A new cryo-STEM method for imaging food colloids in the scanning electron microscope.

A new cryo-scanning transmission electron microscopy (cryo-STEM) technique for imaging casein micelles in a field emission scanning electron microscope is presented. Thin films of micellar casein suspensions on lacey carbon grids were prepared using a modified sample holder developed by Gatan UK. Bright and dark field images were obtained at -135°C showing casein micelles in their frozen hydrated state and in the size range 30-500 nm. Results were compared favorably with published images of casein micelles obtained with conventional cryo-transmission electron microscopy, suggesting that cryo-STEM is a useful alternative technique for visualizing food colloids close to their native state.

Efficacy of whey protein gel networks as potential viability-enhancing scaffolds for cell immobilization of Lactobacillus rhamnosus GG.

This study investigated cell immobilization of Lactobacillus rhamnosus GG in three separate protein products: native, denatured and hydrolysed whey protein isolate (WPI). Treatments were assessed for their ability to enhance probiotic survival during storage, heat stress and ex vivo gastric incubation. Spatial distribution of probiotic cells within immobilized treatments was evaluated by atomic force and confocal scanning laser microscopy, while cell viability was enumerated by plate count and flow cytometry (FACS). Microscopic analysis of denatured treatments revealed an oasis of immobilized cells, phase-separated from the surrounding protein matrix; an environmental characteristic analogous to hydrolysed networks. Cell immobilization in hydrolysed and denatured WPI enhanced survival by 6.1+/-0.1 and 5.8+/-0.1 log10 cycles, respectively, following 14 day storage at 37 degrees C and both treatments generated thermal protection at 57 degrees C (7.3+/-0.1 and 6.5+/-0.1 log(10) cfu/ml). Furthermore, denatured WPI enhanced probiotic protection (8.9+/-0.2 log(10) cfu/ml) following 3h gastric incubation at 37 degrees C. In conclusion, hydrolysed or denatured WPI were the most suitable matrices for cell immobilization, while native protein provided the weakest safeguard against thermal and acid stress, thus making it possible to envision whey protein gel networks as protective substrates for cell immobilization applications.