Most of the head lettuce production in the US comes from California and Arizona. The total production is over 100,000 acres. In some areas, lettuce is grown all year on an industrial scale. Almost all lettuce is harvested and packed in the field by workers using “mule trains” as shown in Figure 3d-1. As the mule train moves through the field, heads of lettuce from as many as 20 rows at a time are cut, trimmed, wrapped in plastic film, boxed, and palletized. Immediately after coming from the field, the lettuce is moved to a central location where it is rapidly vacuum cooled and then shipped all over the country in reefers. This process is a very fast, efficient operation that has been perfected over many years by a steady year-long demand for head lettuce. Cooled, packaged head lettuce can be on the way to market within several hours of harvest. For use with produce such as head lettuce, vacuum coolers are extremely fast and effective. Vacuum coolers are expensive and uncommon in many produce growing regions. However, in regions where suitable crops are grown in large enough volume, vacuum coolers are widely used.

In recent years, several equipment manufacturers have begun making smaller, less expensive vacuum coolers. Some of these units are small enough to be portable and can be utilized in the field for vegetables and certain floral crops.

Figure 3d-1. Harvesting head lettuce with a “mule train.”

Source: M. Boyette.

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Figure 3d-1. Harvesting head lettuce with a “mule train.”

Source: M. Boyette.

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The process of vacuum cooling is unique among produce cooling methods because it does not rely on heat transfer to remove the heat. Instead, vacuum cooling operates on the principle of evaporative cooling from a small amount of moisture contained in the produce. To thoroughly cool a head of lettuce with forced convection would require many hours because the air spaces between the leaves act as insulation for the conduction of heat. However, with vacuum cooling, the cooling can be completed in 20 to 30 minutes because the air spaces act as conduits that allow the vacuum to reach all parts of the head. The cooling is just as fast in the center of the head as on the outside. Being wrapped in plastic and inside a fiberboard carton is no hindrance to vacuum cooling each head of lettuce.

Vacuum cooling installations are available in a variety of sizes, although the basic components are similar. There is a vacuum chamber that is a large, reinforced metal tube with doors on one or both ends that are sealed to be airtight. Some large vacuum coolers have internal tracks or roller conveyors for loading and unloading. There is also a vacuum pump connected to the vacuum chamber that is operated by a large electric motor or a diesel engine. The pump is capable of reducing the air pressure inside the vacuum chamber to below 0.20 atmospheres. There are controls for monitoring and controlling the entire process.

The evaporation of water requires the input of heat energy. With a vacuum cooler, the heat comes from the sensible heat in the produce. This is known as the energy of vaporization. For water, this decreases with increasing temperature as shown in Figure 3d-2. It ranges from 1071 Btu/lb at 40°F to 1037 Btu/lb at 100°F (the normal range of postharvest cooling).

At normal atmospheric pressure (1 atmosphere), the boiling point temperature of water is 212°F. However, the boiling temperature of water is very dependent on pressure as shown in Figure 3d-3. The lower the air pressure, the lower the boiling temperature. The lowered air pressure causes the water in the produce to evaporate rapidly, although at lower pressures and temperature, it still requires over 1000 Btu/lb. The only source of heat energy available is the sensible heat in the water and produce. Therefore, as the water evaporates, the produce is cooled and the sensible heat is changed to latent heat. The generated water vapor is continuously evacuated from the vacuum chamber into the atmosphere. The final product temperature can be controlled precisely through the regulation of the final internal pressure and time in the chamber.

Figure 3d-2. Values of energy of vaporization as a function of temperature.

Source: M. Boyette.

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Figure 3d-2. Values of energy of vaporization as a function of temperature.

Source: M. Boyette.

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Figure 3d-3. Relationship between temperature and pressure.

Source: M. Boyette.

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Figure 3d-3. Relationship between temperature and pressure.

Source: M. Boyette.

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A typical vacuum cooling cycle begins with the field warm produce being loaded into the vacuum chamber and closing the door. The vacuum pump is started and the pressure is reduced to the saturation pressure that corresponds to the initial temperature of the produce. For example, from Figure 3d-3, if the initial temperature was 85°F, the pressure would be reduced to approximately 0.05 atmospheres. This initial evacuation stage usually requires 7 to 10 minutes depending on the size of the vacuum chamber and the capacity of the vacuum pump. The flash point of the vacuum cooling process occurs when the vacuum chamber pressure reaches the saturation pressure that corresponds to the initial produce temperature. Until this pressure is reached, little or no cooling is achieved. Upon reaching the flash point, water begins evaporating rapidly and lowering the sensible temperature. (At 85°F, the energy of vaporization is 1045 Btu/lb.) Cooling continues until it reaches the pre-determined product storage temperature. The process is finished by opening the ventilation valve that allows air to be re-admitted into the chamber. The door is then opened and the produce is removed and sent on its way to market.

