Ascorbic acid and hydrogen peroxide (Fenton’s reagent) induced changes in gelatin systems Asgar Farahnaky, David A. Gray, John R. Mitchell, Sandra E. Hill * School of Biosciences, Division of Food Sciences, Sutton Bonington Campus, University of Nottingham, Loughborough, Leics LE12 5RD, UK Received 19 April 2002; revised 23 May 2002; accepted 4 October 2002 Abstract The effect of Fenton’s reagent (hydrogen peroxide and ferrous sulfate) and ascorbic acid on the viscosity and molecular weight of gelatin solutions has been studied and compared. Fenton’s reagent (0.025 M H 2 O 2 and 0.0025 M FeSO 4 ) lowered the activation energy for depolymerisation of gelatin (49.3 kJ/mol) compared with heat degradation without additives (72.8 kJ/mol). Ascorbic acid (1%) showed no change in the gelatin viscosities compared to the control when heated in dilute solution (15 g gelatin in 100 ml water). In contrast, storing gelatin powders containing ascorbic acid at 80 8C for 6 h at different relative humidities (moisture contents of 8.7 –48% dry basis) caused an increase in molecular weight as assessed by SEC/MALLS. Protein solubility decreased and browning colour intensity increased at RHs of 50% and higher. The greatest changes occurred at RH of 50% where 40% of the gelatin was not soluble in water or 1% sodium dodecyl sulfate. These changes could be due to Maillard cross-linking, however, at moisture levels that equated to the gelatin polymer being in the glassy state no change in M w was observed at the temperatures and times used. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Gelatin; Ascorbic acid; Fenton’s reagent; Maillard cross-linking; Degradation; Molecular weight 1. Introduction The functional properties of polymers depend on their molecular weight. Typically during processing of food polymers molecular weights are decreased by thermal degradation, acid hydrolysis, shear or similar mechanisms. Maillard reactions or the generation of free radicals during processing or storage can also alter the molecular weight of biopolymers and so result in a loss in their desired functional properties. Free radicals are capable of attacking food components; such as carbohydrates and proteins (Gregory, 1995). Possible consequences of free radical reactions in food include formation of off-flavours, loss of nutrients and, of particular interest for this study, degradation of macromolecules and cross-linking of food polymers (Aruoma, 1991; Karel, 1975; Kocha, Yamaguchi, Ohtaki, Fukuda, & Aoyagi, 1997). Ascorbic acid (AA) is commonly used in food products and is notable for its potential to be involved in both Maillard reactions and free radical cycles. It is added to food products as an antioxidant or enzyme inhibitor (Pischetsrie- der, 1996). During processing or heating of food, AA is degraded by oxidation reactions that lead to products such as dehydroascorbic acid (DHA), 2,3-diketo-gulonic acid, xylosone, threose and oxalic acid (Pischetsrieder, 1996). Previous work indicated that starch was depolymerised by factors known to induce free radical generation e.g. sulfite, UV light and hydrogen peroxide (Floor, Schenk, Kieboom, & Vanbekkum, 1989). Ascorbic acid also seems to be implicated in reactions causing depolymerisation of starch even though changes in pH were controlled (Sriburi & Hill, 2000; Sriburi, Hill, & Mitchell, 1999). The marked degradation of starches by free radical attack and the previous findings that the Maillard reaction products could cross-link proteins (Hill & Easa, 1998; Nagaraj, Shipanova, & Faust, 1996) led to the present study. Gelatin, a random coil protein, was incubated with hydroxyl free radicals, formed by Fenton’s reagent (ferrous sulfate and hydrogen peroxide) and ascorbic acid. Changes to gelatin solutions were assessed by viscosity measurements that were then linked to molecular weight changes and activation energies. The effect of ascorbic acid was assessed on gelatin 0268-005X/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. PII: S0268-005X(02)00093-0 Food Hydrocolloids 17 (2003) 321–326 www.elsevier.com/locate/foodhyd * Corresponding author. Tel.: þ44-115-95-16145; fax: þ 44-115-95- 16142. E-mail address: sandra.hill@nottingham.ac.uk (S.E. Hill).