Phospholipids and poly(glutamic acid)/hydrolysed gluten: Interaction and kinetics Abdellatif Mohamed a, * , Rogers E. Harry-O’Kuru b , S.H. Gordon a , Debra E. Palmquist c a Cereal Products and Food Science, National Center for Agricultural Utilization Research, 1815 N. University Street, Peoria, IL 61614, United States b New Crops and Processing Technology Research Units, National Center for Agricultural Utilization Research, 1815 N. University Street, Peoria, IL 61614, United States c Area Statistician, National Center for Agricultural Utilization Research, 1815 N. University Street, Peoria, IL 61614, United States article info Article history: Received 6 June 2008 Received in revised form 3 September 2008 Accepted 30 October 2008 Keywords: Phospholipids Wheat gluten Interactions Kinetics Activation energy Poly(glutamic acid) DSC FT-IR abstract The effect of poly(glutamic acid) (PGA) and hydrolysed wheat gluten (HG) on the thermal and kinetics properties of lysophosphatidylcholine (LPC) was determined using DSC. A model system containing PGA or HG was added to 40% LPC aqueous suspension. The results from the study showed reduced DH values as a function of PGA molecular weight, which signified some kind of penetration of PGA or HG into the LPC bi-layer but not enough to minimise the order of LPC vesicle structure. The ability of LPC mole- cules to rearrange itself into a bi-layer again was evident in the emergence of crystallisation profiles in the course of the cooling cycle. The calculated activation energy (E a ) of pure LPC vesicle disruption (heat- ing) and formation (cooling) was 696.6 and 520.4 kJ/mol, respectively. Overall, the trend of lower E a as a function of PGA molecular weight was apparent during both heating and cooling cycles. Although, both PGA and HG reduced the E a , HG was less effective in reducing E a . The purported molecular interaction between phospholipids and gluten has been confirmed using FT-IR spectroscopy. The FT-IR results indi- cated that interactions occurred in LPC/HG blends and in model mixtures consisting of LPC/PGA with dif- ferent molecular weight. Published by Elsevier Ltd. 1. Introduction Wheat lipids are divided into two groups: non-polar and polar. Non-polar lipids are dominated by triglycerides, and they originate from the embryo and the endosperm liposomes (Morrison, Mann, Soon, & Coventry, 1975). The polar lipids are found mainly in the cell membranes and mostly consist of phospholipids such as lyso- phosphatidylcholine (LPC). Although, lipids and in particular phos- pholipids, are present in small quantities in wheat, they have a significant effect on the final texture of food products. As indicated in the literature reports, polar lipids, i.e., phospholipids and glyco- lipids interact primarily with wheat gluten protein (Morrison, 1978). This is significant because wheat gluten network formation during dough mixing and throughout baking, is the single most important factor that determines dough qualities, in the form of gas retention and bread loaf volume (Chung & Pomeranz, 1977; Chung, Pomeranz, Finney, & Shogren, 1978; Mecham, 1971; Pomeranz, 1971; Pomeranz, 1980). Surfactants are amphyphilic compounds capable of containing both hydrophilic and hydrophobic moieties. Polar lipids (surfac- tants) can self-assemble in clusters called ‘‘micelles”, which occur above a critical concentration, referred to as CMC and form a bi-layer, where the polar ends face the aqueous environment ( Morrison, 1978). Wheat phospholipids (micelles) are capable of interacting with wheat gluten by means of both polar and non-polar ends. Phos- phorous NMR of wheat gluten showed that phospholipids are organ- ised into a lamellar liquid crystalline phase (Didier, Christine, Serge, Charles, & Daniel, 1987). The interaction between phospholipids and gluten, as shown by NMR, does not occur in the same way as in cell membranes, additionally, they are sensitive to temperature and the effect of mechanical work during dough mixing. The interruption of these interactions is the result of the expul- sion of the phospholipids into the water phase from the protein network due to extensive mechanical work or heating and cooling (Didier et al., 1987). Some of the free lipids become bound after flour is wetted and formed into dough ( Morrison, 1978). The inter- action between phospholipids and wheat gluten or poly(glutamic acid) can be estimated by determining the effect of the polypep- tides on the phospholipids peak temperature. The effect of proteins on the kinetics of phospholipids thermal properties may present additional information regarding the degree of the detected inter- action. Although, melting is not considered a kinetic process, Ozawa (1970) demonstrated that, non-isothermal DSC data can be used to determine activation energy (E a ) with the assumption that peak temperature determined by DSC is also the temperature of the maximum reaction rate. Reaction rate for a solid A to pro- duce a solid product B can be expressed as follows: dx dt ¼ Ze Ea=RT ð1  xÞ n ð1Þ 0308-8146/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.foodchem.2008.10.069 * Corresponding author. Tel.: +1 309 681 6331; fax: +1 309 681 6685. E-mail address: a.mohamed@ars.usda.gov (A. Mohamed). Food Chemistry 114 (2009) 1056–1062 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem