops quickly.

Where the cooling comes from

Turning liquid water into vapour takes a large amount of energy, the latent heat of vaporisation. At ordinary temperatures it is roughly 2,450 kilojoules per kilogram of water. In a vacuum chamber there is almost no air to bring heat in, so most of that energy comes out of the product itself as a drop in its temperature. The product effectively cools itself by giving up some of its water.

This leads to a useful rule of thumb. Agriculture Handbook 66, citing work by W. R. Barger of the USDA in 1963, states that water loss of about 1 percent causes about 6 °C (11 °F) of product cooling. A 1963 USDA report found temperature drops of approximately 10 °F per 1 percent of moisture evaporated, regardless of crop. That is about 5.6 °C, and the two sources agree closely.

Illustrative calculation. Here is a rough calculation, using approximate values, to show where the rule comes from. Leafy vegetables are mostly water, so their specific heat (the energy needed to warm one kilogram by one degree) is close to that of water, roughly 4 kilojoules per kilogram per degree. If 1 percent of a 1-kilogram head of lettuce evaporates, that is 10 grams of water. At about 2,450 kJ/kg, it takes about 24.5 kJ to evaporate. Taking 24.5 kJ out of 1 kg of material with a specific heat of 4 kJ/kg·K lowers its temperature by about 6 °C. This simple sum matches the measured rule.

It follows that cooling lettuce from about 24 °C to about 2 °C, a drop of 22 °C, means evaporating roughly 3.5 to 4 percent of its weight. That matches the handbook's figure that produce loses 2 to 4 percent of its weight during vacuum cooling, depending on its starting temperature. The warmer the product goes in, the more water it must lose.

Why the chamber needs a condenser

A practical detail explains much of the equipment design. At very low pressure, water vapour takes up a vast volume. At 0 °C, one kilogram of saturated water vapour occupies about 206 cubic metres, from standard steam tables. A load of produce can release many kilograms of water. Pumping all that vapour out of the chamber would need enormous pumps. Instead, vacuum coolers use refrigerated condensing coils inside or attached to the vessel. These turn the vapour back into liquid water (or frost) so that it no longer takes up space. The vacuum pumps then only have to remove air and any vapour that does not condense.

Agriculture Handbook 66 points designers to methods for sizing vacuum pumps, refrigeration capacity and condensing coils. It suggests assuming a refrigerant evaporating temperature of −9 to −7 °C (15 to 20 °F) when estimating compressor horsepower.

The equipment

A typical produce vacuum cooler has these main parts:

Early commercial installations were large fixed cylinders at packing and shipping points. A 1958 USDA report on the marketing of California lettuce describes "a large air-tight cylinder, usually holding 320 cartons of lettuce". Modern coolers range from small units for a single pallet to multi-door plants, and some are portable so they can be moved between growing regions with the season.

The cycle is a batch process. Product is loaded and the doors are sealed. The pumps draw the pressure down. The product "flashes" and cools. The chamber is held at the target pressure until the product reaches the desired temperature. Then air is let back in and the load is removed. Wikipedia's article on the subject notes that the chamber must be evacuated continuously to keep the process going, and that surface area and the product's sensitivity to water loss both affect how well cooling works.

History

Lettuce, ice and the Salinas Valley

Before vacuum cooling, lettuce shipped from California was cooled with ice. A 1958 USDA Marketing Research Report by Alvin Z. Macomber, Marketing Costs and Margins for California Lettuce, describes the older system. Field crews cut lettuce and loaded it into large baskets holding 700 to 900 heads. Trucks hauled it up to 50 miles to packing sheds. There it was trimmed, graded and packed into wooden crates of 4 to 5 dozen heads, in alternating layers with ice. The crates were loaded into precooled refrigerator railway cars, and 10,000 to 20,000 pounds of "top ice" was added.

Vacuum cooling changed this. The USDA report says that commercial shipments of vacuum-cooled cartons began in 1948. A California Department of Food and Agriculture document on lettuce production places the first attempt at vacuum cooling lettuce near Salinas in 1946. Another historical account describes the first lots being cooled at a pilot plant in Salinas in 1947. Reviews of the technology commonly date the first commercial vacuum cooling plant for lettuce to Salinas, California, in 1948.

