Consumers are increasingly choosing ready-to-eat foods with fewer preservatives. However, such products also spoil more quickly. One of the most popular examples is potato salad whose shelf life is affected by its ingredients, packaging and storage temperature.

Food scientists at Tallinn University of Technology (TalTech) studied the bacterial communities that develop in packaged potato salad at different temperatures and how they affect the product's shelf life. According to the study's lead author, Inga Sarand, potato salad should be stored as cold as possible, preferably at 2–4 degrees Celsius, even though manufacturers recommend storing it at 2–6 degrees Celsius.

"Even a temperature increase of just a few degrees can give spoilage bacteria an advantage, accelerating spoilage and reducing the salad's quality," Sarand said.

Potato salad contains both raw and cooked ingredients, making it a microbiologically complex product. Although the acidic environment created by the mayonnaise slows bacterial growth, boiled potatoes and eggs reduce that protective effect. The microorganisms responsible for spoilage may originate in the raw ingredients or be introduced during processing and the composition of those bacterial communities varies from batch to batch.

To extend shelf life, potato salad is often sold in modified atmosphere packaging (MAP). In this type of packaging, air is replaced with a mixture of carbon dioxide and nitrogen, which inhibits the growth of oxygen-dependent microbes and mold while helping preserve the product's characteristics for longer.

The study found that storage temperature has a major impact on which bacteria begin to grow in the salad. When potato salad in MAP packaging was stored at 0–4 degrees Celsius, its flavor and pH remained stable throughout its designated seven-day shelf life and the salad maintained good quality for up to 10 days.

By contrast, at 6 degrees Celsius, spoilage bacteria multiplied rapidly and by the seventh day the salad's acidity had increased significantly. In potato salad that was not packaged using MAP, pH began to decline by the sixth day regardless of storage temperature.

In addition to conventional microbiological methods, the researchers used DNA-based analyses that allowed them to identify bacteria at the species level and gain a more detailed understanding of the spoilage process.

The researchers found that spoilage is primarily caused by cold-tolerant lactic acid bacteria, which give the salad a sour taste. At lower temperatures, however, the lactic acid bacterium Dellaglioa algida became dominant, suppressing the growth of other spoilage bacteria and helping the salad stay fresh longer.

The study confirms that, in addition to packaging, potato salad's shelf life is influenced by storage temperature and the product's initial bacterial community. Even small differences in temperature can determine which bacteria become dominant and how quickly the product's quality deteriorates.

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

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