Consumers increasingly prefer ready-to-eat foods with few preservatives that require little or no preparation. However, such products are highly perishable. Potato salad is one of the most popular examples in Estonia, and its shelf life depends on its ingredients, packaging and storage temperature.

Food scientists at Tallinn University of Technology investigated bacterial communities developing in packaged potato salad stored at different temperatures and examined how these communities affect the product's shelf life. According to the study's lead author, Inga Sarand, potato salad should ideally be stored at 2–4°C, even though manufacturers recommend a storage temperature of 2–6°C.

"Even a difference of just a few degrees can give spoilage bacteria an advantage, accelerating spoilage and reducing the quality of the potato salad," said Sarand.

Potato salad contains both raw and heat-treated ingredients, making it microbiologically complex. Although the acidic environment of mayonnaise inhibits bacterial growth, boiled potatoes and eggs reduce this protective effect. Spoilage bacteria may originate from raw ingredients or be introduced during food processing, and the composition of the bacterial community varies from one production batch to another.

To extend shelf life, potato salad is often sold in modified atmosphere packaging (MAP). In MAP, air is replaced with a mixture of carbon dioxide and nitrogen, which suppresses the growth of aerobic microorganisms and mould while helping preserve the product's quality for longer.

The study showed that storage temperature has a major influence on which bacteria become dominant in the salad. When MAP-packaged potato salad was stored at 0–4°C, its sensory quality and pH remained stable throughout the recommended seven-day shelf life, and the salad maintained its quality for up to ten days. In contrast, at 6°C, spoilage bacteria multiplied rapidly, leading to a marked increase in acidity by the seventh day. In potato salad stored without MAP, the pH began to decrease by the sixth day regardless of storage temperature.

In addition to conventional microbiological methods, the researchers used DNA-based analyses to identify bacteria at the species level and gain a much more detailed understanding of the spoilage process. They found that spoilage was mainly caused by psychrotrophic lactic acid bacteria, which give the salad a sour taste. At lower temperatures, the lactic acid bacterium Dellaglioa algida became dominant and delayed spoilage by suppressing the growth of other spoilage bacteria.

The findings confirm that, alongside packaging, both storage temperature and the initial bacterial community present in the product play an important role in determining shelf life. Even small differences in storage temperature can determine which bacteria become dominant and how quickly the quality of potato salad deteriorates.

The study was published in the International Journal of Food Microbiology.