http://web.archive.org/web/20070509153848/http://www.fda.gov/bbs/topics/CONSUMER/CON00043.html
The Canning Process: Old Preservation
Technique Goes Modern
by Dale Blumenthal
The steamboat Bertrand was heavily laden
with provisions when it set out on the
Missouri River in 1865, destined for the
gold mining camps in Fort Benton, Mont. The
boat snagged and swamped under the weight,
sinking to the bottom of the river. It was
found a century later, under 30 feet of
silt a little north of Omaha, Neb.
Among the canned food items retrieved from
the Bertrand in 1968 were brandied peaches,
oysters, plum tomatoes, honey, and mixed
vegetables. In 1974, chemists at the
National Food Processors Association (NFPA)
analyzed the products for bacterial
contamination and nutrient value. Although
the food had lost its fresh smell and
appearance, the NFPA chemists detected no
microbial growth and determined that the
foods were as safe to eat as they had been
when canned more than 100 years
earlier.
The nutrient values varied depending upon
the product and nutrient. NFPA chemists
Janet Dudek and Edgar Elkins report that
significant amounts of vitamins C and A
were lost. But protein levels remained
high, and all calcium values "were
comparable to today's products."
NFPA chemists also analyzed a 40-year-old
can of corn found in the basement of a home
in California. Again, the canning process
had kept the corn safe from contaminants
and from much nutrient loss. In addition,
Dudek says, the kernels looked and smelled
like recently canned corn.
The canning process is a product of the
Napoleonic wars. Malnutrition was rampant
among the 18th century French armed forces.
As Napoleon prepared for his Russian
campaign, he searched for a new and better
means of preserving food for his troops and
offered a prize of 12,000 francs to anyone
who could find one. Nicolas Appert, a
Parisian candy maker, was awarded the prize
in 1809.
Although the causes of food spoilage were
unknown at the time, Appert was an astute
experimenter and observer. For instance,
after noting that storing wine in airtight
bottles kept it from spoiling, he filled
widemouth glass bottles with food,
carefully corked them, and heated them in
boiling water.
The durable tin can--and the use of pottery
and other metals--followed shortly
afterwards, a notion of Englishman Peter
Durand. Soon, these "tinned" foods were
used to feed the British army and
navy.
21 Billion Cans a Year
Canned foods are more than a relic dug from
the past. They make up 12 percent of
grocery sales in the United States. More
than 1,500 food products are
canned--including many that aren't
available fresh in most areas, such as
elderberry, guava, mango, and about 75
different juice drinks. Consumers can buy
at least 130 different canned vegetable
products--from artichokes and asparagus to
turnips and zucchini. More than a dozen
kinds of beef are canned, including beef
burgers and chopped, corned and barbecued
beef.
According to a recent study cosponsored by
the U. S. Department of Agriculture and
NFPA, canned foods provide the same
nutritional value as fresh grocery produce
and their frozen counterparts when prepared
for the table. NFPA researchers compared
six vegetables in three forms: home-cooked
fresh, warmed canned, and prepared
frozen.
"Levels of 13 minerals, eight vitamins, and
fiber in the foods were similar," says
Dudek. In fact, in some cases the canned
product contained high levels of some
vitamins that in fresh produce are
destroyed by light or exposure to air.
The Canning Process
Food-spoiling bacteria, yeasts and molds
are naturally present in foods. To grow,
these microorganisms need moisture, a
low-acid environment (acid prevents
bacterial growth), nutrients, and an
appropriate (usually room) temperature.
Dennis Dignan, Ph. D., chief of FDA's food
processing section, explains that foods are
preserved from food spoilage by controlling
one or more of the above factors. For
instance, frozen foods are stored at
temperatures too low for microorganisms
(bacteria, yeasts and molds) to grow. When
foods are dried, sufficient moisture is not
available to promote growth.
It is the preservation process that
distinguishes canned from other packaged
foods. During canning, the food is placed
in an airtight (hermetically sealed)
container and heated to destroy
microorganisms. The hermetic seal is
essential to ensure that microorganisms do
not contaminate the product after it is
sterilized through heating, says Dignan.
