* Corresponding author. Fax: #46-46-222-46-22. E-mail address: kristina.lodaite@livstek.lth.se (K. Lodaite). International Dairy Journal 10 (2000) 829}834 One-dimensional syneresis of rennet-induced gels Kristina Lodaite*, Karin O G stergren, Marie Paulsson, Petr Dejmek Food Engineering, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden Received 5 September 2000; accepted 12 January 2001 Abstract The kinetics of one-dimensional syneresis of rennet-induced, skim milk gels of di!erentthicknesses(3,6.5,10.5,20and35mm)atpH 6.40 and 6.00 were studied. Using a laser displacement transducer, it was possible to follow the initial stages of syneresis within 10 s of surface wetting. The syneresis rate was about 50% faster at pH 6.00 than at pH 6.40 and the initial syneresis rate increased linearly with gel thickness, an e!ect which was not predicted by theories based on elastic deformation (syneresis pressure) of the network during syneresis.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Milk gels; Syneresis; Rennet; Shrinkage; Curd 1. Introduction Gels formed of milk by renneting and/or acidi"cation under quiescent conditions may subsequently show syneresis, i.e., expel liquid (whey) while the gel (curd) contracts. Syneresis is accompanied by a rearrangement of the network. After the gel has been formed, casein micelles still have many reactive sites at their surface and therefore, more bonds between paracasein micelles can be formed. The formation of new bonds leads to more compact packing of the micelles and to the expulsion of whey from the gel. The process is fairly slow since the paracasein micelles are incorporated in the network; to get more pronounced syneresis, more bonds have to be broken. If the gel is constrained the shrinkage is imposs- ible and the rearrangement of the gel leads to the forma- tion of denser and less dense areas elsewhere, i.e., the formation of wider pores (Walstra & Jenness, 1984; Walstra, van Dijk, & Geurts, 1985; Walstra, Geurts, Noomen, Jellema, & van Boekel, 1999). The tendency of the gel to shrink is very small (Walstra & Jenness, 1984; van Dijk & Walstra, 1986; van den Bijgaart, 1988; Walstra et al., 1999). In practice, the syneresis is enhanced by the mechanical pressure exerted on the gel (cutting, stirring, and cheddaring). Empirically, syneresis has been widely studied. The rate and extent of syneresis depend on several factors, such as, "rmness of the gel to cutting, surface area of the curd, pressure, pH, temperature and the composition of the milk (Lawrence, 1959; Patel, Lund, & Olson, 1972; Lelievre, 1977; Marshall, 1982; Pearse, Mackinlay, Hall, & Linklater, 1984; van den Bijgaart, 1988; Pearse & Mackinlay, 1989; Walstra et al., 1985; Walstra et al., 1999; Lomholt & Qvist, 1999; Daviau, 2000). The kinetics of syneresis is complex and still not well understood. A deeper understanding of syneresis and the impact of the relevant factors can be investigated using models based upon measured material parameters such as porosity, permeability and appropriate rheological coe$cients of the casein matrix and well-de"ned bound- ary conditions. Several fundamental studies on syneresis of rennet-induced casein gels have been reported (van Dijk, 1982; Walstra et al., 1985; van Dijk & Walstra, 1986; van den Bijgaart, 1988; van Vliet, van Dijk, Zoon, & Walstra, 1991). The rate of curd shrinkage is determined by the rheological properties of the matrix and by the matrix resistance to the liquid #ow. Purely viscous #ow through a porous medium is described by Darcy's law: "! k  dp dh , (1) where k is the permeability,  the #ow rate relative to the solid phase/unit area,  the liquid viscosity and dp/dh the pressure gradient of the liquid in the direction of #ow. 0958-6946/01/$-see front matter  2001 Elsevier Science Ltd. All rights reserved. PII:S0958-6946(01)00027-9