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Chapter 21A
Examination of Canned Foods
The incidence of spoilage in canned foods is low, but when it occurs it must be investigated properly. Swollen cans often indicate a spoiled product. During spoilage, cans may progress from normal to flipper, to springer, to soft swell, to hard swell. However, spoilage is not the only cause of abnormal cans. Overfilling, buckling, denting, or closing while cool may also be responsible. Microbial spoilage and hydrogen, produced by the interaction of acids in the food product with the metals of the can, are the principal causes of swelling. High summer temperatures and high altitudes may also increase the degree of swelling. Some microorganisms that grow in canned foods, however, do not produce gas and therefore cause no abnormal appearance of the can; nevertheless, they cause spoilage of the product.
Spoilage is usually caused by growth of microorganisms following leakage or underprocessing. Leakage occurs from can defects, punctures, or rough handling. Contaminated cooling water sometimes leaks to the interior through pinholes or poor seams and introduces bacteria that cause spoilage. A viable mixed microflora of bacterial rods and cocci is indicative of leakage, which may usually be confirmed by can examination. Underprocessing may be caused by undercooking; retort operations that are faulty because of inaccurate or improperly functioning thermometers, gauges, or controls; excessive contamination of the product for which normally adequate processes are insufficient; changes in formulation or handling of the product that result in a more viscous product or tighter packing in the container, with consequent lengthening of the heat penetration time; or, sometimes, accidental bypassing of the retort operation altogether. When the can contains a spoiled product and no viable microorganisms, spoilage may have occurred before processing or the microorganisms causing the spoilage may have died during storage.
Underprocessed and leaking cans are of major concern and both pose potential health hazards. However, before a decision can be made regarding the potential health hazard of a low-acid canned food, certain basic information is necessary. Naturally, if Clostridium botulinum (spores, toxin, or both) is found, the hazard is obvious. Intact cans that contain only mesophilic, Gram-positive, sporeforming rods should be considered underprocessed, unless proved otherwise. It must be determined that the can is intact (commercially acceptable seams and no microleaks) and that other factors that may lead to underprocessing, such as drained weight and product formulation, have been evaluated.
The preferred type of tool for can content examination is a bacteriological can opener consisting of a puncturing device at the end of a metal rod mounted with a sliding triangular blade that is held in place by a set screw. The advantage over other types of openers is that it does no damage to the double seam and therefore will not interfere with subsequent seam examination of the can.
Table 1. Useful descriptive terms for canned food analysis.
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Flat - a can with both ends concave; it remains in this condition even when the can is brought down sharply on its end on a solid, flat surface. | |||
Flipper - a can that normally appears flat; when brought down sharply on its end on a flat surface, one end flips out. When pressure is applied to this end, it flips in again and the can appears flat. | |||
Springer - a can with one end permanently bulged. When sufficient pressure is applied to this end, it will flip in, but the other end will flip out. | |||
Soft swell - a can bulged at both ends, but not so tightly that the ends cannot be pushed in somewhat with thumb pressure. | |||
Hard swell - a can bulged at both ends, and so tightly that no indentation can be made with thumb pressure. A hard swell will generally "buckle" before the can bursts. Bursting usually occurs at the double seam over the side seam lap, or in the middle of the side seam. |
The number of cans examined bacteriologically should be large enough to give reliable results. When the cause of spoilage is clear-cut, culturing 4-6 cans may be adequate, but in some cases it may be necessary to culture 10-50 cans before the cause of spoilage can be determined. On special occasions these procedures may not yield all the required information, and additional tests must be devised to collect the necessary data. Unspoiled cans may be examined bacteriologically to determine the presence of viable but dormant organisms. The procedure is the same as that used for spoiled foods except that the number of cans examined and the quantity of material subcultured must be increased.
Equipment and materials
Remove labels. With marking pen, transfer subnumbers to side of can to aid in correlating findings with code. Mark labels so that they may be replaced in their original position on the can to help locate defects indicated by stains on label. Separate all cans by code numbers and record size of container, code, product, condition, evidence of leakage, pinholes or rusting, dents, buckling or other abnormality, and all identifying marks on label. Classify each can according to the descriptive terms in Table 1. Before observing cans for classification, make sure cans are at room temperature.
Classification of cans. NOTE: Cans must be at room temperature for classification.
Table 2. Incubation times for various media for examination of low acid foods (pH > 4.6). | |||
Medium | No. of tubes | Temp. (°C) | Time of incubation (h) |
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Chopped liver (cooked meat) | 2 | 35 | 96-120 |
Chopped liver (cooked meat) | 2 | 55 | 24-72 |
Bromcresol purple dextrose broth | 2 | 55 | 24-48 |
Bromcresol purple dextrose broth | 2 | 35 | 96-120 |
After culturing and removing reserve sample, test material from cans (other than those classified as flat) for preformed toxins of C. botulinum when appropriate, as described in Chapter 17.
Table 3.Schematic diagram of culture procedure for low-acid canned foods
a LVA, liver-veal agar; NA, nutrient agar; CMM, cooked meat medium; BCP, bromcresol purple dextrose broth.
Table 4. Incubation of acid broth and malt extract broth used for acid foods (pH 4.6)
Medium | No. of tubes | Temp. (°C) | Time of incubation (h) |
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Acid broth | 2 | 55 | 48 |
Acid broth | 2 | 30 | 96 |
Malt extract broth | 2 | 30 | 96 |
Table 5. Pure culture scheme for acid foods (pH 4.6).
a NA, nutrient agar; SAB, Sabouraud's dextrose agar.
Check incubated medium for growth at frequent intervals up to maximum time of incubation (Table 2). If there is no growth in either medium, report and discard. At time growth is noted streak 2 plates of liver-veal agar (without egg yolk) or nutrient agar from each positive tube. Incubate one plate aerobically and one anaerobically, as in schematic diagram (Table 3). Reincubate CMM at 35°C for maximum of 5 days for use in future toxin studies. Pick representatives of all morphologically different types of colonies into CMM and incubate for appropriate time, i.e., when growth is sufficient for subculture. Dispel oxygen from CMM broths to be used for anaerobes but not from those to be used for aerobes. After obtaining pure isolates, store cultures to maintain viability.
Table 6. Classification of food products according to acidity | |
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Low acid--pH greater than 4.6 | Acid pH 4.6 and below |
Meats | Tomatoes |
Seafoods | Pears |
Milk | Pineapple |
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Other fruit |
Sauerkraut | |
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Pickles |
Berries | |
Citrus | |
Rhubarb | |
Table 7. Spoilage microorganisms that cause high and low acidity in various vegetables and fruits | ||
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Spoilage type | pH groups | Examples |
Thermophilic | ||
Flat-sour | >5.3 | Corn, peas |
Thermophilic(a) | >4.8 | Spinach, corn |
Sulfide spoilage(a) | >5.3 | Corn, peas |
Mesophilic | ||
Putrefactive anaerobes(a) | >4.8 | Corn, asparagus |
Butyric anaerobes | >4.0 | Tomatoes, peas |
Aciduric flat-sour(a) | >4.2 | Tomato juice |
Lactobacilli | 4.5-3.7 | Fruits |
Yeasts | <3.7 | Fruits |
Molds | <3.7 | Fruits |
a The responsible organisms are bacterial sporeformers. |
Table 8. Spoilage manifestations in low-acid products | ||
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Group of organisms | Classification | Manifestations |
Flat-sour | Can flat | Possible loss of vacuum on storage |
Product | Appearance not usually altered; pH markedly lowered, sour; may have slightly abnormal odor; sometimes cloudy liquor | |
Thermophilic anaerobe | Can swells | May burst |
Product | Fermented, sour, cheesy or butyric odor | |
Sulfide spoilage | Can flat | H2S gas absorbed by product |
Product | Usually blackened; rotten egg odor | |
Putrefactive anaerobe | Can swells | May burst |
Product | May be partially digested; pH slightly above normal; typical putrid odor | |
Aerobic sporeformers | Can flat or swollen | Usually no swelling, except in cured meats when nitrate and sugar present; coagulated evaporated milk, black beets |
Table 9. Spoilage manifestations in acid products | ||
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Type of organism | Classification | Manifestation |
Bacillus thermoacidurans (flat, sour tomato juice) | Can flat | Little change in vacuum |
Product | Slight pH change; off-odor | |
Butyric anaerobes (tomatoes and tomato juice) | Can swells | May burst |
Product | Fermented, butyric odor | |
Nonsporeformers (mostly lactic types) | Can swells | Usually burst, but swelling may be arrested |
Product | Acid odor |
Table 10. Laboratory diagnosis of bacterial spoilage | ||
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Underprocessed | Leakage | |
Can | Flat or swelled; seams generally normal | Swelled; may show normal defects(a) |
Product appearance | Sloppy or fermented | Frothy fermentation; viscous |
odor | Normal, sour or putrid, but generally consistent from can to can | Sour, fecal; generally varying from can to can |
pH | Usually fairly constant | Wide variation |
Microscopic and cultural | Cultures show sporeforming rods only | Mixed cultures, generally rods and cocci; only at usual temperatures |
Growth at 35 and/or 55°C.
May be characteristic on special growth media, e.g., acid agar for tomato juice.
If product misses retort completely, rods, cocci,yeast or molds, or any combination of these may be present. | ||
History | Spoilage usually confined to certain portions of pack | Spoilage scattered |
In acid products, diagnosis may be less clearly defined; similar organisms may be involved in understerilization and leakage. | ||
a Leakage may be due not to can defects but to other factors, such as contamination of cooling water or rough handling, e.g., can unscramblers, rough conveyor system. |
Table 11. pH range of a few selected commercially canned foods | |||
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Food | pH range | Food | pH range |
Apples, juice | 3.3 - 3.5 | Jam, fruit | 3.5 - 4.0 |
Apples, whole | 3.4 - 3.5 | Jellies, fruit | 3.0 - 3.5 |
Asparagus, green | 5.0 - 5.8 | Lemon juice | 2.2 - 2.6 |
Beans | Lemons | 2.2 - 2.4 | |
Baked | 4.8 - 5.5 | Lime juice | 2.2 - 2.4 |
Green | 4.9 - 5.5 | Loganberries | 2.7 - 3.5 |
Lima | 5.4 - 6.3 | Mackerel | 5.9 - 6.2 |
Soy | 6.0 - 6.6 | Milk | |
Beans with pork | 5.1 - 5.8 | Cow, whole | 6.4 - 6.8 |
Beef, corned, hash | 5.5 - 6.0 | Evaporated | 5.9 - 6.3 |
Beets, whole | 4.9 - 5.8 | Molasses | 5.0 - 5.4 |
Blackberries | 3.0 - 4.2 | Mushroom | 6.0 - 6.5 |
Blueberries | 3.2 - 3.6 | Olives, ripe | 5.9 - 7.3 |
Boysenberries | 3.0 - 3.3 | Orange juice | 3.0 - 4.0 |
Bread | Oysters | 6.3 - 6.7 | |
White | 5.0 - 6.0 | Peaches | 3.4 - 4.2 |
Date and nut | 5.1 - 5.6 | Pears (Bartlett) | 3.8 - 4.6 |
Broccoli | 5.2 - 6.0 | Peas | 5.6 - 6.5 |
Carrot juice | 5.2 - 5.8 | Pickles | |
Carrots, chopped | 5.3 - 5.6 | Dill | 2.6 - 3.8 |
Cheese | Sour | 3.0 - 3.5 | |
Parmesan | 5.2 - 5.3 | Sweet | 2.5 - 3.0 |
Roquefort | 4.7 - 4.8 | Pimento | 4.3 - 4.9 |
Cherry juice | 3.4 - 3.6 | Pineapple | |
Chicken | 6.2 - 6.4 | Crushed | 3.2 - 4.0 |
Chicken with noodles | 6.2 - 6.7 | Juice | 3.4 - 3.7 |
Chop suey | 5.4 - 5.6 | Sliced | 3.5 - 4.1 |
Cider | 2.9 - 3.3 | Plums | 2.8 - 3.0 |
Clams | 5.9 - 7.1 | Potato salad | 3.9 - 4.6 |
Cod fish | 6.0 - 6.1 | Potatoes | |
Corn | Mashed | 5.1 | |
Cream style | 5.9 - 6.5 | White, whole | 5.4 - 5.9 |
On-the-cob | 6.1 - 6.8 | Prune juice | 3.7 - 4.3 |
Whole grain | Pumpkin | 5.2 - 5.5 | |
Brine-packed | Raspberries | 2.9 - 3.7 | |
Vacuum-packed | 6.0 - 6.4 | Rhubarb | 2.9 - 3.3 |
Crab apples, spiced | 3.3 - 3.7 | Salmon | 6.1 - 6.5 |
Cranberry | Sardines | 5.7 - 6.6 | |
Juice | 2.5 - 2.7 | Sauerkraut | 3.1 - 3.7 |
Sauce | 2.3 | Juice | 3.3 - 3.4 |
