Vacuum or evaporative cooling technology has been used in the food industry for many years. Lettuce, spinach, cabbage and other leafy vegetables have been vacuum cooled since the early 1940s. Typical installations were portable or semi-portable, in some cases enclosed trailers, to facilitate handling the product in or near the fields. The product then was driven to a cooperative facility where large chambers would hold loads of produce, just picked and crated.
Cooling the product this quickly enhanced shelf life and enabled the product to be shipped long distances to the market and still maintain the field freshness. The process then evolved to transporting a product to a process plant and cooled prior to distribution.
Vacuum cooling serves a vital role in today’s market. But there’s a new twist: Current technology has made the process more efficient. Manual operation has turned into push-button automation. Instead of co-op facilities using the technology, now processing plants are using vacuum cooling to provide the finished product to foodservice outlets or retail stores.
Foods such as potatoes, soups, sauces, fruits, chick peas, some meat products and, of course, vegetables are ideal for vacuum cooling. Most can be cooked and cooled in the same vessel, thereby minimizing handling. Retort capacities can be sized for 3,200 lbs. of product and kettles for 50-800 gal.
Companies currently using this cook/cool technology in retorts include St. Clair Foods in Memphis, Tenn., which processes potatoes for salads and other items. Cedars Mediterranean Foods, Ward Hill, Mass., processes chick peas for hummus. The retorts were supplied by Allpax Products LLC Covington, La., as shown in Figure 1 (right).
Lee Industries Inc., Philipsburg, Pa., furnished kettles for the purpose of cooking and cooling soups, sauces and other liquid products. This technology is used by several Fortune 500 international and domestic companies (see Figure 2 below).
Why vacuum cooling versus other forms of cooling/refrigeration? If some foods are cooked in the traditional ways, they then have to be transported to refrigerated rooms, cooled either by mechanical refrigeration using Freon, ammonia or air blast coolers. All of which may do an adequate job, but it takes time to reach required temperatures not to mention the additional handling or transportation time. Added handling and time may adversely affect quality, texture or appearance.
An additional advantage of vacuum cooling is the deaeration or removal of air from the product, which will improve the quality and increase shelf life by retarding bacterial growth. Typically moisture-rich products can be vacuum cooled from 240 o F to 40-35 o F in 12-20 minutes depending on the size of the vacuum system, vessel capacity and product quantity. Since the cooking and cooling occur in the same vessel, the product can then be pumped or transported to the packaging station or next stage of the process. This saves time and handling while maintaining the product quality.
Where a product is heat sensitive, vacuum is used -- not to cool, but to maintain a constant temperature to prevent damage, maintain quality and to ensure the proper consistency during heating/cooking. Some soups or broths may gel at too low a temperature. Some products such as custards and puddings are cooked at temperatures of 100-160 degrees Fahrenheit, but the temperature must be held below the point at which caramelizing will occur. The vacuum system, therefore, is designed to maintain that critical pressure/temperature point.
The processing equipment can be controlled by a programmable logic controller to operate at that point or higher. In certain cases where different foods are processed the system can be programmed for different recipes to accommodate variable temperature and pressure as well as product quality and quantity.
Vacuum cooling is achieved by rapid evaporation of moisture in the product. In most vegetables, the moisture is contained within the structure of the food; in soups and sauces, the moisture may be part of the recipe added before or after the evaporation.
Evaporation occurs under vacuum, whereby the boiling temperature of the product’s moisture is reduced by lowering the pressure below atmospheric. At standard atmospheric pressure, 760 torr or 14.69 psia, water boils at 212 degrees Fahrenheit. However, at a reduced pressure, in a vacuum, water boils at a much lower temperature. For example, at 370 torr (or 7.35 psia) water boils at 179 degrees Fahrenheit, at 6.3 torr boiling occurs at 40 degrees Farhenheit.
Without control, a vacuum system could reach freezing at 32 degrees Fahrenheit or 4.8 torr. This relationship is referred to as the vapor pressure of water at various temperatures. Data is widely published for water and other liquids.
Energy in the form of heat, which is required to change water from a liquid to vapor, comes from the product itself. Evaporation of 1 lb. of water from the product will cool approximately 1,000 lbs. of product by 1o F. Evaporation is frequently referred to as "flash" or "flash cooling." The actual cooling of the product is the result of the latent heat in BTUs or amount of thermal energy absorbed by the liquid in the flash process. Thermal energy is transferred from the product resulting in a loss of the sensible heat in the product and a resultant decrease in its temperature.