Aassalamu alikum how are you I hope you are all well. All the praise to Allah subhanahu oatala.I am Abu Saeid Studying MS in Food Processing and Preservation in Hajee Muhammad Danesh Science and Technology University, Dinajpur. Stay in Zia hall room no#226# 1st floor in HSTU campus, Basher hat, Dinajpur.


Thursday, June 9, 2016

Preservation of foods by canning (thermal/heat processing)

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)*

202× 214

211× 301
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.
Can body
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:
3.09-3.20 mm
1.50-1.55 mm
Body hook
1.88-1.98 mm
Cover hook
2.01-2.11 mm
Minimum 45%
*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.
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.
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)

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