WATER ACTIVITY OF SKIMMED MILK POWDER IN THE TEMPERATURE RANGE OF 20 – 45 °C
J. ·TENCL Mendel University of Agriculture and Forestry, Brno, Czech Republic Received May 24, 1999 Accepted July 16, 1999 Abstract ·tencl J.: Water Activity of Skimmed Milk Powder in the Temperature Range of 20 – 45 °C. Acta Vet. Brno 1999, 68: 209–215. Water activity data foradsorption and desorption of moisture from skimmed milk powder were investigated at temperatures in the range of 20 – 45 oC and moisture content of the material tested from 3.2 to 20 % (wet basis). The experimental procedure used was a gravimetric dynamic method with continuous registration of sample weight changes. Four mathematical models of sorption isotherms (Chung-Pfost, Halsey, Henderson, andOswin) were evaluated to determine the best fit for the experimental data. The modified Oswin equation was a good model for moisture adsorption and desorption of skimmed milk powder. Water sorption capacity decreased as temperature increased. The critical value of equilibrium moisture content of milk powder tested, corresponding to the water activity equal 0.6, was 11 % (wet basis) at the temperatureof 20 oC. Repeated rehydration of the material brought an increase in the original equilibrium moisture content 3.2 % (wet basis) to 6.3 – 8.1 % (wet basis) in relation to the temperature. It was also demonstrated that an increase in equilibrium moisture content was very small (about 4 % wet basis) in the range of water activity 0.1 to 0.9. Higher levels of water activity than 0.9 resulted in amarked increase of equilibrium moisture content and susceptibility to spoilage by microorganisms. The hysteresis effect between moisture adsorption and desorption was insignificant. Adsorption, desorption, microbial food stability, milk powder, modeling, water activity
The moisture sorption isotherm is an extremely valuable tool for food scientists and technologists because it can be used topredict potential changes in food stability; it can be used for storing method determination, packaging selection and ingredient selection. The moisture sorption isotherms of foodstuffs show usually the equilibrium relationship between water activity (aw) and moisture content (MC) of the food at constant temperatures and pressures. A critical aw also exists below which no microorganisms can grow(Beuchat 1981). For most foodstuffs, this is in the range of 0.6-0.7 aw. In general, dehydrated foods have aw’s less than 0.6; semi-moist foods, such as cereal grains, raisins, dates, syrups, and intermediate-moisture pet foods usually have aw between 0.62 and 0.92. Cheeses, jams, jellies, meat, fish etc. have aw’s greater than 0.92. Thus, with knowledge of the moisture sorption isotherm, we canpredict the maximum moisture that the food can be allowed to gain during storage (Kieslingerová and Bartl 1993). Of course, higher aw’s can be allowed if other factors such as pH, salt, antimicrobial agents, and temperature are taken into consideration. Table 1 lists minimum aw values for growth and toxin production by pathogens (Beuchat 1981). Generally, temperature has important influence on aw.Investigations of water adsorption/desorption isotherms have been the subject of study for numerous products due to the development of modern techniques for their processing and storage. Procedures for obtaining water sorption isotherms in foods were described in detail by
Address for correspondence: Doc.Ing. Jiﬁí ·tencl, CSc. Department of Postharvest Technology and Engineering Mendel University ofAgriculture and Forestry Zemûdûlská 1, 613 00 Brno, Czech Republic Phone: ++420 5 45132116 Fax: ++420 5 45212044 E-mail: firstname.lastname@example.org http://www.vfu.cz/acta-vet/actavet.htm
Table 1 Minimal aw limits for some microorganisms of significance to public health Microorganism Growth Aspergillus clavatus A. flavus A. ochraceus A. ochraceus A. parasaticus Bacillus cereus Byssochlamys nivea...