Physical sample structure as predictive factor in growth modeling of Listeria innocua in a white cheese model system.

Growth of Listeria innocua at 9 °C was investigated in white cheeses manufactured from ultra-filtrate milk concentrate added varying amounts of skimmed milk powder, NaCl and glucono-delta-lactone. Characterization of the white cheese structures was performed using nuclear magnetic resonance (NMR) T2 relaxation parameters (relaxation times constants, relative areas and width of peaks) and their applicability as predictive factors for maximum specific growth rate, √µ(max) and log-increase in 6 weeks of L. innocua was evaluated by polynomial modeling. Inclusion of NMR parameters was able to increase the goodness-of-fit of two basic models; one having pH, undissociated gluconic acid (GA(u), mM) and NaCl (% w/v) as predictive factors and another having pH, GA(u) and a(w) as predictive factors. However, the best model fit was observed using √µ(max) as response for the model including pH, GA(u), aw and Width T21 revealing the lowest relative root mean squared errors of 14.0%. As the T2 relaxation population T21 is assigned to represent immobilized bulk water protons and the width T21 the heterogeneity of this water population, growth of L. innocua in white cheese seemed to be dependent on the heterogeneity of the immobilized bulk water present in cheese.

Water properties in cream cheeses with variations in pH, fat, and salt content and correlation to microbial survival.

Water mobility and distribution in cream cheeses with variations in fat (4, 15, and 26%), added salt (0, 0.625, and 1.25%), and pH (4.2, 4.7, and 5.2) were studied using (1)H NMR relaxometry. The cheese samples were inoculated with a mixture of Listeria innocua, Escherichia coli O157 and Staphylococcus aureus, and partial least-squares regression revealed that (1)H T(2) relaxation decay data were able to explain a large part of the variation in the survival of E. coli O157 (64-83%). However, the predictions of L. innocua and S. aureus survival were strongly dependent on the fat/water content of the samples. Consequently, the present results indicate that NMR relaxometry is a promising technique for predicting the survival of these bacteria; however, the characteristics of the sample matrix are substantial.

Water properties and structure of pork sausages as affected by high-pressure processing and addition of carrot fibre.

The effects of high-pressure processing (HPP) and addition of carrot fibre on pork sausages have been studied using NMR T2 relaxometry and measurements of water-binding capacity (WBC) by centrifugation. Significant effects of temperature (raw, 40, 50, or 60 °C), holding time (1s, 3, 6, or 9 min), and addition of carrot fibre on the distribution and mobility of water were found. However, the effect of carrot fibre could not be explained by structural changes in the sausages when examined by confocal laser scanning microscopy (CLSM). Correlations between T2 relaxation measurements and WBC determined by centrifugation revealed that T2 relaxation times were able to explain more than 90% of the variation in WBC for both non-pressure and pressure-treated sausages. However, only 49% of the variation was explained for pressure-treated sausages with carrot fibre, indicating that combining addition of fibre and high pressure treatment causes non-coherent changes in T2 NMR relaxation times.

Molecular water motions of skim milk powder solutions during acidification studied by 17O and 1H nuclear magnetic resonance and rheology.

The molecular motion of water was studied in glucono-δ-lactone-acidified skim milk powder (SMP) solutions with various pH values and dry matter contents. NMR relaxometry measurements revealed that lowering the pH in SMP solutions affected 17O and 1H T2 relaxation rates almost identically. Consequently, the present study indicates that the proteins present in the samples do not affect the 1H relaxation behavior markedly, even at relatively high SMP concentrations (15-25%). Comparison of rheological measurements and NMR measurements suggested that the collapse of κ-casein during acidification could contribute to the initial decrease in 17O and 1H relaxation rate in the pH range between 6.6 and 5.5 for 15% SMP and in the pH range between 6.6 and 5.9 for 25% SMP. However, below pH 5.5 the viscosity and 17O and 1H NMR relaxation rates did not correlate, revealing that the aggregation of casein micelles, which increases viscosity below pH 5.5, does not involve major repartitioning of water.

Effect of starter culture and fermentation temperature on water mobility and distribution in fermented sausages and correlation to microbial safety studied by nuclear magnetic resonance relaxometry.

Water mobility and distribution in fermented sausages produced with differences in pH development as a result of the use of three different starter cultures (T-SPX, F-1, or F-SC-111) and two fermentation temperatures (24 degrees C, or 32 degrees C) were studied using low-field proton NMR relaxometry. Changes in the distribution and mobility of water in fermented sausages upon fermentation and drying were detectable by NMR T(2) relaxation, and the progress in the drying process could be followed as a shift towards faster relaxation times. In addition, the distribution of water in the sausages was significantly affected by the pH decline. The sausages were spiked with Listeria monocytogenes, Salmonella, and Escherichia coli VTEC, and partial least squares regressions revealed that 90% of the variation in reduction of Salmonella and VTEC could be explained by the NMR T(2) relaxation decay. Consequently, the study demonstrated that NMR relaxometry is a promising technique for elucidating process parameters and microbial safety in the production of fermented meat products.

Protein conformational modifications and kinetics of water-protein interactions in milk protein concentrate powder upon aging: effect on solubility.

Protein conformational modifications and water-protein interactions are two major factors believed to induce instability of protein and eventually affect the solubility of milk protein concentrate (MPC) powder. To test these hypotheses, MPC was stored at different water activities (a(w) 0.0-0.85) and temperatures (25 and 45 degrees C) for up to 12 weeks. Samples were examined periodically to determine solubility, change in protein conformation by Fourier transform infrared (FTIR) spectroscopy and principal component analysis (PCA), and water status (interaction of water with the protein molecule/surface) by measuring the transverse relaxation time (T(2)) with proton nuclear magnetic resonance ((1)H NMR). The solubility of MPC decreased significantly with aging, and this process was enhanced by increasing water activity (a(w)) and storage temperature. Minor changes in protein secondary structure were observed with FTIR, which indicated some degree of unfolding of protein molecules. PCA of the FTIR data was able to discriminate samples according to moisture content and storage period. Partial least-squares (PLS) analysis showed some correlation between FTIR spectral feature and solubility. The NMR T(2) results indicated the presence of three distinct populations of water molecules, and the proton signal intensity and T(2) values of proton fractions varied with storage conditions (humidity, temperature) and aging. Results suggest that protein/protein interactions may be initiated by unfolding of protein molecules that eventually affects solubility.