Article pubs.acs.org/JAFC
Water Properties in Cream Cheeses with Variations in pH, Fat, and Salt Content and Correlation to Microbial Survival Sandie M. Møller,*,† Tina B. Hansen,§ Ulf Andersen,# Søren K. Lillevang,# Anitha Rasmussen,# and Hanne C. Bertram† †
Department of Food Science, Faculty of Science and Technology, Aarhus University, Kirstinebjergvej 10, DK-5792, Årslev, Denmark DTU Food, National Food Institute, Division of Microbiology and Risk Assessment, Technical University of Denmark, Mørkhøj Bygade 19, DK-2860, Søborg, Denmark # Arla Strategic Innovation Centre, Arla Foods, DK-8220 Brabrand, Denmark §
ABSTRACT: 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 1H 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 1H T2 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. KEYWORDS: cream cheese, transverse relaxation, low-field NMR, water self-diffusion, Listeria innocua, Staphylococcus aureus, E. coli O157
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INTRODUCTION Pathogenic bacteria are unwanted in food, and extensive efforts are made to prevent the presence of these bacteria. However, from time to time, pathogenic bacteria are detected in different foods, and it is therefore of interest to make sure that these bacteria cannot grow to such a number that they pose a threat to human health. Since 1953 when Scott established a correlation between water activity and bacterial growth, overall water activity in food items has been used as an indicator of whether bacterial growth is possible.1 However, a theory proposed by Hills et al.2 suggests that the microstructure in food gives rise to different local water activities within the food. This theory was confirmed for a variety of model porous systems, proposing a new model for water activity, whereby the microstructure of the food should be taken into account when an overall water activity is calculated.2 This theory is based on fast exchange of water between bulk, surface, and bound states, and the form of the theory is similar to that describing NMR water relaxation rates in the fast-exchange limit.3 1H NMR T2 relaxation can be used to investigate the availability of water in foods, where each state of water present has its own intrinsic water relaxation time. In cheese systems NMR relaxometry has been applied for monitoring changes in water mobility and water distribution during production of mozzarella and imitation cheese4,5 and to characterize fat and water states in cheese.6,7 Furthermore, NMR relaxometry has in dairy products been applied for the prediction of creaminess in cream cheese,8 to characterize acidified milk drinks,9 for the estimation of water-holding capacity in fresh cheese,10 and to investigate the initial stage of cheesemaking, gel formation, gels, and syneresis development.11−14 © 2012 American Chemical Society
Few studies have attempted to use NMR relaxometry to investigate the growth and survival of different pathogenic bacteria.3,15,16 Hills et al.3 found that in the slow, diffusive limit of NMR relaxation, it was possible to monitor the microscopic redistribution of water between different pores in the beds of porous media, and these changes were correlated with microbial recovery rates. In addition, a recent study by Møller et al.15 found that 90% of the variation in the reduction of Salmonella and Escherichia coli VTEC in fermented sausages could be explained by NMR T2 relaxation data. These findings are intriguing and reveal that NMR relaxometry contains information of importance for bacterial growth potential in foods. The objective of the present study was (i) to investigate the mobility and distribution of water in a model system consisting of different cream cheeses using 1H NMR relaxometry, (ii) to monitor the behavior of Listeria innocua, Escherichia coli O157, and Staphylococcus aureus over a period of 6 months and calculate survival parameters, and (iii) to investigate if any correlation could be established between the 1H NMR relaxometry data and survival parameters of L. innocua, E. coli O157, and S. aureus, respectively.
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MATERIALS AND METHODS
Microorganisms and Inoculum Preparation. L. innocua (DSM 20649), E. coli O157 (ATCC 43888), and S. aureus (ATCC 6538) were maintained as frozen suspensions at −80 °C. Cultures were grown in tryptone soy broth (Oxoid CM 0129, Basingstoke, U.K.) at Received: Revised: Accepted: Published: 1635
October 26, 2011 January 24, 2012 January 25, 2012 January 25, 2012 dx.doi.org/10.1021/jf204371v | J. Agric. Food Chem. 2012, 60, 1635−1644
Journal of Agricultural and Food Chemistry
Article
Table 1. Sample Codes for the 11 Cream Cheese Products, Experimental Design, and Analyzed Chemical Composition, pH, and Water Activity (aw) experimental design
a
analyzed chemical composition
sample
fat (%)
salt (%)
pH
fat (%)
protein (%)
dry matter (%)
water (%)
salta (%)
pH
aw
N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11
4 26 4 26 4 26 4 26 15 15 15
0 0 0 0 1.25 1.25 1.25 1.25 0.625 0.625 0.625
4.2 4.2 5.2 5.2 4.2 4.2 5.2 5.2 4.7 4.7 4.7
4.0 24.5 4.0 24.4 3.9 25.4 3.9 24.1 14.7 14.4 14.6
12.8 4.8 12.5 4.8 12.3 4.8 12.4 4.9 11.1 11.1 11.2
21.8 33.3 21.1 33.5 22.7 34.4 22.2 34.1 31.0 31.0 30.9
78.5 67.0 78.4 66.9 77.5 65.2 77.8 65.9 69.3 69.2 69.4
0.17
