It is important to recognize that all non-sterile foods contain microorganisms; nonetheless, only bacteria will be mentioned here. The composition and level of bacteria are highly influenced by a variety of internal (such as bacteriological quality, food matrix, pH, water activity, and fabrication) and external (such as humidity, packaging and temperature) features, in addition to synergistic and antagonistic interactions between these factors.
For bacteria, the food matrix is important since bacteria need nutrients and water to thrive, but the type of packaging, availability of oxygen, and temperature during processing and storage highly affect composition and level of bacteria.
A certain level of pathogenic bacteria is needed to cause human food poisoning, but the amount required to cause illness depends greatly on an individual’s physical condition. There are interactions between, for instance, condition of the immune system, use of medicines or alcohol, pregnancy and age. It appears to be a very complex and almost impossible task to contemplate bacteria in food products, and the presence of spoilage and pathogenic bacteria result in huge economic losses due to discharged food and food poisoning. Luckily, the majority of bacteria are harmless and some are even helpful.
Traditionally, lactic acid bacteria (LAB) have been used in the production of food, largely added for technological purposes. Typical applications are in dairy products, but also in the production of meat products and wine. More recently, LAB are applied as protective cultures, which should not influence the sensory and technologically properties of food products. LAB are generally recognized as safe (GRAS) organisms and well-known for their ability to produce a number of antimicrobial compounds, including bacteriocins. None of these compounds need, by themselves, to elicit a complete inhibitory effect, but together, or in the presence of other inhibiting factors, they can contribute to a hurdle effect.
Bacteriocins are antibacterial peptides, which may influence the growth of related bacteria in their near environment. They are, therefore, active metabolizing bacteria strains, which can produce bacteriocins during growth in the food product and are advantageous to achieving a long-term inhibiting effect in the product. Further good news is that Listeria monocytogenes is susceptible to some LAB bacteriocins, and consequently, by applying an efficient bacteriocin-producing starter culture, safety is enhanced.
L. monocytogenes is very resistant to various physical and chemical influences, and therefore, is widely spread in nature and thereby present in all types of raw materials – and in humans. Generally, L. monocytogenes resists both high and low temperatures, can resist freezing and drying for quite a long time, and is very tolerant to pH — being able to survive at both high and low pH. Furthermore, L. monocytogenes is known to aggregate biofilm, being stubbornly resistant to antimicrobial agents, which consequently may result in secondary contamination of food products.
Outbreaks of listeriosis are linked to cheese and raw materials, as well as to finished products manufactured from meat, poultry, and fish. Mortality is more than 50 percent for people in the risk groups with compromised immune systems, whereas it is only a few percentage points for individuals with a healthy immune system. Therefore, although L. monocytogenes is widespread in the environment and serious infections are rare, death rates due to listeriosis are high with outbreaks.
Many hurdles are traditionally used in the manufacture of ready-to-eat food products, such as salting, addition of nitrite, organic acids, cold-smoking, and packaging in either modified atmosphere or vacuum. Still, lightly preserved food products are high-risk products when it comes to growth of unwanted bacteria. The present hurdles are not usually sufficient to control the indigenous flora in raw materials, manufacture, and post-treatment/handling of the products, especially since L. monocytogenes remains a microbial risk, particularly in modified atmospheres or vacuum-packed products.
Because of recent outbreaks of listeriosis in Sweden and Denmark, awareness is high. There have been approximately 30 deaths within less than two years (in a total population of about 15.5 million people) due to cooked, sliced meat products and cold-smoked fish products. Therefore, manufacturers of ready-to-eat foods prove the safety of their products by conducting challenge tests to demonstrate that the hurdles are sufficient to meet the EU Regulation EC No 1441/2007. The purpose of the regulation is to prevent making useless (and expensive) microbiological analyses yet to achieve protection of the consumers. The demand is that the level of L. monocytogenes should be <100 CFU/g on the last day of shelf life, which makes sense since a few initial L. monocytogenes may easily grow to high levels during cold storage of ready-to-eat products. The challenge test is conducted by accredited labs, which, when they receive the products, add approx. 100 CFU L. monocytogenes/g product (a cocktail of three serotypes selected by the Swedish authorities), store the products at demanded temperature, and analyze in triplicate three times over the requested storage period.
Below are the results of a challenge test on skinless, emulsion sausages (frankfurters) with and without added Lyocarni BOX-74 (Carnobacterium and Lactobacillus sakei) on the surface after cooking and cooling:
Comparing the two red graphs showing the development of applied L. monocytogenes, it is clear that, by application of a protective culture (figure on the left), it was possible to decrease the level of L. monocytogenes. On the contrary, in a product without culture (figure to the right), L. monocytogenes grew very well.
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