Thermally induced milk fouling: Survival of thermophilic spore formers and potential of contamination

Thermophilic spore formers are prevalent contaminants in powdered milk products. In this study, we investigated whether fouling layers formed during the heat treatment of milk have protective effects on entrapped thermophilic spores. First, we developed an experimental set-up to produce fouling layers and incorporate spores. It was possible to produce fouling layers that contained a constant count of thermophilic spores. Afterwards, the fouling layer was subjected to a cleaning procedure with alkaline, enzymatic, or acidic solutions and a peracetic acid disinfectant. No treatment could reduce the spore counts below 3 log in the contaminated fouling. Moreover, it was demonstrated that spores could transmit into the product stream in dairies. This was shown by submerging a contaminated fouling layer in UHT milk and measured the spore count afterwards. This work emphasises that milk fouling is a reservoir for thermophilic spores and additionally can protect entrapped spores against cleaning procedures.