container. If atmospheric pressure is normally 14.7 pounds per square inch on the
surface of the earth, a decrease in this pressure by 1 pound will create a partial
vacuum. Vacuum may be referred to in terms of inches, and the requirements are
usually in federal and military specifications. Two inches of vacuum equals a reduction
of approximately 1 pound of pressure. Vacuum is commonly measured in inches of
mercury.
c. Purpose. A condition of partial vacuum is desirable in canned foods for
chemical, physical, and biological reasons.
(1) Chemically, it is important to remove the oxygen from air in the
headspace of containers. When air is flushed from a container with steam, and the
steam condenses, the oxygen content is very low. In addition, a vacuum in a container
of food helps protect color and flavor of the product, assists in retaining vitamins,
prevents the food product from becoming rancid due to oxidation, and helps retard
corrosion of tin plate. (Oxidation causes can corrosion and a rancid condition in food.)
(2) Physically, a vacuum is important in holding the closures on glass jars,
keeping the ends concave in cans, and reducing the pressure within containers while
they are being treated.
(3) Biologically, a vacuum is of value because it restricts the growth of
organisms requiring air for growth (aerobic). This is particularly important for products
processed at temperatures below the lethal level of some spoilage microorganisms.
However, the lethal process in canning is designed to kill spoilage microorganisms;
therefore, the biological importance of a vacuum in a container is of less significance
than for the other factors.
2-4.
MEASURING VACUUM
a. Vacuum Gauges. The food industry uses several types of vacuum gauges,
most of which puncture the can to determine the vacuum inside. The most commonly
used puncture type is the end-puncture gauge (see Figure 2-1), with the side-puncture
gauge being used much less frequently. Vacuum may be measured by many different
scales--millimeters of mercury, inches of mercury, feet of water, atmospheres, and
pounds per square inch. The most common measurement for gauges used by the food
industry is inches of mercury.
b. Metal Rulers. Tray pack vacuum is measured by using metal rulers of two
different sizes. Vacuum gauges are not used with tray packs. The length of a tray pack
can be measured by placing a 12 inch metal ruler on the can. The width of a tray pack
can is measured by placing a 9 7/8 inch metal ruler on the can. Both ends of the ruler
must be in contact with the lid at the inside edge of the double seam. A normal tray
pack lid will be slightly concave at the center of the lid where the labeling surface is. If
the vacuum is normal, you will be able to see light between the metal ruler and the
labeling surface. (See Figure 2-2.)
MD0708
2-4
Previous Page
