Approximately 9 billion chickens are processed in the U.S. each year, of which 80% are marketed as fresh product. It is estimated that 2% to 4% of this meat is lost as a result of poultry spoilage. Therefore, spoilage is of great concern to the poultry industry.
The primary causes of poultry products spoilage are as follows:
- Prolonged distribution or storage time
- Inappropriate storage temperature
- High initial bacterial counts
- High post-rigor meat pH
Dealing with spoilage factors
Companies are able to prevent prolonged storage times by properly rotating their stock. Product that is to be sold in locations far from the processing plant should be transported at temperatures that are below freezing (i.e. 26 F), but not sufficient to freeze the muscle tissue (deep chill).
Inappropriate storage temperatures or fluctuations in storage temperature are the most avoidable causes of spoilage. Temperature abuse can occur during distribution, storage, retail display or handling of the product by the consumer. Processors can determine whether product has been temperature abused by monitoring temperature or evaluating bacterial populations throughout the distribution system.
Initial bacterial counts on broiler carcasses may have a direct effect on the shelf-life of fresh product as well. The initial number of bacteria on poultry is generally a function of grow-out procedures, production practices, and plant and processing sanitation. Higher numbers of spoilage bacteria on the chicken immediately after processing, translates to more rapid spoilage.
High post-rigor meat pH is often caused by stress on the birds during grow-out or transportation. This reduces the shelf-life of the meat by up to six days and is due to the fact that spoilage bacteria multiply much more rapidly on meat that is at a pH of 6.2 than on meat that is at a normal post-rigor pH of 5.4-5.6.
Bacteria responsible for spoilage
Research demonstrates that the populations of bacteria high in number on the carcass immediately after processing are not the ones that grow under refrigeration and spoil carcasses. Instead, the bacteria found after carcasses spoil are very difficult to find on carcasses at the time of processing. Just after processing, the spoilage bacteria are present in very low numbers, but they can multiply rapidly to cause spoilage odors and slime.
These spoilage bacteria are called psychrotrophic bacteria (psychro=cold; trophic=able to grow) because they are able to multiply under cold conditions. Fresh poultry products held long enough at refrigerator temperatures will spoil as a result of the growth of psychrotrophic bacteria.
In contrast, the bacteria that exist in higher numbers at the time of processing on the skin of chickens and in their intestinal tracts are primarily mesophiles (meso=middle; phile=love). These bacteria do not multiply to an appreciable degree at refrigerator temperatures. Salmonella, E. coli and other bacteria found on chickens are mesophiles. When a company conducts an “Aerobic Plate Count” or “Total Plate Count” on a chicken carcass, it is measuring the mesophiles.
The figure “Mesophilic and psychrotropic bacterial growth during cold storage at 4 C on fresh poultry” shows how these populations of bacteria behave on carcasses during refrigeration.
Origin of spoilage bacteria
Spoilage bacteria on the carcass immediately after processing come from the feathers and feet of the live bird, the water supply in the processing plant, the chill tanks and processing equipment. These spoilage bacteria are not usually found in the intestines of the live bird. High populations of Acinetobacter (108cfu/g) have been found on the feathers of the bird and may originate from the deep litter. Other spoilage bacteria, such as Cytophaga and Flavobacterium, are often found in chill tanks but are rarely found on carcasses.
The psychrotrophic spoilage bacteria on chicken carcasses immediately after slaughter are generally Acinetobacter and pigmented pseudomonads. Although strains of nonpigmented Pseudomonas produce off-odors and off-flavors on spoiled poultry, initially, they are difficult to find on carcasses and P. putrefaciens (Shewanella putrefaciens) is rarely found.
Russell et al. (1995) conducted a study to identify the bacterial species responsible for spoilage of poultry from various locations around the U.S. The species isolated are presented in the figures. The bacterial genera most isolated in high numbers on spoiled poultry was Pseudomonas fluorescens, putida, or fragi or Shewanella (formerly a Pseudomonas) putrefaciens. Identification of the genus and species most responsible for spoiling poultry is important because, once identified, it is easier to understand the mechanisms by which they produce spoilage.
High numbers (105 cfu/cm2) of psychrotrophic spoilage bacteria are required on poultry surfaces before off-flavors, off-odors and appearance defects are able to be detected organoleptically. Researchers have reported that higher numbers of bacteria (3.2x107 to 1x109 cfu/cm2) were required to produce slime than were needed for odor to become noticeable.
Causes of spoilage defects
Spoilage is caused by the accumulation of metabolic by-products or the action of extracellular enzymes produced by psychrotrophic spoilage bacteria as they multiply on poultry surfaces at refrigeration temperatures. Some of these by-products become detectable as off-odors and slime, as bacteria utilize nutrients on the surface of meats.
Off-odors do not result from breakdown of the protein in skin and muscle, as previously thought, but from the direct microbial utilization of low molecular weight nitrogenous compounds such as amino acids, which are present in skin and muscle.
Concentrations of free amino acids increase as proteolysis occurs throughout the storage period. It has been demonstrated that measurement of these free amino acids, due to the production of aminopeptidases and subsequent breakdown of protein, may be used to rapidly determine the bacteriological quality of beef.
Development of off-odors and slime
Microorganisms appear first in damp pockets on the carcass, such as folds between the foreleg and breast of a carcass, and their dispersion is promoted by condensation which occurs when a cold carcass is exposed to warm, damp air.
An ester-like odor, which was described as a “dirty dishrag” odor, can develop on cut-up chickens. In most cases, off-odor precedes slime formation and is considered the initial sign of spoilage. Immediately after off-odors are detected, many small, translucent, moist colonies may appear on the cut surfaces and skin of the carcass. Eventually, meat surfaces become coated with tiny drop-like colonies (see photo), which increase in size and coalesce to form a slimy coating.
In the final stages of spoilage, the meat may begin to exhibit a pungent ammoniacal odor in addition to the dirty dishrag odor, which may be attributed to the breakdown of protein and the formation of ammonia or ammonia-like compounds. Various authors have reported that degradation of meat by pseudomonads results in the formation of slime.
Effects of cold storage
Under refrigeration (< 5 C), psychrotrophic bacterial populations are able to multiply on broiler carcasses and produce spoilage defects; however, the mesophilic bacteria that predominate on the carcass initially remain the same or decrease in number. This phenomenon may be explained by examining the metabolic changes that occur in these groups of bacteria as they are exposed to refrigerator temperatures.
Cellular lipids. Typically, mesophilic bacteria cease to proliferate below a certain environmental temperature because as temperature decreases so does their cellular absorption of nutrients. Psychrotrophic bacteria species typically exhibit no such temperature-induced difference. This is why the psychrotrophic spoilage bacteria are able to grow rapidly at refrigerator temperatures.
Lipase production. Research has demonstrated that the amount of lipase produced by psychrotrophic bacteria increases as a result of exposure of the bacteria to cold temperatures. This means that Pseudomonas is able to breakdown fat equally well when on chicken in the refrigerator or at room temperature. This capability makes it well suited to spoil chicken.
Proteolytic activity. Research has shown that production of proteolytic enzymes by Pseudomonas fluorescens was higher when this bacterium was cultured at lower temperatures. This means that, as with fat, Pseudomonas is able to breakdown protein more effectively at refrigeration temperature than at room temperature. Again, this makes the bacterium ideally suited to spoil chicken.
Shelf life: Counting the right bacteria
From time to time premature spoilage will occur. In order for companies to assess this problem, they often conduct aerobic plate counts on products. This microbiological method is inappropriate for this purpose because measuring mesophilic bacteria on chicken (APC count) does not indicate what is happening with spoilage bacteria. APC counts may miss up to 99.9% (3 logs) of spoilage bacteria on the surface of the product.
To measure spoilage bacteria, samples should be plated and incubated at 7 C for 10 days. In this way, the bacteria that grow and produce colonies on the plate will be the ones responsible for spoiling the product.