Adoption was rapid. According to the 1958 USDA report:

Field packing and a reshaped industry

Vacuum cooling made it practical to pack lettuce in the field, straight into fiberboard cartons, instead of hauling loose heads to sheds for packing with ice. The USDA report describes both "ground-pack" crews and harvesting machines that worked as moving packing houses. Cutters placed heads on the machine, packers graded and packed them into cartons on board, and the stapled cartons went straight onto a truck heading for the vacuum-cooling plant. The report states plainly that the field-packing method "was made feasible by vacuum-cooling".

The economic effects were significant. The report found that the major change in the lettuce industry over the previous ten years had been the transition from shed packing and icing to field packing and vacuum cooling. Harvesting and packing costs fell by 22 percent between 1950 and 1953, before rising 4 percent by 1956. Vacuum cooling plus standard rail refrigeration replaced top icing as the typical way to ship lettuce. The industry also reorganised: specialised vacuum-cooling companies chilled the lettuce and loaded it onto railcars or trucks, jobs the packing houses used to do.

The same report records how the trade judged the quality. Vacuum cooling brought down lettuce temperature quickly. At suitable pressures the inside of a head could reach 35 °F in 20 to 30 minutes, which helped keep it fresh for longer. There was no bruising from ice and less decay. On the other hand, ice-packed lettuce arrived at the shop looking crisp and fresh, while vacuum-cooled lettuce "may appear somewhat dry". That trade-off, speed and cleanliness against moisture loss, still shapes how the technique is used.

Industry histories credit particular growers, such as Bud Antle, with championing vacuum cooling in the early 1950s. Such accounts come from industry hall-of-fame write-ups rather than independent histories.

Later developments

Research continued on improving the process. Agriculture Handbook 66 cites work by Cheyney, Kasmire and Morris, published in California Agriculture in 1979, showing that leafy vegetables can be vacuum cooled in 20 to 30 minutes "even when the product is wrapped in plastic film". This mattered as the trade moved toward film-wrapped heads. It requires packaging that lets water vapour escape, such as perforated film or permeable wraps.

From the 1990s onward, researchers in Europe and Asia, notably Da-Wen Sun's group at University College Dublin, extended the method to cooked meats, bakery goods, ready meals and fish. Karl McDonald and Da-Wen Sun's 2000 review in the Journal of Food Engineering, "Vacuum cooling technology for the food processing industry: a review", and Sun and Liyun Zheng's 2006 follow-up, "Vacuum cooling technology for the agri-food industry: past, present and future", are standard references in that literature.

Which products suit vacuum cooling

The rule: lots of surface, loose water, open structure

Vacuum cooling works best on products that give up water vapour quickly from a large surface area relative to their mass. Agriculture Handbook 66 says vacuum and water-spray vacuum cooling "are usually reserved for crops, such as leafy vegetables, that release water vapor rapidly, allowing them to be quickly cooled". Wikipedia's summary describes the ideal product as porous and containing free water.

Dense products with a thick skin or waxy surface, such as apples, citrus, melons and tomatoes, lose water mainly through a small surface. They cool slowly and unevenly under vacuum, and forced-air cooling or hydrocooling suits them better.

Commodity notes from Agriculture Handbook 66

The commodity chapters of Agriculture Handbook 66 give practical advice on many crops. A selection:

The pattern is clear. Leafy, open, high-water crops are ideal. Crops with tighter structure or a waxy surface can be vacuum cooled if they are wetted to supply extra water to evaporate. Sensitive crops like basil should not be chilled near freezing by any method.

Cut flowers

Wikipedia notes vacuum cooling's use with flowers as well as fruits and vegetables. In practice, many cut flowers are cooled with forced air. Agriculture Handbook 66 cites cooling times of about an hour for cut flowers in forced-air tunnels.

