Modelling of the kinetics of colour change in hazelnuts during air roasting Ali Do gan Demir a , Jes us Mar  ıa Fr  ıas Celayeta a , Kevin Cronin a, * , Kamal Abodayeh b a Department of Process Engineering, University College, Cork, Ireland b Prince Sultan College, P.O. Box 66 833, Riyadhv 11586, Saudi Arabia Received 19 October 2001; accepted 4 March 2002 Abstract Thecolourchangesthatoccurinhazelnutsduringthedryroastingprocesswereexperimentallystudiedinordertodeterminethe magnitudesoftheparametersforacorrespondingcolourchangemodel.Roastingtimesvaryingfrom5to1900minandanumber ofroastingtemperaturesintherangefrom100to180 °Cwereanalysed.Theorderofthereactionwasfoundbyplottingisothermal curves of colour change against time at a number of different roasting temperatures. Statistical analysis of the data demonstrated thatthecolourparameter, L wasbestrepresentedbyafirst-orderprocess.TherateconstantwasassumedtohaveanArrhenius-type dependence on temperature. The activation energy (E a ) and the frequency (pre-exponential) term (k 1 ) were determined as 64.5 kJmol 1 and33416.5s 1 respectively for the conditions studied. Theoretical equations, based on the exponential integral function, to predict first-order quality changes when product temperature is varying exponentially were developed. These equations were validated by comparison with the experimental colour data extracted from the non-isothermal region of the roasting process. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Hazelnut roasting; Colour responses; Quality kinetics; Exponential integral 1. Introduction The world tree nut market is segmented into two majorcategories;in-shellnutsandshelledtreenuts.The term ‘in-shell’ describes tree nuts that are produced and marketed with the shell intact. Their processing pri- marily involves exterior cleaning and preparation for seasonal market. However the most common form of nuts used in the food-processing sector are shelled nuts. Shelled nuts undergo processes including blanching, dicing, coating, roasting and grinding to fit product formulation needs or give them increased consumer appeal.Roastingisoneofthemostimportantprocesses giving the product the necessary alterations to become value-added nuts. There are two types of nut roasting: dry and oil. Dry roasting involves the heating of nuts using air convection or radiant microwave heat. In the oil method, raw nutmeats are immersed in boiling oil, and then drained to remove excess oil (Somogyi, Bar- rett, & Hui, 1996). Currently, over 4,000,000 tons of nuts are commercially produced throughout the world. 700,000 tons of that figure is hazelnut (Corylus avellana L.), production ( € Ozdemir & Devres, 1999). There are several objectives to hazelnut roasting; one is to ensure that the centre of every nut reaches some minimumtemperaturetodestroyanytoxinsorallergens that may exist (M€ uller et al., 2000). Wigotzki, Stein- heart, and Paschke (2000) showed that the IgE binding activity of main hazelnut allergens decreased after 15 min roasting at temperatures between 100 and 185 °C andwasnolongerdetectableat170 °C.Roastingat100 °Cforupto90minhadnoinfluenceontheallergenicity of hazelnut proteins. Another objective of hazelnut roasting is to give the product surface a variety of col- ours such as a whitened, golden yellow, dark roast or very dark roast ( € Ozdemir & Devres, 2000a). Roasting alters and significantly enhances the flavour, colour, texture and appearance of hazelnuts. Roasting also inactivates enzymes that speed up nutrient loss and de- stroys undesirable microorganisms and food contami- nants. Therefore from the quality and safety point of view, the times and temperatures applied are very im- portant factors in hazelnut roasting. Journal of Food Engineering 55 (2002) 283–292 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +353-21-4902644; fax: +353-21- 4270249. E-mail address: k.cronin@ucc.ie (K. Cronin). 0260-8774/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0260-8774(02)00103-6