How to tackle undesirable microbes in foods, in particular bacterial spores?

High rates of spoilage before the end of shelf life of foods? Consumer complaints? Or worse, cases of foodborne illness due to consumption of your products? This must be avoided and safeguarded in production. As all foods placed on the market must be safe and of high quality, clearly, control of microbes that may cause food spoilage or that lead to foodborne illness has high-level priority in the food industry.

To control contaminants in fresh ready-to-eat foods, the food industry uses different conditions to prevent outgrowth of undesirable microbes. For example, low storage temperatures and short distribution chains. In processed foods, undesirable microbes are often eliminated by applying heat or alternative treatments that kill them. However, bacterial spores, which are the hardiest forms of life on earth, may survive such treatments. Their ubiquitous presence in the environment means that their presence in food ingredients and in food processing facilities is inevitable. As a result, sporeformers may lead to a reduced shelf life of products due to spoilage and in the case of foodborne pathogenic sporeformers, consumption of products in which growth has occurred may even lead to illness or death. Clearly, contaminants originating from ingredients or the processing environment must be controlled during and following production to achieve the required shelf life and assure food safety. But how to do that?

1. Analyse the number of pathogenic spores which are present in ingredients and finished product

To prevent potential problems with spores in finished food products that undergo heat treatments, the ingredients generally have specifications for spores. Many different methods exist to detect spores of different types of sporeformers, as not all spores behave the same. Different growth media may be used and incubation may take place at specific temperatures (e.g. psychrotrophic, mesophilic or thermophilic species) or in the presence or absence of oxygen. In addition, a distinction can be made in the heat resistance of spores by applying different heat treatments (e.g. 10 min 80°C, 30 min 100°C, 30 min 106°C). When setting specifications for spore concentrations in ingredients, it is critical to apply meaningful methods with relevant detection limits. Ideally, the methods allow for a link between the sporeformers present in ingredients and the potential defect rates in finished products after production. Knowledge about spore detection, problem species, impact of processing on different spore types and growth potential in finished products is crucial for such assessments.

2. Identify the source of contamination

Sporeformers may enter the production chain via ingredients. Another contamination route is via processing equipment containing biofilms that shed spores into the product. This may happen in holding tanks, on transport belts, around seals and even in heating equipment, such as the regenerative section of pasteurizers or in evaporators (by thermophilic species). When spores cause problems in finished product, a ‘track-and-trace’ approach of the problem microbes can identify the source of the contamination. By assessing the genetic make-up of the problem organism and of isolates in ingredients and along the production lines, a source can be found.

3. Define acceptable levels of pathogenic spores in finished products

Even low levels of spores in finished product may lead to spoilage or foodborne illness if they survive and if outgrowth can occur: one per packaging unit may be enough! When spores are present in ingredients, reduction to acceptable levels can be achieved by inactivation (e.g. by heat treatments) or by removal (in liquid products e.g. by bactofugation or ultrafiltration). The efficacy of such treatments can be assessed experimentally. To do so, it is critical to use spores with representative properties. The efficacy can also be calculated using modelling approaches when relevant data are available. If adjustment of processing conditions does not suffice, reduction at source may be necessary.

4. Select conditions that prevent outgrowth

Spores can ‘wake up’ via the process of germination in nutritious environments. This may happen during food production or in finished products. When conditions in the product (e.g. water activity, pH) and during storage (e.g. temperature) are favourable for outgrowth, the sporeformers can multiply. Traditional preservatives that are effective in preventing germination and outgrowth of spores (for instance sorbate and benzoate) are used less nowadays. New formulations and novel processing methods are being developed, for instance based on clean label preservatives and combinatory treatments. The efficacy of such formulations must be evaluated in products using relevant strains of sporeformers and spores thereof. This requires knowledge of species to inoculate and often high-throughput testing of conditions, to ensure efficacy of treatments.

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