High microbiological quality characterizes a large proportion of Norwegian milk and milk products. Nevertheless, a continuous focus and increased competence on the milk microbiota is important, as reduced milk quality results in significant economic losses to the dairy industry. Bacteria can reduce product quality by altering the taste, texture and/or color, and some species are potentially pathogenic to humans. This project has increased the knowledge on the composition and dynamics of the bacterial microbiota along the production chain of Norwegian consumption milk. The results from the project are useful in the dairy industry's work to assure high quality and for risk assessment. It also has the potential to reduce waste, during and after production of milk and dairy products, due to bacterial contamination. This project has combined the knowledge of two research groups at the Norwegian University of Life Sciences, with different expertise in the food area, namely quality and food safety.
The project has collected milk samples from different stages in the production line of full fat consumtion milk, from arrival of raw milk at the dairy processing plant to end of shelf life of milk cartoons. The milk samples were collected from two different dairies (A and B) over a period of one year to capture seasonal variations in the bacterial composition. In an article published by the project team in ?The International Journal of Food Microbiology?, the levels of cultured bacteria and presumptive Bacillus cereus spp., some of which are potentially deteriorating or pathogenic contaminants in milk and dairy products, were relatively stable in the milk samples, both before and after pasteurization. The bacteria from the milk samples were further characterized by microbiological methods. In addition, molecular biology techniques, such as 16S rRNA gene sequencing, were used to identify the total composition of bacterial species in the milk samples, which is not possible with traditional cultivation methods. This method can also identify non-culturable species. Sequencing of 16S rRNA genes in the milk samples showed that the bacterial composition was significantly affected by the processing stage (raw milk, pasteurized milk, milk in carton). Bacillus and Pseudomonas were the most common bacterial genera in raw milk, pasteurized milk and in milk from cartons at the production day. There was a seasonal variation in the bacterial composition; Pseudomonas species dominated during the autumn and winter months, while Bacillus species dominated in the spring and summer months. During the shelf life, temperature (4 ° C versus 8 ° C) was the most important factor that influenced the bacterial composition. Storage at 8 °C during the shelf life of the milk cartons significantly changed the bacterial composition, with increased relative levels of particular psycrotrophic B. cereus spp. No significant change in the microbiota was observed during storage at 4 °C. This is consistent with results from cultivation, where increased levels of B. cereus spp. was observed during storage at 8 ° C while levels remained unchanged at 4 ° C.
A new method for detailed taxonomic characterization of B. cereus spp. isolates was developed in the project. This method, which was recently published by the project team in the journal ?Food Microbiology?, will simplify characterization of B. cereus isolates. By using the new method, it was found that the composition of B. cereus spp. population was changed during pasteurization, through different seasons and during storage of milk at 8 ° C. The project also investigated the deterioration and virulence properties of B. cereus isolates from milk at genome, protein and metabolite-level. So far, the project has mapped the enzymatic activity (ability to deteriorate milk) and the ability to produce toxins of more than 300 B. cereus spp. milk isolates. The project has also investigated the ability of B. cereus spp. isolates to deteriorate and produce toxins during growth in milk at different temperatures. There is currently limited knowledge about bacterial behavior in mixed species co-culture, which is how they normally occur in nature and thus in raw materials and food products. In a master's thesis linked to the project, it was studied how co-cultivation of a psycrotrophic B. cereus together with Pseudomonas spp. or Paenibacillus (which are also common bacterial contaminants in milk) affect growth in milk as well as production of quality-deteriorating or pathogenic substances. This project has so far generated five master's theses and four scientific articles. Two scientific articles are in progress and will be sent for publication in the beginning of 2019. During the project, three major information meetings together with collaborating dairies and a major information meeting together with dairies in TINE group is being planned.
This project has shown that factors such as season, processing, dairy plant, and storage temperature significantly influence the composition of the milk microbiota. None of more than 300 B. cereus milk isolates produced emetic toxin (causing vomiting). The same type of B. cereus was found in raw milk and in carton milk suggesting that the contamination was due to heat resistant spores surviving the pasteurization. It was not possible to rule out that contamination also occurs during processing. This project has resulted in four scientific peer-reviewed papers (two more in progress) and five master theses. The knowledge gained throughout this project will be implemented in the dairy industrys hygienic work and in their strategies for reducing losses along the value chain caused by bacterial contamination. This will contribute to reduced loss of resources along production chains, ensure food safety and promote food production in a more cost-effective and environmentally sustainable way.
High microbiological quality characterizes Norwegian milk products, nevertheless, nearly 1.7 % of the milk produced by the largest Norwegian dairy product cooperative is downgraded due to inadequate microbiological quality, causing huge financial losses. Reduced shelf-life of will also negatively affect usage further down the production chain (industry and large-scale household). The presence of bacteria in milk can be problematic for two main reasons: i) spoilage by altering flavor, texture or color ii) certain species are capable of causing foodborne disease.
The project will consist of three work packages from which the results from each will be used in the others. Samples will be collected from several points at production lines of two dairy plants throughout the year. The project will combine new molecular biology tools and valuable conventional microbiological techniques and virulence assays, as well as newly applied chemical analysis methods.
Knowledge on the effects of bacterial mixed cultures is still in its infancy for both the academic and industrial societies. Therefore the project will study co-cultures of isolated bacterial strains to address the effects on growth and on production of deteriorative and virulence factors.
Updated and in-depth knowledge on the bacterial flora in the products before pasteurization and throughout shelf life, will enable the industry to determine possible contamination routes. Information about the ability of the bacterial flora to deteriorate food quality and cause foodborne disease, and possible variations throughout the year, would also benefit the industry in their work for safe products of best possible microbial quality.
The project combines the competence of two research groups of different approaches within the food area, quality and safety, and will provide the groups with valuable isolates, method experience and collaboration.