Properly canned foods can be stored
unrefrigerated indefinitely without fear of
their spoiling or becoming toxic.
Canning for a New Age
Dignan also notes that foods packaged in
materials other than metal cans are
considered "canned" by food processing
specialists if the food undergoes the
canning preservation process. Thus, today a
canned food may be packaged in a number of
other types of containers, such as glass
jars, paperboard cans, and plastics that
can be formed into anything from pouches to
soup bowls to serving trays.
For example, FDA consumer safety officer
Tom Gardine, holding up a small, plastic
container of half-and-half for his morning
coffee, says, "This is a canned food." He
explains that the coffee creamer was heated
to destroy bacteria and sealed to prevent
microorganisms from entering the sterile
container. Until it is opened, the creamer
is intended to be stored on the shelf, not
in the refrigerator.
Meals for today's U. S. military come in
plastic pouches--a new version of the
heavier C-rations in metal cans. Such
flexible pouches aren't as popular with
American civilians as they are with
Europeans. Many Americans, instead, are
buying their canned foods in plastic
containers that come with a peel-off metal
top and plastic lid--ready for the
microwave. Barriers (made of sophisticated
synthetic materials) that provide an
airtight seal are sandwiched in these
plastic layered containers. They are used
for applesauce, pudding, and other foods
that can be stored on supermarket or home
shelves for years.
Then there are containers made of new
transparent plastic materials like
polyethylene terephthalate--used for peanut
butter and catsup. Packages made of
paperboard layers have been designed in the
shape of boxes to contain such foods as
fruit juices, tomato sauce, and even
milk.
Even the tin can is changing. For years,
the three-piece can (made from a top, a
bottom, and a body formed from a plate
soldered into a cylinder) was the only can
around. Now there are two-piece cans, which
eliminate the side seam and one seamed end.
These cans are made by feeding metal into a
press that forms the can body and one end
into a single piece.
In the traditional three-piece cans, a
welded side seam has replaced the
lead-soldered side seam in all but 3.7
percent of American cans, says NFPA
official Roger Coleman. The welding process
uses electrodes that apply pressure and
electric current to overlapping edges at
the side seam. These new seams eliminate
concern about lead leaching into metal
canned foods. In the 3.7 percent of U. S.
cans where lead still is used, it is often
for dry foods (such as coffee) packaged in
cans, according to Coleman. Leaching is not
a concern here.
Many imported cans, however, still bear
lead-soldered side seams. To tell whether a
can has been soldered with lead, first peel
back the label to expose the seam. The
edges along the joint of a lead-soldered
seam will be folded over. Silver-gray metal
will be smeared on the outside of the seam.
A welded seam is flat, with a thin, dark,
sharply defined line along the
joint.
Turning Up the Heat
Foods with a naturally high acid
content--such as tomatoes, citrus juices,
pears, and other fruits--will not support
the growth of food poisoning bacteria. In
tests, when large numbers of food poisoning
bacteria are added to these foods, the
bacteria die within a day. (The exact
amount of time depends upon the bacteria
and amount of acidity.) Foods that have a
high acid content, therefore, do not
receive as extreme a heat treatment as
low-acid foods. They are heated
sufficiently to destroy bacteria, yeasts
and molds that could cause food to
spoil.
Canners and food safety regulators are most
concerned about foods with low acid
content, such as mushrooms, green beans,
corn, and meats. The deadly Clostridium
botulinum bacterium, which causes botulism
poisoning, produces a toxin in these foods
that is highly heat-resistant. The
sterilization process that destroys this
bacteria also kills other bacteria that may
poison or spoil food.
Low-acid canned foods receive a high dose
of heat--usually 107 degrees Celsius (250
degrees Farenheit) for at least three
minutes. (The amount of time the food is
heated, though, depends upon the size of
the container and the product.) The canned
food is heated in a retort, a kind of
pressure cooker.
The coffee creamer on Gardine's desk,
however, was packaged differently. Although
both the half-and-half and plastic
container were sterilized with heat, they
were heated separately and then brought
together in a sterile environment where the
container was filled and sealed. The
advantage of this "aseptic processing," a
type of canning, is that higher
temperatures with reduced heating times
prevent deterioration in the quality of the
food.
Aseptic processing is the "wave of the
present and the future," says Gardine. It
is now used for liquids, and scientists are
on the way to perfecting the method for
canning stews and chowders. However, says
Gardine, because solid foods may be more
difficult to keep sterile during the
filling and sealing period, FDA is being
especially cautious in approving uses for
aseptic processing.
Finessing the Attack on Food Spoilers
Another critical element in the canned food
process is sealing products in air-tight
containers. It is essential that air be
removed from the container before sealing.
Air could cause the can to expand during
heating, perhaps damaging the seals or
seams of the container.
A telltale sign of loss of this vacuum--and
a possibly contaminated product--is a can
with bulging ends. (See accompanying
article.) If a seal is not airtight,
bacteria may enter the can, multiply, and
contaminate the product.
The hermetic seal finesses the canning
process. The bacteria in a food and
container are killed through heating, and
at the same time new bacteria are kept from
contaminating the food.
The distinction between the canning process
and food handling before processing is an
important one for food processors and
regulators. Last February, 22 students at
Mississippi State University became ill
after eating omelets made with canned
mushrooms imported from China. Similar
outbreaks followed in New York and
Pennsylvania, affecting more than 100
people. FDA identified the culprit as
staphylococcal enterotoxin, a poison
produced by the bacteria Staphylococcus
aureus.
FDA's investigation suggests that poor
sanitation caused the problem, and that the
mushrooms were contaminated with
staphylococcal enterotoxin even before they
were canned. The canning process did not
destroy the substance because food
preservation processes are not normally
designed to destroy staphylococcal
enterotoxin, a highly heat-resistant
toxin.
Since this incident, FDA and the Peoples
Republic of China have been working
together to determine the source of the
contamination. However, FDA authorities
still are preventing mushrooms canned in
China from entering the United States. And,
says Gardine, FDA is focusing attention on
sanitation procedures in imported
foods.
Surpassing Napoleon
The canned food principle that won Nicolas
Appert his prize of 12,000 francs has
endured over the years. What might surprise
Appert, however, is how his discovery is
making food shopping and storing easier for
the 20th century consumer.
Those who order coffee at fast food
restaurants now also are served canned
half-and-half, which has been transported
and stored without concern about
refrigeration. Hikers can take flexible
pouches of canned food on backpacking trips
without having to worry about saving water
to reconstitute freeze-dried meals. And, in
this society of microwave owners, Americans
who don't have time to prepare a
well-balanced meal can pick up a plastic
container filled with a canned, nutritious
dinner.
Dale Blumenthal is a staff writer for FDA
Consumer.
How to Recognize Can Defects
"Never eat food from a tin can with bulging
ends" was a maxim many grew up with.
Bulging was one of several clues that might
indicate contamination of food packaged in
metal cans. Guidelines have been adapted
for recognizing defects in cans made of
plastic and other materials, as well. The
guidelines are:
Metal Cans
* an obvious opening underneath the double
seam on the top or bottom of the can
* a can with bulging ends
* a fracture in the double seam
* a pinhole or puncture in the body of the
can
* an unwelded portion of the side seam
* a leak from anywhere in the can
Plastic Cans
* any opening or non-bonding in the
seal
* a break in the plastic
* a fractured lid
* a swollen package
Paperboard Cans
* a patch in the seal where bonding or
adhesive is missing
* a slash or slice in the package
* a leak in a corner of the package
* a swollen package
Glass Jars
* a pop-top that does not pop when opened
(indicating loss of the vacuum)
* a damaged seal
* a crack in the glass of the jar
Flexible Pouches
* a break in the adhesive across the width
of the seal
* a slash or break in the package
* a leak at a manufactured notch used for
easy opening
* a swollen package
(Taken from a chart for retailers developed
by FDA and NFPA and published by the
Association of Official Analytical
Chemists.)
--D. B.
