Survival of probiotic microorganisms in dried foods is optimal for water activity (aw) values between 0.1 and 0.3. Encapsulating and adding low-molecular weight additives can enhance probiotic viability in intermediate aw food products, but the effectiveness of sub-lethal homogenization is still not proven. This study evaluates the effect of 10% (w/w) trehalose addition and/or 100 MPa homogenization on Lactobacillus salivarius CECT 4063 counts and antioxidant properties of apple slices dried to different water activity values (freeze-drying to a aw of 0.25 and air-drying at 40 °C to a aw of 0.35 and 0.45) during four-week storage. Optical and mechanical properties of dried samples were also analyzed. Freeze-drying had the least effect on the microbial counts and air drying at 40 °C to a aw of 0.35 had the greatest effect. Antioxidant properties improved with drying, especially with convective drying. Decreases in both microbial and antioxidant content during storage were favored in samples with higher water activity values. Adding trehalose improved cell survival during storage in samples with a water activity of 0.35, but 100 MPa homogenization increased the loss of viability in all cases. Air-dried samples became more translucent and reddish, rather rubbery and less crispy than freeze-dried ones.
Keywords: Lactobacillus salivarius spp. salivarius; antioxidants; freeze-drying; high pressure homogenization; hot air drying; trehalose; water activity.