ISSN:
1476-5535
Keywords:
Bacterial growth retardation
;
Suppression of the activity of water
;
Protein hydration
;
Deuterium NMR relaxation
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Process Engineering, Biotechnology, Nutrition Technology
Notes:
Summary Food microbiologists have long known that suppression of the activity of water,a w, can retard microbial growth in food systems. Traditionally,a w, suppression has been achieved by addition of salts or humectants to foods. To limit the amount of preservatives added to food products, studies were initiated to assess the feasibility of using proteins to suppressa w to a practical value for retarding bacterial growth and to determine the optimum environmental condition for maximizing this effect for milk proteins. New expressions were developed relating observed longitudinal and transverse NMR relaxation rates, in the absence of cross-relaxation, to protein hydration $$\bar \upsilon _w $$ , to the protein activity coefficient, γp, and to the correlation time of the bound water, τc. From γp, the second virial coefficient of the protein,B o, can be found. By use of $$\bar \upsilon _w $$ andB o,a w could then be directly evaluated at any protein concentration. Resulting expressions were tested by2H-NMR relaxation measurements made as a function of protein concentration, for: β-lactoglobulin A (the major whey protein) under nonassociating (pH 6.0) and associating (pH 4.65) conditions; and for casein (the major milk protein) in the micellar (with added Ca2+) and submicellar (without Ca2+) forms. Values ofa w calculated from these2H-NMR data show that casein, at all the concentrations and temperatures examined, suppressesa w more than does β-lactoglobulin A because of a largerB o. In turn, micellar casein suppressesa w to a larger extent than does submicellar casein because of a larger $$\bar \upsilon _w $$ . Extrapolation ofa w at 4°C to a concentration ten times that in normal milk yields a value, ofa w of less than 0.95, at whichSalmonella and some strains ofClostridium botulinum no longer grow. These results are in agreement with what is known about storageability of condensed milk. Generalizations regarding the types of proteins and cosolutes to be used for suppressinga w will be discussed. Structural information on these proteins calculated from τc will also be presented.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01569521
