ELSEVIER hr. Iloir\, Journal 7 (I 997) 647 653 c 199X Elsevier zyxwvutsrqponm Sc ie nc e Ltd All rig hts re se rve d . Printe d m G re a t Brita m PII: SO958-6946(97)00059-9 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC 0958-hY46/YXi$lY.00+0.00 Determination of Protein Content of Whey Powder Using Electrical Conductivity Measurement Y. Zhuang, W. Zhou *, M. H. Nguyen and J. A. Hourigan Shof of Food Scirnrrs. University of’ WcJstem Sydney -Huwke.shur.v, Richmond, NSW 2753. Austruliu (Received 17 October 1995; accepted 22 September 1997) zyxwvutsrqponmlkjihgfedcbaZYXWVU ABSTRACT A new method was developed to determine the protein content of commercial whey powder using electrical conductivity (EC:) measurement. This method is based on a multi-point measurement technique and a mathematical model that can identify the contributions of protein and non-protein constituents to EC. Twenty-four samples were analysed separately by the Kjeldahl and EC methods under different conditions. A significant correlation (r=0.96) was observed and no significant difference was found between these two methods (PC 0.05). The results demonstrate that the protein content of whey powder can be estimated by measuring changes in the conductivity of its dilute solution. 8” 1998 Elsevier Science Ltd. All rights reserved NOTATION AP c Abnormal point G zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA K A4 PC W x Y i r7 ;: PI, P,, Adjustment value for the estimated protein concentration (gmL_‘) Weight of sample (g) Estimated protein concentration (gmL_‘) Moisture content (weight %) Protein content (weight O/o> Estimated total water volume (mL) EC reading (ms cm-‘) Contribution of sample’s protein to EC relative to the standard sample (ms cm-‘) The ith point corresponding to the EC measurement of the sample Specific volume of sample (mL g.-‘) Coefficient of Cj Integer in Ki calculation Weight of total whey protein in whey powder (g) Weight of total soy protein in soy protein isolate (g) NP Weight of total non-protein substances (g) Subscripts: i The point corresponding to each EC measurement of the sample ci The point corresponding to each EC measurement of the standard sample min Minimum max Maximum. INTRODUCTION Progress in protein research, as well as the application of protein analysis to dairy husbandry and “Corresponding author. quality control in the dairy industry, was for a long time hampered because no rapid and accurate method of analysis was available (Grappin and Ribadeau- Dumas, 1992). The most popular method for the determination of protein content is the Kjeldahl method. Two indirect methods are dye-binding and infra-red. The Kjeldahl method is still the standard against which other methods are compared (Grappin and Ribadeau-Dumas, 1992). Today, the Kjeldahl method for protein determination evokes increasing environmental concern (Schmitter and Rihs, 1989). Also. it is time-consuming and not economically feasible for many smaller companies (Richard and Paquin, 1990). For process quality control, a simple method for measuring protein content should be developed that is rapid, accurate, continuous, economical and in which no chemicals are involved. The aim of this study was to develop such a method to determine the protein content of commercial whey powders. The electrical conductivity of milk and whey is due mainly to its soluble salt fraction, and the contribution of proteins is of minor importance (Mucchetti ct al., 1994). Lactose, an uncharged sugar, does not conduct current. Fat is a non- conductor and hinders the conduction of electricity by occupying volume and by impeding the mobility of ions (Prentice, 1962). Therefore, the conductivity of milk and whey is determined primarily by Hi and Cl _ (Schultz and Sydow, 1957) and also by other ions (Jenness and Koops, 1962). The contribution of the whey proteins to overall conductivity is difficult to quantify but is assumed to be small, considering their molecular size (Mucchetti et al., 1994). However, the conductivity of a strong electrolyte increases approximately in proportion to its concentration (Chen and Soucie. 1986) and in contrast, the conductivity of a weak 641