High pressure processing of spoilage fungi as affected by water activity in a diluted apple juice concentrate

High Pressure Processing (HPP) is a method used to extend shelf life of foods by subjecting products in their final packaging to extreme pressure. This leads to protein degradation in microbial cells, membrane degradation, and eventual cell death. Survival of spoilage organisms, especially spore-formers, has not been closely considered in regard to the effect of water activity (a(w)). The purpose of this study was to determine the effect of pH and a(w) on HPP apple juice to prevent fungal spoilage during shelf life. Apple juice concentrate was adjusted to a(w) 0.94 (39.3 degrees Brix), 0.96 (32.2 degrees Brix), 0.98 (22.0 degrees Brix), or 1.00 (7.1 degrees Brix) and pH 4.6 or pH 7.0. Polyethylene terephthalate bottles were filled with concentrate and inoculated with Penicillium spp., Aspergillus niger, Byssochlamys spectabilis, Rhodotorula mucilaginosa, Candida parapsilosis, Torulaspora delbrueckii, or Brettanomyces bruxellensis. Samples were HPP treated at 450 MPa for 1.5 min and for pressure-resistant species, at 600 MPa for 1.5 min or 3 min to resemble industrial processing conditions. Fungi were more resistant to HPP at lower a(w) and higher pH. C. parapsilosis was more resistant to HPP than the other organisms tested. Fungi, with the exception of C. parapsilosis, experienced a 4.26- or greater log reduction at a(w) of 0.98 and above at either pH. Their reductions ranged from 0.5 to 5.3 log reduction at 0.94 a(w) and 0.96 a(w) with pH 4.6. The reductions were 0.1-4.5-log at 0.94 and 0.96 a(w) at pH 7.0. In general, shelf life was a matter of weeks at ambient temperatures and 7.0 pH, even when processing pressures were increased to 600 MPa. To reduce risk of fungal spoilage, HPP products should be at or above 0.98 a(w) and below 4.6 pH and stored at refrigerated conditions.