Preservation
of foods by canning (thermal/heat processing)
Brief
history: During Napoleon in 1790, France was in war and
facing problems of supplying meat to solidiers due to putrefaction of meat
during transit and storage. Napoleon declared a prize of 12,000 Francs for
inventing useful methods of preserving meats and other foodstuffs for longer
period. Nicolas Appart, a Franch confectioner, heated foods in sealed air-tight
containers and able to preserve foods for longer period until the container is opened.
Nicolas Appert thus invented the process of canning and received the award from
Napoleon. Henceforth the process of canning is also called Appertising after
the name of Nicolas Appert.
Principle
of canning:
Canning may be defined
as the preservation of foods in hermetically sealed containers by the
application of heat. The minimum requirement for the process for any product is
that it should be adequate to destroy the most heat resistant bacteria (Clostridium blotulinum) likely to be
present in foods. The pH of food has a very important role to play in the
thermal processing or canning of foods. In
acid foods the microorganisms are less resistant, and it is easier to kill
them. While in non-acid foods (High pH) the microorganisms are more
heat-resistant and thus difficult to kill them and ultimately require more
severe heat treatment during canning. The spores of Clostridium blotulinum do not germinate at pH 4.4 or below and are
more easily killed at pH below 4.5. The
lower limit of growth of Clostridium
blotulinum is at pH 4.5. Food with pH value 4.5 or above, steam pressure is
used to process at high temperatures [10 psig=116oC; 15 psig=121oC;
20 psig=127oC; (10 psig=20 psi abs.)]. On the contrary foods with pH
below 4.5 may be processed at atmospheric pressure at 212oF/100oC.
The process time for canned foods vary with the nature of the products, the
capacity of container, sterility desired and other factors.
Containers
for canning /Heat processing:
A.
Tinplate
containers:
The most important
container still in use for the packaging of foods for heat processing is the
open top tinplate can. The properties of tinplate which make it an ideal
construction material are : strength and rigidity, ability to be worked at high
speed (construction of can bodies and ends , and formation of double seam and
side seam); Good corrosion resistance under normal storage conditions; Attractive
appearance; Ability to withstand high pressure and high processing
temperatures; Ease of decoration.
Tinplate which is
usually used for manufacture of tin cans consists of 9 layers comprising a
central core of base steel covered on each surface by as layer of tin-iron
alloy, a layer of oxide and finally a thin film of oil. Lacquered tinplate has
another layer, usually on one side. The relative thickness of the various
layers is approximately-Base steel 10,000, Alloy 5, Tin 50, Oxide 0.10, and oil
0.20
The thickness of
tinplate may range from 0.2 mm to 0.9 mm and sometimes up to 1.4 mm. The tin
used in tinplate manufacture is at least 99.8% pure and produces a highly
adherent coating on the steel base. The thickness of the tin coating is
measured in g/m2 [1.4 to 15 gms per sq. M on one side only]. The
coating, previously measured as lb/base box [0.12 to 1.35 lb/base box, where a
base box is 31.360 in2 [or 20.23 m2].
The tinplate can be
produced by either hot-dipping or electro-deposition process. The tinplate
surface is chemically treated to produce an oxide layer which improves
corrosion and tarnish resistance. Several types of lacquer are used in tinplate
cans for foods. These are:
·
Modified epoxy coating: often used in
cans for products such as jams and meats; they have aluminum pigments and are
therefore grey in color.
·
Normal epoxy coating: appear almost
transparent when applied to tin cans; mainly used for corrosive [acid
containing] pigmented products e.g. acidified beet roots.
·
Phenolic coating: used for meat cans; they
have little color like the epoxy-type coatings.
·
Oleoresinous coating: used for products
which require a low cost acid resistance coating.
·
Sulphur resistance coating: most
commonly used lacquer for cans for processed foods. They are required for
vegetables, meat, fish etc.
·
Vinyl and modified vinyl coatings: used
as top coats to protect cans for products such as beer and carbonated beverages
which are especially sensitive to metal pickup or are highly corrosive.
B.
Aluminium
containers:
As a material for making
cans aluminum has certain advantages: it is light in weight; it resists
atmospheric corrosion; it is not stained by sulphides released from some foods
during heating; it is non-toxic; it may be shaped into containers by several
differential methods.
Unfortunately aluminium
also has some deficiencies: it is difficult to solder; it is not as strong in
equivalent thickness as tinplate; it severely bleaches some products; it
usually gives shorter shelf-lives with wet foods than do tinplate cans; it
cannot be made into cans simply as tinplate.
C.
Glass
containers:
Glass containers are
still used extensively for the heat preservation of a wide range of foods,
particularly acid products which require only mild pasteurization heat
treatments or which are too corrosive for metal containers.
D.
Flexible
film pouches:
Significant advances
have occurred in recent years in the development of flexible pouches suitable
for retort operations. Despite a number of inherent advantages compared to the
metal containers [light weight, rapid heating and cooling], widespread use of
reportable pouches will require more rapid production speeds, lower pouch costs
and greater consumer acceptance.
Can
and can seam formation:
Can
formation: A conventional tinplate container is composed of 3
parts, the cylindrical body and 2 ends or lids. The ends are sealed onto the
body in a 2-stage seaming operation called double seaming. Thus a can contains
2 end seams and a side seam which is usually soldered but which can be welded
or cemented.
The size of can is
specified by two dimensions, the diameter and the height: can size [Dia ×Ht.]
|
Product
volume (mL)
|
Can
names
|
Can
dimension (inches)*
|
|
120-125
|
|
202×
214
|
|
200
|
|
211×
301
|
|
800
|
A
2
|
401×
411
|
*202×214=2
inch Dia x 2
inch High
The
components of double seam:
If the double seam or
side seams of a can are not well formed, air or water-borne bacteria may leak
into the can and cause the contents to spoil. For this reason the canner must
maintain a constant watch on the closing operation to ensure that the seams are
within the acceptable standards set by the can manufacturer.
Fig:
The major component of a double seam of tin cans
Dimensions
of a double seam:
In a well- formed seam
the depth of counter sink should be approximately the same as the length of the
outer wall and thickness should not be greatly in excess of combined thickness
of the five layers of tinplate. The actual dimensions, particularly the thickness,
will be influenced by the gauge of tinplate used in fabrication of the cans.
The most commonly thickness of tinplate is 0.28 to 0.305 mm [11 to 12 thou].
Considering the tinplate thickness of 0.28 mm the dimensions of double seam
should be approximately as follows:
|
Length
|
:
|
3.09-3.20
mm
|
|
Thickness
|
:
|
1.50-1.55
mm
|
|
Body
hook
|
:
|
1.88-1.98
mm
|
|
Cover
hook
|
:
|
2.01-2.11
mm
|
|
Overlap*
|
:
|
Minimum
45%
|
|
wrinkle
|
:
|
0-2
|
*minimum overlap is
determined by:
Where, BH=Body hook
length, CH=Cover hook length; L= Seam length; EPT= End plate thickness, BPT= Body
plate thickness. For plate thickness above or below 0.28 mm [11 thou] the seam
thickness will be correspondingly higher or lower.
Unit
operations in canning fruits and vegetables
[Typical
commercial canning steps/operation]
1.
Harvesting:
The
fruits should be harvested at the proper stage of maturity. That is it should
not be so ripe of so soft. The vegetable should be uniform in color, quality
and tender.
2.
Receiving:
Should be receive with minimum handling. Should be stored in proper place for
future use.
3. Soaking and washing:
Fruits/vegetables may be washed with water in different ways: these are:
Soaking in water; washing by agitation; washing by sprays. However, washing by
means of sprays of water is by far the most satisfactory method.
4. Sorting and grading: The
fruits/vegetables should be sorted and graded for size, color, maturity,
freedom from blemishes or other defects.
5. Blanching: Pre-heat
treatment is required for some products for short period in hot boiling water
or by steam for 3-5 min. This treatment is called blanching. The blanching
results in: shrinkage of material; inactivate enzymes [peroxidase and catalase
etc,] which are responsible for causing off-flavor or discoloration; reduce
internal strain on can during sterilization.
6. Peeling and coring:
Certain fruits and vegetables should be peeled and cored. Peeling may be
performed by:
a)
Hand
peeling: Using stainless steel knives
b)
By
using heat: Tomatoes may be blanched in steam or
boiling water (30-60sec) and then sprayed with cold water to loosen the skin
c)
Mechanical
peeling: e.g. potato peeler (Cylinder surface is coated with
abrasive materials [emery] and peels removed by abrasive action when the
cylinder rotates)
d)
Lye
peeling: By dipping in
hot solution of 1.5 -2% NaOH for 10- 15 seconds e.g. carrots, orange segments.
7. Filling:
The prepared materials [of known weight] are filled into the washed and clean
cans. This operation may be manual or automatic. Usually the products are
covered with liquid medium such as syrup [usually for fruits], brine [usually
for vegetables] or gravy. Enough liquid medium is added [generally hot] to
cover the material leaving about 1-2 cm head space [i.e. empty portion on the
top of the filled can]
8. Exhausting: The
filled cans are usually passed through steam-exhaust box to remove air from
head space and the contents of the can. During this exhausting the can centre
temperature should be 170oF (77oC). The objective of
operation is: To reduce internal strain on the can during heat processing-
removal of air from the head space and contents results in formation of vacuum
is necessary to increase the shelf life of the cans. The lids of the properly
exhausted cans after closing appear to be concave in shape due partial vacuum
inside the can.
9. Sealing: The
exhausted cans are immediately sealed in a double seamer without delay.
10. Processing [heat treatment/thermal
processing]: This is the most important unit
operation in the canning process. The sealed cans are processed (commercial
sterilization) in boiling water or retort under steam pressure depending upon
the pH of eh foods packed. Acid and high acid foods [pH 2-4.4] can be processed
in boiling water [100oC] for a specified period. While low acid and
medium acid foods [pH 4.5 to above] are heat processed in steam retort under
pressure at high temperature [10 psig=240oF (116oC) 15
psig=250oF (121.1oC) or 20 pisg=260oF (127oC)
at sea level]. The duration of processing depends upon the size of the
container, nature of the products and sterility desired.