The rate at which a certain product can be cooled is directly related to the ratio of surface area to volume, the condition of the surface, density of its tissue, and the amount of required temperature reduction. Typical cooling times range from 20 to 30 minutes at a temperature drop of 45 F° (from 85°F to 40°F). In addition to lettuce and leafy greens, vacuum cooling is especially useful for cooling mushrooms, which are porous and have a low moisture content. Cut flowers also respond very well to vacuum cooling as do broccoli and cauliflower.

The moisture evaporated from the produce during the cooling process results in a measurable weight loss. The average moisture loss is 1% for each 10 degrees of drop in Fahrenheit temperature. For some products, this may not be noticeable. For other types of produce such as leafy greens, the results are noticeable by the unacceptable wilting (loss of cell turgor.) In addition, since produce is most often sold by weight, a 3.6% loss in weight reduces the profit margin. To reduce wilting, hydrovac systems have been developed that spray a light mist of clean water over the produce before or during the vacuum cooling cycle.

Disadvantages. The process of vacuum cooling is not suitable for many produce items, and is limited to produce with a surface that readily allows the evaporation of moisture. Most fruits, which are covered with a thick skin that resists the loss of moisture, are not suitable for vacuum cooling. Bell peppers, eggplant, and tomatoes have been known to split during vacuum cooling. The very rapid evaporation of moisture occurs during vacuum cooling both inside and on the surface of the product. Thus, vacuum cooling can have a significant effect on the internal texture and structure of certain types of produce.

Energy analysis of vacuum cooling. Heads of lettuce are uniform and weigh about one pound. Many larger items that are uniform are packed by count and not by weight. A typical 24-count carton of head lettuce weighs 24 lb and measures 23 in. by 15 in. by 12 in. A full pallet would include 48 cartons with a total pallet weight of approximately 1150 lb net. A standard 48 ft reefer can hold about 24 pallets with a total net weight of approximately 29,900 lb (see Figure 3d-4).

Suppose you have a vacuum cooler that will hold a full reefer load. The lettuce is pulped coming from the field at 85°F and will be cooled to 40°F. How much water weight will be lost during vacuum cooling?

The first question to answer is how many Btu’s must be extracted from the lettuce to reduce its temperature by 40 F°? The specific heat of lettuce is 0.96.

The field heat equation is: Qf = m · Cp (ΔT)

Where:

Qf = total field heat (Btu)

m = mass or produce = 29,900 lb

Cp = specific heat of lettuce = 0.96 Btu/lb·F°

(ΔT) = 85°F − 40°F = 45 F°

\(Qf=m·Cp\ (ΔT)=(29,900\ lb)\ (0.96\ Btu/lb·F°)\ (45\ F°)\)

Qf = 1,291,680 Btu

If the heat of vaporization of water at 85°F is 1045 Btu/lb, then

Qf/1045 Btu/lb = approximately 1236 lb of water.

What is the percent weight loss?

(1236) ÷ (29,900) = 0.038 or about 4.1%

Given that the average moisture loss is 1% for each 10 F° temperature drop (∆T):

∆T = (4.1) (10) = 4.1%

Thus, there is about a 1% weight loss for each 10 F° drop in temperature.

Energy efficiency. Hydrocooling has poor energy efficiency with over half of the cooling capacity wasted into the atmosphere. Vacuum cooling is considerably better. A microwave oven heats food by vibrating the polar molecules like water in the food. A microwave heats a donut on a paper plate but not the paper plate because the paper contains no polar molecules. The same concept works in a vacuum cooler. The produce has water, and it is the only thing that cools. The carton and the metal vacuum tube do not cool. During the vacuum cooling process, the vacuum tube and its doors remain at ambient temperature because they have no water to evaporate. This means that with vacuum cooling, there is no energy wasted cooling objects that are not produce. Beyond the electrical inefficiencies of the motor and the mechanical inefficiency of the vacuum pump, the vacuum cooler approaches 100% energy efficiency.

Figure 3d-4. A full reefer load of boxed head lettuce entering a vacuum cooler. Some large vacuum coolers have doors on both ends so that the cooled produce may be loaded directly into a reefer as another load is entering from the other end.

Source: M. Boyette.

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Figure 3d-4. A full reefer load of boxed head lettuce entering a vacuum cooler. Some large vacuum coolers have doors on both ends so that the cooled produce may be loaded directly into a reefer as another load is entering from the other end.

Source: M. Boyette.

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