Model approach to estimate the probability of accepting a lot of heterogeneously contaminated powdered food using different sampling strategies

• The pathogen may be distributed in ‘nests’ in powdered food.
• The probability of accepting a contaminated lot was investigated.
• For probabilities lower than 0.05, a sample size of 100 g is better than 10 or 1 g.
• Because of low contaminations, a huge number of samples need to be tested.
• Sampling is not effective in controlling pathogens.

Current sampling plans assume a random distribution of microorganisms in food. However, food-borne pathogens are estimated to be heterogeneously distributed in powdered foods. This spatial distribution together with very low level of contaminations raises concern of the efficiency of current sampling plans for the detection of food-borne pathogens like Cronobacter and Salmonella in powdered foods such as powdered infant formula or powdered eggs. An alternative approach based on a Poisson distribution of the contaminated part of the lot (Habraken approach) was used in order to evaluate the probability of falsely accepting a contaminated lot of powdered food when different sampling strategies were simulated considering variables such as lot size, sample size, microbial concentration in the contaminated part of the lot and proportion of contaminated lot. The simulated results suggest that a sample size of 100 g or more corresponds to the lower number of samples to be tested in comparison with sample sizes of 10 or 1 g. Moreover, the number of samples to be tested greatly decrease if the microbial concentration is 1 CFU/g instead of 0.1 CFU/g or if the proportion of contamination is 0.05 instead of 0.01. Mean contaminations higher than 1 CFU/g or proportions higher than 0.05 did not impact on the number of samples. The Habraken approach represents a useful tool for risk management in order to design a fit-for-purpose sampling plan for the detection of low levels of food-borne pathogens in heterogeneously contaminated powdered food. However, it must be outlined that although effective in detecting pathogens, these sampling plans are difficult to be applied since the huge number of samples that needs to be tested. Sampling does not seem an effective measure to control pathogens in powdered food.