11. Cooling: The
processed cans are promptly cooled in water to prevent further cooking to temp.
About 100oF (38oC) before storing. The cooling water
should be clean and should contain a residual chlorine content of about 1-2ppm
12. Labeling, coding and warehousing:
The cooled cans are wiped if necessary, labeled, cased and stored in cool dry
warehouse before dispatching for slaes. Alternatively, the labeling of cans may
be done at the time of dispatch. It is important that canned foods be coded
when placed into warehouse. In the event of spoilage, lots may be isolated in
blocks from the main product. In addition, products of poor quality may be isolated
and controlled.
Canning
of fish (Sardine Fish in oil)
Canning in the narrow
sense covers the range of practices beginning with pre-treatment of the fish as
well as preparation of can; the filling and closure of the can and the crucial
techniques of heating the filled cans to such a degree that the adequate
killing of microorganisms is attained without undue damage to fish, and finally
the cooling, cleaning and storage the product.
Procedures:
1. Descaling:
Descaling is done with the help of knife. Care is taken so that the fish is not
affected. All the fins and the tail are removed.
2. Beheading:
The heads are cut off and evisceration is done.
3. Washing:
The dressed fish is washed with water.
4.
Brining: Dip in 25% brine (salt)
solution for 10 min.
5.
Washing: After brining the fish is
quickly washed.
6.
Packing in cans: Pack the fishes in the
can (e.g. can capacity 120 gms) by putting fish in head-to-tail position with
belly upright.
7.
Pre-cooking: Pre-cook the fish in can
for 20 min.
8.
Draining of water: The water which comes
out from the fish due to pre-cooking is drained out.
9.
Addition of oil: Refined groundnut or any kind of edible oil is added
to each can(@21mL per 2-3 oz of fish)
10.
Exhausting: The filled cans are
exhausted for 7-8 min. this could be achieved by steam exhausting or heating
the filled cans at the can centre temperature of about 77oC.
11.
Seaming: The exhausted cans are double
seamed and the sealed cans are immediately inverted. Then the cans are washed
with detergent (1-1.5% Na3PO4 solution at 80oC)
12.
Processing (Retorting): The sealed cans
are heated in retort at 10 psig for 1 hour.
13.
Cooling: The cans are cooled in cooling
water after processing.
14.
Labeling and storing: The cans are then
labeled and stored.
Canning
of vegetables (Green peas)
Selection
of variety: For canning purpose the two varieties
are important e.g.
i.
The early smooth seeded
ii.
Later and sweet wrinkled-seed variety.
The most important smooth seeded variety is
“Alaska”. The most widely used wrinkled seeded varieties are “Advancer”,
“Horsford Market Marden” etc.
Procedures:
1. Collect
green tender pods of peas.
2. Shell
peas and soak for 1 hr in water.
3. Separate
into 3 quality grades by floating methods (in brine);
Fancy-Float in brine of 30-60o
salometer
Extra standard-Float in brine of
36-42o salometer
Standard- Float in 42-46o
salometer
[Note: 26.5% salt solution=100o
salometer]
4. Blanch
for about 5 min in boiling water
5. Fill
into 8 oz plain cans
6. Top-up
with boiling brine containing 2% salt and 3% sugar, leaving 3/16 inch head
space.
7. Exhaust
for about 7 min. (until centre of the can attains 82oC)
8. Seal
and process for 40 min at 240oF(116oC)
9. Cool
quickly and label
10. Store
Spoilage
of canned foods
The most obvious sign of spoilage of canned foods is
swelling of the ends, usually proceeding through Flipper, Springer, and Soft swell
and Hard swell stages (these are described below). However, some types of
spoilage (e.g. flat sour) are characterized by the ends remaining in a concave
position.
The canned food spoils as result of:
·
Tinplate corrosion, particularly with
acid foods, due to hydrogen evolution.
·
Chemical reactions e.g. non-enzymic
browning reactions (CO2 swells), severe nitrate detining etc.
·
Incorrect retort operation, particularly
during cooling.
·
Under-exhausting and over-filing:
leading to excessive pressure development during heat processing.
·
Microbial growth: As a result of no heat
process or under-processing , pre-process spoilage, post-process contamination
as a result of defective seams, or insufficient coding, and
·
Fluctuation in atmospheric pressure.
Flipper:
A flipper has flat ends, one of which become convex when side of can is struck
or temperature of content increased
Springer-
Both ends of cans bulged, but one or both ends will stay concave if pushed in
Soft
swell- Both ends bulged, gas pressure is low enough that
the ends can be dented by pressure of finger.
Hard
swell- Has high pressure that ends are too hard to dent by
hand.
Flat
sour spoilage: The ends of can remain flat (or
concave) but there is development of lactic acid (sour) during spoilage by flat
sour bacteria (This type of spoilage is found in tomato juice: caused by
thermophile species of bacterial such as Bacillus
coagulans)
