The physicochemical parameters during dry heating strongly influence the gelling properties of whey proteins Muhammad Gulzar a,b,c , Valérie Lechevalier a,b , Saïd Bouhallab a,b , Thomas Croguennec a,b,⇑ a AGROCAMPUS OUEST, UMR 1253, F-35042 Rennes, France b INRA, UMR 1253, F-35042 Rennes, France c University of Veterinary and Animal Sciences, Abdul Qadir Jilani Road, Lahore, Pakistan article info Article history: Received 24 January 2012 Received in revised form 23 April 2012 Accepted 6 May 2012 Available online 14 May 2012 Keywords: Dry-heating Whey protein powder pH Water activity Temperature of dry heating Gelling properties abstract In this study we determined the composition (proportion of native proteins, soluble and insoluble aggre- gates) and quantified the gelling properties (gel strength and water holding capacity) of pre-texturized whey proteins by dry heating under controlled physicochemical conditions. For this purpose, a commer- cial whey protein isolate was dry heated at 80 °C (up to 6 days), 100 °C (up to 24 h) and 120 °C (up to 3 h) under controlled pH (2.5, 4.5 or 6.5) and water activity (0.23, 0.32, or 0.52). Gelling properties were quan- tified on heat-set gels prepared from reconstituted pre-texturized proteins at 10% and pH 7.0. The forma- tion of dry-heat soluble aggregates enhanced the gelling properties of whey proteins. The maximal gelling properties was achieved earlier by increasing pH and water activity of powders subjected to dry heating. An optimized combination of the dry heating parameters will help to achieve better gelling properties for dry heated whey proteins. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Protein dry heating is intensively used in pharmaceutical and food industry for viral and microbial decontamination of thermo- sensible proteins. It was noticed that prolonged dry heating also in- duces denaturation and aggregation of proteins, which could lead to an improvement in the functional properties (gelling, foaming, and emulsifying) of proteins (Kato et al., 1990; Mine, 1997; Watan- abe et al., 2000; Matsudomi et al., 2001; Desfougeres et al., 2011). Despite the numerous studies conducted on dry heating of egg white proteins, the process of dry heating is still not well con- trolled at industrial scale resulting in the variability of functional properties of final products. Functional properties of dry heated proteins are affected by powder physicochemical parameters such as pH (Mine, 1996, 1997; Matsudomi et al., 2001) and water activ- ity, a w (Hammershoj et al., 2006; Van der Plancken et al., 2007) but also conditions for dry heating (Hammershoj et al., 2006). In con- trast to egg white proteins, the impact of dry heating on the func- tional properties of whey proteins was sparsely investigated (Ibrahim et al., 1993; Li et al., 2005) although whey proteins are extensively used to produce various ingredients for food and non-food industries. In addition the natural pH range for whey pro- teins, from neutral (sweet whey) to acidic (acid whey), was out of the pH range covered in the studies on the dry heating of egg white proteins for functional improvement purposes. Recent works showed that the dry heating in acidic pH range (pH 2.5–6.5) gives pH-specific aggregates (Gulzar et al., 2011) and also improves the emulsifying and foaming properties of food proteins (Li et al., 2005; Desfougeres et al., 2008). Then, dry heating under acidic con- ditions constitutes an opportunity to explore for the devise of new whey protein ingredients. In this work we assessed the impact of physicochemical param- eters of powder like pH (neutral to acidic), water activity (a w ) and processing parameters (temperature and time of dry heating) on the composition (aggregation level of proteins) and gelling proper- ties (gel strength and water holding capacity of heat set gels) of whey proteins (WPI) during the course of dry heating. The aim of the study was to get predictive models for the gelling properties of dry heated whey protein from its composition and the physico- chemical parameters used for dry heating (pH and a w of whey pro- tein powders and heat treatment). The pH, a w and temperature (time) ranges under investigation were from 6.5 to 2.5, from 0.23 to 0.52 and from 80 °C (up to 6 days) to 120 °C (up to 3 h), respec- tively. The pH range covered the diversity of whey proteins pro- duced in the industry and extended toward acidic pH (up to pH 2.5), where whey proteins are heat treated in solution to get spe- cific aggregates (Bolder et al., 2006; Oboroceanu et al., 2010). An a w of 0.23 and heat treatment of 80 °C (up to 6 days) corresponds 0260-8774/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2012.05.006 ⇑ Corresponding author at: AGROCAMPUS OUEST, UMR 1253, F-35042 Rennes, France. Tel.: +33 2 23 48 59 27. E-mail address: thomas.croguennec@agrocampus-ouest.fr (T. Croguennec). Journal of Food Engineering 112 (2012) 296–303 Contents lists available at SciVerse ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng