Microbial spoilage of canned foods

Microbial spoilage of canned foods should not be seen as a common problem since it only happens if the manufacturer fails to follow the correct processing procedure. The main causes are:

 Underprocessing

Underprocessing is the application of heat to an insufficient extent or degree to attain commercial sterility. This may be the result of errors in dimensioning the heat treatment process; excessive contamination of the product prior to starting the application of heat (making the heat treatment insufficient); changes in the formulation of the product (with a consequent effect on heat transfer); equipment malfunctioning; flaws in the sterilization time/temperature control.

In low acid foods, underprocessing is characterized by the survival of mesophilic or facultative thermophilic spores. Anaerobic spoilage caused by putrefactive clostridia is the most common form of deterioration of low acid foods and is characterized by putrid odor. This type of deterioration has serious implications from a public health standpoint, due to the risk of the presence of  C. botulinum and its toxins.

In acid foods, the survival of spores of mesophilic bacteria is not sufficient to characterize as under-processing, since the heat treatment commonly applied to this category of products is milder. The survival of a small number of non-acid-tolerant mesophilic spores is acceptable, since they are incapable of germinating and growing at the pH of the product. Underprocessing, in this case, is characterized by the survival of

spores of acid-tolerant bacteria and/or spores of heat-resistant molds. A common presence are anaerobic, mesophilic  butyric bacteria, such as  C. pasteurianum, which produces butyric acid (butyric odor), CO₂ and H₂ (blowing). Also common is the “ flatsour”  facultative thermophile  B. coagulans (mainly in tomato products), which does not cause blowing of the packages.

The most common  heat-resistant molds are  Byssochlamys fulva,  Neosartorya fisheri and  Talaromyces flavus, which cause discoloration, moldy odor, the presence of mycelia and, eventually, slight blowing. In the case of gross underprocessing, cells of non- spore forming bacteria, molds and yeasts may also survive. In the latter case, in order to characterize underprocessing as the cause of spoilage, it is necessary to verify the absence of leakage, since mixed cultures of bacteria, molds and yeasts are also characteristics of deterioration caused by leakage.

 Leakage

The term leakage is used to refer to recontamination of the product after having been subjected to heat treatment. It is the result of the entry of microorganisms or contaminated air or water through the processed package. This may occur as a result of flaws in the closure or sealing system of the packages; use of defective packages; inappropriate handling of the packages, with consequent damage to the closure or sealing system; cooling in excessively contaminated water. In foods produced by the aseptic process (in which the product and the packaging are sterilized separately and filling is done in an aseptic environment), recontamination may occur during the cooling step (which precedes filling), in the filling area, in the sealing or closing area, or even, by filling a non-sterile packaging.

Deterioration by leakage is characterized by the presence of a  mixed microflora, which may contain both spore forming bacteria, as well as molds, yeasts and non-spore forming bacteria (rods, cocci, coccobacilli). Depending on the predominant microflora, there may or there may not be gas production (blowing).

Spoilage by strictly thermophiles

Deterioration caused by strictly thermophiles occurs when the product remains for a prolonged period of time at high temperatures, thereby allowing germination of the spores that survived processing. It can be caused by slow cooling and/or storage at temperatures higher than 37°C. To characterize this deterioration, it is necessary to verify whether the surviving spores are strictly or facultative thermophiles. For this purpose, the culture must be isolated and purified and, departing from the pure culture, promote the production of spores, subject to thermal shock and check the temperature at which germination occurs (at 35°C and 55°C or only at 55°C). Spores that germinate exclusively at 55°C are strictly thermophiles, the survival of which in small numbers is considered normal in canned foods, if the product is not intended for storage at temperatures above 37°C. Spores that germinate both at 35 as at 55°C are  facultative thermophiles and should not survive heat treatment.

The most common changes are: a) deterioration of the “flat-sour” type, caused by  G. stearothermophilus, which results in a pH reduction without blowing; b) anaerobic spoilage without H₂S production, caused by  T. thermosaccharolyticum, which results in cheese odor and pronounced blowing of the packages; c) anaerobic thermophilic spoilage with H₂S production, caused by  D. nigrificans, which results in darkening of the content (reaction of H₂S with iron).

Microbial multiplication before heat treatment

The permanence of a formulated product for prolonged periods of time, prior to sterilization, may allow the multiplication of microorganisms, resulting in an incipient spoilage process. This process will be interrupted by the heat treatment, but the changes that have already occurred will not be reversed, resulting in altered products, containing a large number of dead cells and, in some cases, with packages without vacuum or slightly blown by the build-up of CO₂.

Non-microbial causes of spoilage

Not always is the spoilage of canned foods of microbial origin but can be the result of other causes, such as: a) Chemical interaction between the food and the packaging, which occurs principally between the acids of the foods and the metal of the packages. This phenomenon causes corrosion with the production and build-up of hydrogen gas (blowing). In more advanced stages, micro holes may be observed in the can or the lids of glass packages. b) Enzyme reactions, which affect mainly foods produced by the aseptic process, treated at high temperature for a short time (UHT or HTST). These enzyme-triggered changes result from the non-activation or regeneration of enzymes that are constituents of the food and may cause changes like liquefaction, coagulation, discoloration or off-odors. This type of deterioration does not represent a risk to human health, does not bring about undesirable changes to the packaging and is generally characterized by the presence of a reduced number of microbial cells when the product is observed directly under the microscope.

Useful terms

BAM/FDA useful terms which describe the conditions of packages and product in the analysis of spoiled canned food (Landry et al., 2001)  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 pres-sure. 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.

Exterior can condition: leaking, dented, rusted, buckled, paneled, bulged.

Internal can condition: normal, peeling, slight, moderate or severe etching, slight, moderate or severe blackening, slight, moderate or severe rusting, mechanical damage.

Product odor: putrid, acidic, butyric, metallic, sour, cheesy, fermented, musty, sweet, fecal, sulfur, off-odor.

Product liquor appearance: cloudy, clear, foreign, frothy.

Liquid product appearance: cloudy, clear, foreign, frothy.

Solid product appearance: digested, softened, curdled, uncooked, overcooked.

Consistency: slimy, fluid, viscous, ropy.

References

Silva, N.D .; Taniwaki, M.H. ; Junqueira, V.C.A.;  Silveira, N.F.A. , Nasdcimento , M.D.D. and Gomes ,R.A.R .(2013) . Microbiological examination methods of food and water a laboratory Manual. Institute of Food Technology – ITAL, Campinas, SP, Brazil .

Landry, W.L., Schwab, A.H. & Lancette, G.A. (2001) Examination of Canned Foods. In: FDA (ed.)  Bacteriological Analytical Manual, Chapter 21A. [Online] Silver Spring, Food and Drug Administration. Available from:  http://www.fda.gov/Food/Sci-enceResearch/LaboratoryMethods/BacteriologicalAnalytical-ManualBAM/ucm109398.htm [Accessed 10th November 2011].