Plant "milks" and their potential microbiological risks in a context of climate change

Geobacillus stearothermophilus is one of the main bacteria responsible for spoilage of food products treated with ultra-high temperature (UHT), due to the extremely heat-resistant spores it can produce. Spores that have survived UHT treatment can germinate and grow in sufficient quantity to render the product unusable (known as spoiled product) after exposure to a temperature above their minimum growth temperature for a certain period of time.

To date, these products can be stored at room temperature for several months without risk of spoilage, as the difference between preparation and processing temperatures is sufficiently great for spores not to survive. Nevertheless, given the expected rise in temperature due to climate change, cases of non-sterility during distribution, transport and storage at room temperature are likely to multiply. All the more so as the ingredients of milk substitutes (plant "milks") are particularly loaded with bacterial spores, and G. stearothermophilus spores in particular. The aim of the study carried out by UMR SECALIM (INRAE ONIRIS) was to quantitatively assess the risks of spoilage of plant-based dairy alternatives in Europe. Quantification took 4 main stages into account: 1) initial contamination of raw materials; 2) thermal inactivation of spores during UHT  processing; 3) distribution in bottles or bricks; 4) germination and growth of spores during distribution and storage.

The risk of spoilage was defined as the probability of G. stearothermophilus reaching its maximum concentration at the time of consumption. The assessment was carried out for 2 countries (as an example for Northern Europe: Poland, and for Southern Europe: Greece), and the spoilage risk was estimated for current climate conditions and a climate change scenario.
All calculations were performed using 2-dimensional Monte Carlo statistical methods to incorporate variability (e.g. variability in the initial level of spore contamination) and uncertainty (lack of data) into probabilistic models. R software was used for this purpose.
According to the results, the risk of spoilage is negligible for the Northern European region, while the risk of spoilage in Southern Europe is already proven under current climatic conditions. On the other hand, under climate change conditions, the 2 regions will be at risk of sppoilage during the summer periods: heat peaks will exceed the minimum temperatures required for spore germination and subsequent growth.
Following these results, the intensity of heat treatment and the use of thermally insulated trucks for distribution were studied as measures to control microbiological risk. These measures led to a significant reduction in risk.

The quantitative assessment developed in this study can help with risk management decisions for these products, by enabling different control options to be tested under different weather scenarios.

SCIENTIFIC PARTNERS: Aristotle University of Thessaloniki, Greece
FUNDING: European Union, Horizon 2020 research and innovation program, Marie Sklodowska-Curie grant agreement, PROTECT project no. 813329.
ASSOCIATED PUBLICATION: Misiou O., Koutsoumanis K., Membré J. M. (2023). Quantitative microbial spoilage risk assessment of plant-based milk alternatives by Geobacillus stearothermophilus in Europe. Food Research International, 166, 112638. doi: https://doi.org/10.1016/j.foodres.2023.112638