Thanks to lactic acid — which kills harmful bacteria during fermentation — fermented foods are arguably among the safest foods that humans eat. But if critical errors are made, there is the  risk of food safety hazards.

“When we talk about fruit and vegetable ferments, there is a very long history of microbial safety with traditionally fermented fruits and vegetables,” says Erin DiCaprio, extension specialist at University of California, Davis, Department of Food Science and Technology. “While outbreaks associated with fermented fruits and vegetables are rare, vegetable and fruit fermentation is not without risk.”

DiCaprio, a food safety expert, detailed proper food safety protocols during a webinar for EATLAC, a UC Davis project putting scientific knowledge and research behind fermentation. DiCaprio shared two documented instances of fermented foods causing a foodborne illness, both from small-scale batches of kimchi. But she emphasized that, when all food safety concerns are mitigated, fermented foods do not pose a risk.

“From the food microbiology standpoint, bacteria really are the most important group of microorganisms because bacteria, certain types of bacteria, are a food safety concern,” she adds. “There are many different types of bacteria that contribute to food spoilage and, of course, there are specific types of bacteria that are used beneficially for fermentation.”

What happens during fermentation that makes food safe? Lactic acid bacteria are created, which convert sugars into lactic acid, acetic acid and CO2. Those antimicrobial compounds help fight off pathogens, competing with other microbes for nutrition sources.

Biological hazards — bacteria, viruses and parasites that can cause foodborne illnesses — are the biggest concerns. Botulism, E. coli and salmonella are the main hazards for fermented foods. Botulism can form in oxygen-free conditions if a fermentation is not successful and acid levels are too low. E. coli and salmonella form when sanitation practices are not followed.

“Commercially, when someone is developing a valid fermentation process, they are typically going to be looking to see that sufficient acid is produced during fermentation, to inactivate some of these acid tolerant (bacteria),” DiCaprio says. “Our traditional vegetable fermentations — things like fermented cucumber, sauerkraut, kimchi — they’ve all been shown to produce sufficient acid to inactivate the sugar toxin producing e-coli, so from a safety standpoint, sufficient acid production is the critical control point for ensuring the safety of a fermented fruit or vegetable. “

There are seven critical factors to keep a ferment safe:

1. High-quality, raw ingredients. “If there are a high number of spoilage microorganisms to start with, it will be really difficult for the lactic acid bacteria to dominate the fermentation,” DiCaprio says.

• Research-based recipe. Following a tested recipe ensures the proper balance of ingredients to keep the food safe.
• Proper sanitation. Cleaning of all utensils and surfaces ensures no pathogens will contaminate the food. This mitigates cross-contamination risk, too.
• Preparation of ingredients. Food particles should be uniform in size, either cut in small slices or shredded. Smaller pieces release more water and nutrients, promoting the growth of lactic acid bacteria.
• Salt concentration. Lactic acid bacteria thrive in a salt brine. The key amount: anywhere from 1-15% salt brine by weight of the ferment.
• Appropriate temperature. A temperature between 65-75 degrees fahrenheit is ideal to keep spoilage microorganisms at bay.
• Adequate time. “It takes a while for significant acid to be produced, so be patient and follow the directions in the recipe,” DiCaprio advises. Most fruit and vegetable ferments take 3-6 weeks to be completed.