Currant juice | 3.0 | Shrimp | 6.8 - 7.0 |
Dates | 6.2 - 6.4 | Soups | |
Duck | 6.0 - 6.1 | Bean | 5.7 - 5.8 |
Figs | 4.9 - 5.0 | Beef broth | 6.0 - 6.2 |
Frankfurters | 6.2 - 6.2 | Chicken noodle | 5.5 - 6.5 |
Fruit cocktail | 3.6 - 4.0 | Clam chowder | 5.6 - 5.9 |
Gooseberries | 2.8 - 3.1 | Duck | 5.0 - 5.7 |
Grapefruit | Mushroom | 6.3 - 6.7 | |
Juice | 2.9 - 3.4 | Noodle | 5.6 - 5.8 |
Pulp | 3.4 | Oyster | 6.5 - 6.9 |
Sections | 3.0 - 3.5 | Pea | 5.7 - 6.2 |
Grapes | 3.5 - 4.5 | Spinach | 4.8 - 5.8 |
Ham, spiced | 6.0 - 6.3 | Squash | 5.0 - 5.8 |
Hominy, lye | 6.9 - 7.9 | Tomato | 4.2 - 5.2 |
Huckleberries | 2.8 - 2.9 | Turtle | 5.2 - 5.3 |
Vegetable | 4.7 - 5.6 | ||
Strawberries | 3.0 - 3.9 | Miscellaneous products | |
Sweet potatoes | 5.3 - 5.6 | Beers | 4.0 - 5.0 |
Tomato juice | 3.9 - 4.4 | Ginger ale | 2.0 - 4.0 |
Tomatoes | 4.1 - 4.4 | Human | |
Tuna | 5.9 - 6.1 | Blood plasma | 7.3 - 7.5 |
Turnip greens | 5.4 - 5.6 | Duodenal contents | 4.8 - 8.2 |
Vegetable juice | 3.9 - 4.3 | Feces | 4.6 - 8.4 |
Vegetables, mixed | 5.4 - 5.6 | Gastric contents | 1.0 - 3.0 |
Vinegar | 2.4 - 3.4 | Milk | 6.6 - 7.6 |
Youngberries | 3.0 - 3.7 | Saliva | 6.0 - 7.6 |
Spinal fluid | 7.3 - 7.5 | ||
Urine | 4.8 - 8.4 | ||
Magnesia, milk of | 10.0 -10.5 | ||
Water | |||
Distilled, CO2 | 6.8 - 7.0 | ||
Mineral | 6.2 - 9.4 | ||
Sea | 8.0 - 8.4 | ||
Wine | 2.3 - 3.8 |
Nitrogen, the principal gas normally present in canned foods during storage, is associated with lesser quantities of carbon dioxide and hydrogen. Oxygen included in the container at the time of closure is initially dissipated by container corrosion and/or product oxidation. Departure from this normal pattern can serve as an important indication of changes within the container, since the composition of headspace gases may distinguish whether bacterial spoilage, container corrosion, or product deterioration is the cause of swollen cans (2). Use of the gas chromatograph for analyzing headspace gases of abnormal canned foods has eliminated the possibility of false-negative tests for different gases. It has also allowed the analyst to determine the percentage of each gas present, no matter what the mixture is. By knowing these percentages, the analyst can be alerted to possible can deterioration problems or bacterial spoilage. A rapid gas-liquid chromatographic procedure is presented here for the determination of carbon dioxide, hydrogen, oxygen, nitrogen, and hydrogen sulfide from the headspace of abnormal canned foods.
The analysis of 2352 abnormal canned foods, composed of 288 different products by a gas-liquid chromatography showed viable microorganisms in 256 cans (3). Analysis of this data showed that greater than 10 percent carbon dioxide in the headspace gas was indicative of microbial growth. Although greater than 10 percent carbon dioxide is found in a container, long periods of storage at normal temperatures can result in autosterilization and absence of viable microorganisms. Carbon dioxide my be produced in sufficient quantities to swell the container. Storage at elevated temperatures accelerates this action. Hydrogen can be produced in cans when the food contents react chemically with the metal of the seam (3).
NOTE: Other gas chromatograph instruments equipped with the appropriate columns, carrier gas, detector and recorder or integrator may also be suitable for this analysis.
Operating conditions: column temperature, 75°C; attenuation, 64/256; carrier gas, argon, with in-let pressure of 40 psig; flow rate, 26 ml/min through gas partitioner and 5 ml/min through flush line; bridge current, 125 mA; column mode, 1 & 2; temperature mode, column; injector temperature, off.
NOTE: Installation of flush system. Injection of gas samples through either sample out port or septum injection port may lead to damaged filaments in detector and excessive accumulation of moisture on columns due to bypassing the sample drying tube. To avoid this, make all injections in the sample in port. To avoid cross-contamination, install a flush line off the main argon line (Fig. 2), and flush sample loop between injections.
Calibration gases of known proportions are commercially available. Construct calibration curves from analysis of pure gases and at least 2-3 different percentage mixtures of gases. Plot linear graph of various known concentrations of each gas as peak height (mm) vs percent gas (Fig. 7).
Prepare gas collection apparatus as illustrated in Figs. 8 and 9. Adjust height of gas collection apparatus to height of can to be examined. Attach male terminal of miniature valve to female Luer-Lok terminal mounted on top of brass block on can-puncturing press. Attach one end of gas exhaust tubing to female terminal of miniature valve. Attach small pinch clamp to other end of gas exhaust tubing and place in beaker partially filled with water. Attach disposable syringe to other female Luer-Lok terminal on miniature valve. Turn 2-way plug so that gas entering from piercer will flow toward disposable syringe. Place sterile gas piercer in position on male terminal mounted on bottom of brass block on can-puncturing press.
Place can under gas press (cans to be cultured should first be cleaned and sterilized). Lower handle until gas piercer punctures can and seals. Hold in position until adequate volume of gas has been collected (minimum of 5 ml); then turn 2-way plug to release excess gas through exhaust tubing. Release handle, remove syringe, and cap immediately. Identify syringe appropriately.
Turn on gas chromatograph and recorder. Let stabilize for about 2 h. Make sure flush line is attached and gas sampling valve is open to allow flushing of sample loop. Turn on chart drive on recorder. Remove flush line, uncap, and immediately attach syringe to Sample-In Injection Port. Inject 5-10 ml of gas and immediately close gas sampling valve. Remove syringe and cap. Reattach flush line onto Sample-In Port and open gas sample valve to allow flushing of system before next injection. Observe chromatogram and switch attenuation from 64 to 256 after carbon dioxide peak has been recorded and returned back to base line. This allows hydrogen peak to be retained on scale. After hydrogen peak returns to base line, switch attenuation back to 64. After instrument has separated gases (about 6 min), determine retention time and peak height for each gas recovered from unknown sample and percent determined from standard graph by comparing retention times and peak heights with known gases, usually associated with headspace gases from abnormal canned food products. Mount chromatogram on mounting paper and identify properly as in Fig. 10. For each sample examined, inject control gases for each type of headspace gas recovered.
1. Association of Official Analytical Chemists. 1990. Official Methods of Analysis, 15th ed. AOAC, Arlington, VA.
2. Vosti, D.C., H.H. Hernandez, and J.G. Strand. 1961. Analysis of headspace gases in canned foods by gas chromatography. Food Technol. 15:29-31.
3. Landry, W.I., J.E. Gilchrist, S. McLaughlin, and J.T. Peeler 1988. Analysis of abnormal canned foods. AOAC Abstracts.
Hypertext Source: Examination of Canned Foods,
Bacteriological Analytical Manual, 8th Edition, Revision A, 1998. Chapter 21A.
Authors: Warren L. Landry, Albert H. Schwab, and Gayle A. Lancette
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Hypertext updated by kwg 2001-JAN-05