How it compares with other cooling methods

Agriculture Handbook 66 includes a comparison table of the six common initial cooling methods, drawn from Thompson and colleagues' 1998 UC publication Commercial Cooling of Fruits, Vegetables, and Flowers. The key figures:

A few practical differences stand out:

• Speed and uniformity. Vacuum cooling is among the fastest methods for suitable products. Because evaporation happens throughout the product, including between the leaves inside a packed carton, it cools evenly. Wikipedia describes the absence of a temperature gradient as a main advantage. Forced air has to push cold air through cartons and pallets, so the centre of a load cools last.
• Packaging. Vacuum cooling works on produce already packed in cartons and on pallets, as long as the packaging lets vapour escape. Hydrocooling needs water-resistant packaging such as waxed fiberboard or plastic crates.
• Water contact and decay. Dry vacuum cooling does not wet the product. Hydrocooling uses water that can spread decay organisms between batches if it is not kept sanitised.
• Moisture loss. This is vacuum cooling's main weakness, which is why water-spray versions exist.

The competing methods in more detail

It helps to know how the alternatives work, because packers often choose between them crop by crop.

Forced-air cooling pulls refrigerated air through packed boxes or bins. In the most common layout, the tunnel cooler, two rows of pallets stand on either side of an air-return channel. A tarp is pulled over the top, and a fan draws air out of the channel, so cold room air is sucked through the packages. Agriculture Handbook 66 says cooling times run from about 1 hour for cut flowers to more than 6 hours for large fruit in airflow-restricting packs. It also explains what controls the speed: the airflow rate and the diameter of the product. Coolers often run at about 1 litre of air per kilogram of produce per second. At that rate, small grapes cool in about 2 hours, while cantaloupes need more than 5. Boxes need about 5 percent of their sidewall area as vents to let the air through. Forced-air cooling also causes some moisture loss. It is negligible for produce with low transpiration, like citrus, but can reach several percent for produce that transpires heavily.

Hydrocooling showers produce with cold water or immerses it. Water carries heat far better than air, so it is much faster. The handbook gives under 10 minutes for small produce like cherries in a well-designed shower cooler and 45 to 60 minutes for melons. It causes no moisture loss and can even refresh slightly wilted produce. But it needs packaging that tolerates water, and the recirculated water must be treated, usually with chlorine, to stop it spreading decay organisms.

Package icing packs crushed or flaked ice in with the produce. It cools quickly and keeps cooling during transport, and it maintains high humidity. It is heavy and expensive, needs water-resistant packaging, and meltwater can damage neighbouring products in a mixed load.

Room cooling simply puts warm produce in a cold store. It takes at least 24 hours and is used mainly where products tolerate slow cooling.

Choosing a method

No single method is best for everything. As a general guide drawn from the handbook's comparisons:

• Leafy, open, high-water crops such as lettuce, spinach and leafy herbs suit vacuum or hydro-vacuum cooling.
• Dense crops that tolerate water, such as many root vegetables, sweet corn and melons, often suit hydrocooling.
• Fruit, fruit-type vegetables and cut flowers, which may not tolerate wetting, usually go to forced-air cooling, the most adaptable method.
• Products whose buyers expect ice, such as broccoli in some markets, are still package-iced.

Cost and scale also matter. Hydrocoolers are the cheapest to buy. Vacuum coolers cost more but use less energy per load and pay off on high-volume leafy crops. Forced-air units are flexible and can often be added to an existing cold store.

A hypothetical cooling cycle

The following example is hypothetical. It is built from the figures above to show how a cycle unfolds, not taken from any single plant's records.

A pallet of field-packed romaine arrives at a cooler at 24 °C on a warm afternoon. The heads are dry and close to the 25 °C threshold at which the handbook advises sprinkling, so the crew have sprinkled them with clean, chlorinated water before closing the cartons. The pallet goes into the chamber and the door is sealed. As the pumps draw the pressure down from 760 mm Hg, nothing much happens at first. When the pressure reaches about 22 mm Hg, the boiling pressure for water at 24 °C, the surface water and the water in the leaves begin to flash into vapour. From then on the product temperature falls steadily, tracking the falling pressure. The refrigerated coils condense the vapour, and frost builds up on them. After about 20 to 30 minutes the pressure is near 5 mm Hg and probes in the cartons read about 1 to 2 °C. The operator lets air back in and the pallet goes into a cold store or straight onto a refrigerated truck. The heads have lost roughly 3 to 4 percent of their starting weight in water. Some of that came from the sprinkled water, not from the leaves.

Energy and economics

Agriculture Handbook 66 describes vacuum coolers as "very energy efficient" and "cost competitive if well utilized". The last phrase matters. A vacuum cooler is a substantial capital investment: a pressure vessel, pumps, a refrigeration plant and controls. It pays off when it runs many cycles a day through a busy season. Lightly used equipment has a high cost per carton.

The energy efficiency comes from the way the cooling happens. In forced-air or room cooling, fans move large volumes of air, and the refrigeration system also has to remove the heat from the fans' motors and from the cold room. In a vacuum cooler, the product cools itself by evaporation. The refrigeration system's main job is to condense the released vapour, and little energy goes into moving air.

Wikipedia's article claims that vacuum cooling uses roughly a quarter of the energy of traditional methods. That figure is not clearly sourced in the article, which has been flagged for promotional content, and should be treated with caution. The handbook's more general statement, that vacuum coolers are very energy efficient compared with other methods and forced-air coolers are the least efficient, is better supported.

Moisture loss, wilting and how to manage them

Because vacuum cooling works by evaporation, every degree of cooling costs water. For a high-value leafy vegetable sold by weight, a 2 to 4 percent loss is a real cost, and too much loss causes visible wilting. Operators use several approaches:

• Pre-wetting. Spraying produce with water before cooling gives the vacuum free surface water to evaporate first, sparing the product's own tissue water. The handbook notes that this minimises moisture loss, and its lettuce chapter recommends sprinkling dry, warm heads before closing the cartons.
• Hydro-vacuum cooling. Some coolers spray water during the cycle, which further limits weight loss. The handbook's lettuce chapter calls this modification "hydrovacuum".
• Controlling the end point. Ending the cycle at the target temperature, instead of holding the product under vacuum for longer than needed, avoids extra losses.
• Avoiding freezing. The pressure must not go so low that the product's surface freezes. At about 4.6 mm Hg water boils at 0 °C. Going lower, or holding too long, risks ice forming in tender leaves. A USDA report on vacuum-cooled lettuce sets the aim as reducing the temperature quickly to 40 °F or lower without causing freezing damage or wilting.
• Suitable packaging. Film wraps and bags must be perforated or permeable to water vapour. Otherwise vapour cannot escape and the product inside will not cool.

Published research on lettuce shows how much the operating conditions matter. In one Turkish study of vacuum-cooled lettuce, weight losses of 9.6, 5.7 and 4.9 percent were measured at final chamber pressures of 7, 10 and 15 millibars. Lower final pressure gave more cooling and more water loss. Another study found that unpackaged lettuce lost about 5 percent of its weight while packaged lettuce lost about 1.8 percent.

Food safety considerations

Water quality

Any water that touches produce must be safe, whether it is used for washing, hydrocooling or pre-wetting before vacuum cooling. Agriculture Handbook 66 states that all water that contacts produce "for washing, hydrocooling, or vacuum cooling must be potable". It recommends 2 to 7 parts per million free chlorine at pH 6 to 7, or an oxidation-reduction potential above 650 millivolts with any oxidising sanitiser. Chlorination prevents cross-contamination between batches but does not sterilise the produce.

Can vacuum cooling draw bacteria into leaves?

Researchers have asked whether the pressure changes in a vacuum cooler could pull bacteria on the leaf surface into the tissue, where washing cannot reach them. A 2008 study by Li, Tajkarimi and Osburn in Applied and Environmental Microbiology found that vacuum cooling significantly increased infiltration of E. coli O157:H7 into lettuce tissue. Treated lettuce held about 2.65 × 10⁶ colony-forming units per gram, against about 1.98 × 10⁵ without vacuum cooling. The authors suggested that the process changes leaf structure, including the stomata. A 2016 study by Vonasek and Nitin in the same journal used multiphoton imaging of intact leaves. It found that vacuum cooling did not significantly increase internalisation of surface bacteria into an intact leaf, though it did increase the number of bacteria associated with stomata. The two studies used different methods, and the question is not fully settled. Both point to the same practical lesson: keeping produce and process water clean before cooling matters.

Cooked foods

For cooked foods, speed of cooling is itself a safety measure. Bacteria multiply fastest in the warm middle range of temperatures, so the faster food passes through that range the better. The EU-funded COOL-MEAT project report notes that guidelines in Europe require cooked meat joints to be chilled from 74 °C to 10 °C within 2.5 hours of leaving the cooking process, with joints no larger than 2.5 kg or 100 mm thick. Large joints are hard to cool that fast with air, and this is one reason vacuum cooling has drawn interest.

Beyond vegetables: bakery, cooked meat and other foods

Bakery products

Bread leaves the oven very hot and must cool before it can be sliced and packed. Conventional cooling on racks takes time and floor space. BAKERpedia, a baking industry reference, describes bread leaving the oven at about 96 °C (205 °F) at sea level and says vacuum cooling can bring it to 30 °C (86 °F) in as little as 3 minutes, at about 22 °C per minute. It then allows roughly 5 to 30 minutes for moisture to even out between crust and crumb before packaging. It also notes that bakers can shorten baking time, since the vacuum stage removes moisture that would otherwise be baked off. Vendors of vacuum equipment make similar claims. These are industry figures rather than independent research.

Peer-reviewed work points the same way, with more modest claims. A Croatian study of pre-baked gluten-free bread found that vacuum cooling brought the core to 30 °C in about 7 minutes, compared with about an hour at room temperature. It reported vacuum cooling to be roughly nine times faster, with up to 2 percent more weight loss than conventional cooling.

Cooked meat

Cooked meat is a harder case. Large joints cool slowly by conventional means, but vacuum cooling them causes heavy weight loss, which means lost product and drier meat. Research reported by University College Dublin's group and others shows the trade-off:

• In a study of large cooked meat joints over 5 kg reported by Wang and Sun, vacuum cooling took under 2 hours. Water immersion took over 5 hours, air blast over 7 hours and slow air over 9 hours. But vacuum cooling caused weight loss of about 10 percent, against 6 to 7 percent for slow air and air blast.
• McDonald and Sun found that slowing the rate of pressure reduction for cooked beef, starting from 100 millibars, lengthened cooling to up to 155 minutes but raised yield by up to 1.8 percent.
• The EU COOL-MEAT project, which ran from October 2010 to September 2012 and was coordinated by Iris Technology Solutions in Spain, developed "immersion vacuum cooling". In this method meat is vacuum cooled while still in part of its cooking liquid. For hams, weight loss was about 4 to 5 percent, compared with 10 to 12 percent for ordinary vacuum cooling and about 6 percent for air blast. Cooling took longer than ordinary vacuum cooling but was about 40 to 50 percent shorter than air blast.

So vacuum cooling of meat is a balance between speed and food safety on one side and yield and texture on the other. Hybrid methods try to get the best of both.

Other foods

Sun and Zheng's 2006 review notes applications to viscous foods, bakery products and fish such as tuna, alongside vegetables and cooked meats. Ready meals, sauces, rice and some confectionery have also been studied. The same physical principle applies throughout: the food must contain free water that can evaporate, and the process must stop before the loss of water harms quality.

Advantages and disadvantages at a glance

Common misconceptions

"The vacuum itself is cold." A vacuum has no temperature of its own and is a poor conductor of heat. The cooling comes from evaporation of the product's own water, not from contact with anything cold.

"Vacuum cooling freeze-dries food." Freeze-drying also uses low pressure, but it starts with frozen food and removes most of its water over hours or days. Vacuum cooling removes only a few percent of the water and is designed to stop just above freezing.

"It works on any food." It works well only on products that release water easily. A melon or an apple will barely cool in the time a carton of lettuce takes.

