Microbio­logical challenges plant-based alternatives

Globally, the market for plant-based dairy and meat alternatives is growing. Little is known about the specific microbiological hazards for applications of plant-based ingredients in new product formulations. Predictive models fall short for the time being. Time for more research.

In the refrigerated section of the supermarket, more and more plant-based products can be found as alternatives for products containing animal proteins. In the development and production of these new plant-based products, many steps have already been taken to improve the taste and texture to meet consumers' wishes. In addition, it is also important that plant-based products are microbiologically stable: for safe consumption, and for a sufficiently long shelf life to minimise food waste. However, in the current transition from animal proteins to plant-based proteins, there is a growing awareness that plant-based products are not comparable with their animal protein-based counterparts in terms of microbiology. 

Contamination risk

Microbial spores (a microbial survival form found in certain bacteria and fungi) are commonly found in the soil of agricultural crops. The basis of microbiological hazards in plant-based products therefore lies at the source of raw materials. Soy, pulses, wheat, or potatoes are exposed to a multitude of microorganisms during cultivation (soil, water and air). If these microorganisms contaminate the crop, they can potentially cause problems in proteins isolated from it. The level and type of contamination varies greatly by product type, processing type and season. 

Researchers at Wageningen Food & Biobased Research (WFBR) sometimes encounter more than 1,000 spores per gram for a number of plant protein ingredients. These spores can, under the right conditions, germinate and grow, which can lead to shelf-life problems (microbial spoilage) and even product safety issues (pathogen outgrowth). To prevent this, production processes and product properties aim to prevent germination and outgrowth. Research should establish whether the conditions are sufficient to ensure microbiologically stable and safe products. Microbial contamination can also occur through post-contamination during the production process, for instance during packaging of the product after heat treatment.

Recalls: practical examples 

Several recent recalls for plant-based dairy and meat alternatives illustrate how difficult it can be to ensure microbiological safety. For example, in the United States early this year there was a recall of baby milk powder based on vegetable proteins. The reason: presence of Cronobacter sakazakii, a potentially pathogenic bacterium. The suspected cause was cross-contamination during the manufacturing process. The bacterial species is part of a group of bacteria (Cronobacter spp.) that can survive well in a dry environment, as is the case with milk powder. Therefore, EU legislation (1) sets strict food safety criteria for this bacterium in baby milk powder. That this bacterium has now also been found in a plant-based version of baby milk powder indicates the importance to gain more knowledge about plant-based alternatives.  

In Australia, an almond drink was recalled from sale in mid-February because botulinum toxin was detected by authorities in a sample of the product. One person became seriously ill with symptoms of botulism after consuming this almond drink. Botulism is a rare but life-threatening disease caused by toxins (poisons) produced by the bacterium Clostridium botulinum. The label did not state that the product should be kept refrigerated. This created conditions (unrefrigerated storage) where spores of this bacterium could germinate and grow and form toxins.    

A third example of a recall early this year involved several vegan cheese products and a vegetable pâté produced in France. These products were recalled because of the possible presence of Listeria monocytogenes. L. monocytogenes is a bacterium usually linked to ready-to-eat products of animal origin such as (soft) cheeses, processed meats and smoked salmon. But it turns out that it is also important to pay attention to controlling Listeria for plant-based dairy and meat alternatives as well.   

Featured

About 4 in a million people get infected with L. monocytogenes each year. Although listeriosis (the disease caused by L. monocytogenes) is relatively rare, the mortality rate of the disease is high: in 20 to 30% of cases, listeriosis leads to the death of the patient. Fatal cases mostly are in vulnerable groups such as the elderly and pregnant women. To minimize the risk of infection among consumers, refrigerated ready-to-eat products must therefore meet a legally established food safety criterion: producers must demonstrate that the limit of 100 colony-forming units of L. monocytogenes per gram is not exceeded during the entire shelf life of the product. To demonstrate this, challenge tests on the specific product are necessary. These are labour-intensive and therefore costly activities.

Validated models necessary

Predictive models can be used to estimate in advance the risk of outgrowth of L. monocytogenes and thus reduce the number of challenge tests required. It is important that these models are validated in a relevant product matrix. However, there is a gap here for plant-based products. Many of the open-source models used, such as Food Safety and Spoilage Predictor (FSSP) and Combase, have not been validated for plant-based products. In addition, some parameters of plant-based products, such as pH, do not fall within the range of the predictive model. With this in mind, a public-private partnership (PROSPECT) (2) was set up in which Wageningen University & Research is working with Corbion, Vivera, VanLoon Group, Zwanenberg and Chr. Hansen to develop improved models for prediction of L. monocytogenes growth in plant-based products. 

Within the project, the researchers are looking at validating available growth-prediction models for L. monocytogenes and enriching existing datasets with experimentally determined growth in the product. Various storage temperature scenarios are being considered. In addition, miniaturized products have been developed that mimic the composition of plant-based meat alternatives. This makes it possible to efficiently generate new data for growth of L. monocytogenes in relevant matrices of variable composition (plant protein source, pH, water activity, organic acids). The miniaturized matrices appear to yield good results and contribute to the development of more accurate growth models for Listeria in plant matrices.

Cooperation with WUR

There is not only a need for more knowledge about safety of plant-based products. Also (publicly accessible) predictive models for microbial spoilage, specifically developed and validated for plant-based products, are desired. More knowledge contributes to preservation of product quality during storage, and reduction of food loss. Wageningen University & Research is therefore setting up a collaboration aimed at developing knowledge on microbial spoilage in plant-based products: SHIELT: Shelf-life 4 plant-based alternatives. In particular, the research focuses on plant-based alternatives for meat products and the effect of mild preservation technologies on microbial spoilage. This knowledge will then be used to develop predictive models for microbial spoilage in plant-based products. For more information:

www.wur.eu/shielt

www.wur.nl

Sources

1 COMMISSION REGULATION (EC) No. 2073/2005 of November 15, 2005 on microbiological criteria for foodstuffs.
2 www.wur.nl/en/show/PROSPECT-to-predict-the-growth-of-Listeria-monocytogenes-in-plant-based-foods.htm

Photo burgers: ©Mila Naumova/shutterstock.com, photo science: ©LookerStudio/shutterstock.com