"Vacuum cooling and vacuum packing are the same thing." Vacuum packing removes air from a package to slow spoilage. Vacuum cooling is a short process in a chamber that chills food by evaporation. They share a word but not a purpose.

"Lower pressure is always better." Going too low, or holding too long, increases water loss and can freeze delicate tissue. Good practice targets a final temperature, not the lowest possible pressure.

Frequently asked questions

How cold does vacuum-cooled produce get?

At the end of a typical cycle the chamber pressure is about 4.6 mm Hg, where water boils at 0 °C. Produce held at that pressure long enough will approach 0 °C. In practice operators aim for a target just above freezing that suits the crop.

How long does it take?

For leafy vegetables the handbook gives 20 to 30 minutes. The 1958 USDA report said lettuce could reach 35 °F internally in that time. The handbook's comparison table gives a broader range of 0.3 to 2 hours across products. Bread can be cooled in minutes. Large cooked meat joints take an hour or two.

Why does the produce lose weight?

The cooling comes from evaporating water, so some water has to leave the product. About 1 percent of the product's weight evaporates for every 6 °C of cooling. Pre-wetting and water-spray systems let some of that water come from added water instead of the plant tissue.

Is vacuum-cooled lettuce less nutritious?

The sources consulted here do not report that vacuum cooling reduces nutritional value. Rapid cooling generally slows the deterioration that lowers quality after harvest. The main quality effect is moisture loss, which operators manage.

Can vacuum cooling be done at home?

Not practically. It needs a strong vessel that can hold a near-vacuum, vacuum pumps and a refrigerated condenser. Laboratory and small commercial units exist, but they are specialised equipment.

Why is lettuce not simply cooled in water?

For head lettuce, water held inside the tightly packed head encourages decay, so the handbook advises against hydrocooling it. Looser, non-heading lettuces can be hydrocooled. Vacuum cooling also lets lettuce be packed in the field in fiberboard cartons, which would be weakened by soaking.

Summary

Vacuum cooling uses a basic fact of physics: water boils at lower temperatures when the pressure is lower, and boiling takes heat. Seal warm, wet, leafy produce in a steel chamber, pump the pressure down to a few millimetres of mercury, and the produce chills itself by evaporating a small share of its water. Roughly 1 percent of its weight is lost for every 6 °C of cooling. Introduced commercially in Salinas, California, around 1948, the method spread through the western lettuce industry within a decade. It replaced ice-packed crates with field-packed cartons and changed how the crop was harvested and shipped. Today it remains the preferred way to precool lettuce and many other leafy crops. Its use has widened to bread, cooked meat and other foods, where speed helps quality and food safety. Its limits are as clear as its strengths: it suits only products that release water readily, it costs some weight and moisture, and it needs costly equipment that pays off only when kept busy.

Sources

• Vacuum cooling – Wikipedia
• USDA Agricultural Research Service – The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks (Agriculture Handbook 66), including J. F. Thompson, "Precooling and Storage Facilities"
• Macomber AZ (1958). Marketing Costs and Margins for California Lettuce. USDA Marketing Research Report No. 225 (AgEcon Search)
• Barger WR (1963). Vacuum cooling: a comparison of cooling different vegetables. USDA Marketing Research Report 600 (AgEcon Search)
• California Department of Food and Agriculture – Lettuce Production in California
• McDonald K, Sun D-W (2000). Vacuum cooling technology for the food processing industry: a review. Journal of Food Engineering 45(2):55–6500041-8)
• UC Davis Postharvest Center – Commercial Cooling of Fruits, Vegetables, and Flowers (Thompson et al.)
• CORDIS – COOL-MEAT: A novel method for improving the vacuum cooling of cooked meats, final report summary
• Li H, Tajkarimi M, Osburn BI (2008). Impact of vacuum cooling on Escherichia coli O157:H7 infiltration into lettuce tissue. Applied and Environmental Microbiology 74(10):3138–3142 (PMC)
• BAKERpedia – Advantages of vacuum cooling
• Quality of vacuum cooled gluten free bread (CROSBI record)