Correlation of the fractal enzymatic browning rate with the temperature in mushroom, pear and apple slices R. Quevedo a, * , F. Pedreschi b , J.M. Bastias c , O. Díaz a a Departamento of Acuicultura y Recursos Agroalimentarios, Universidad de Los Lagos, FITOGEN Program, Chile b Pontifica Universidad Cat olica de Chile, Department of Chemical Engineering and Bioprocesses, Av. Vicu~ na Mackena, 4869 Santiago, Chile c Departamento de Ingeniería en Alimentos, Universidad del Bio, Av. Andr es Bello s/n, Chill an, Chile article info Article history: Received 5 June 2015 Received in revised form 11 August 2015 Accepted 17 August 2015 Available online 19 August 2015 Keywords: Browning rate Fractal method Arrhenius Mushroom Pear Apple abstract A Fractal Kinetic method was applied to characterize enzymatic browning activity in samples from mushrooms, pears and apples. In this study, three independent repetitions were fulfilled to describe enzymatic browning kinetic by using two colorimetric methods: (1) the Mean method, when calculating an averaged intensity color (L* value) and assuming that colors are distributed homogeneously on the fruit surface during browning, and (2) the Fractal Kinetic method, that describes a non-homogenous color distribution on the food surface during the reaction. Samples of apples (‘Gala’), pears (‘Pack- ham’) and mushrooms (Agaricus bisporus) were cut and stored at four different temperatures (i.e., 5, 15, 25 and 35  C). Four environmental test chambers, all equipped with a computer system vision, were used. The images were photographed every 15 s during a period of 4 h and saved as a Tiff Format. The results of this study showed that the enzymatic browning kinetic rate was higher when the Fractal Ki- netic method was applied. The Arrhenius Law and the log-logistic model were applied in order to establish a relationship between the enzymatic browning rates and the temperature reactions. The re- sults revealed that the Arrhenius law can also be applied when using the Fractal Kinetic method. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Enzymatic browning is a reaction that affects the appearance, quality and sensory properties of fruits and other vegetables, thus becoming one of the main study areas for food processors and re- searchers (Soliva-Fortuny, Elez-Martinez, Sebastian-Caldero, & Martin-Belloso, 2002). Enzymatic browning is caused by structural damage to the fruit tissue and is originated by enzymes, such as the polyphenol oxidase (ppo) and phenolic substrates, which react to each other, producing darker pigments (brown) on the surface (Quevedo, Ronceros, Garcia, Lop ez, & Pedreschi, 2011; Song et al., 2007). Modeling of browning is essential to evaluate the effectiveness of the processes used to obtain fresh-appearance food and also to determine shelf life model. However, enzymatic browning modeling is often complex, not only due to the biochemical vari- ability of the fruit (Degl’Innocenti, Degl’Innocenti, Guidi, Pardossi, & Tognoni, 2005) but also due to variation in its kinetic behavior, which can be found even in the same fruit (Quevedo et al., 2014). Enzymatic browning is a reaction where colors are distributed in a non-homogenous way on the fruit surface. For instance, when physical stress (e.g., a cut) takes place in apple tissues, the compartmentalization of some cells fails (Marangoni, Marangoni, Palma, & Stanley, 1996) and this process is followed by the mix- ing of polyphenol substrates (e.g., catechin, polyphenols) with polyphenol oxidase and/or phenol peroxidases (Quevedo, Jaramillo, Díaz, Pedreschi, & Aguilera, 2009), with more emphasis in some areas than others. A practical method used to quantify these kinds of reactions is the browning index, using the average of color parameters (generally L* luminance). Nonetheless, in essence, it is known that enzymatic browning is displayed as a heterogeneous distribution of colors on the fruit surface. Recently, some researchers have pro- posed procedures based on color to quantify the heterogeneity of food reactions. For example, segmenting the space color in enzy- matic browning (Yoruk, Yoruk, Balaban, & Marshall, 2004); appearance of brown spotting in mushrooms (Aguirre, Frias, Barry- Ryan, & Grogan, 2009); applying the fractal kinetic method (Quevedo, Díaz, Caqueo, Ronceros, & Aguilera, 2009; Quevedo, Díaz, Ronceros, Pedreschi, & Aguilera, 2009; Quevedo & Ronceros * Corresponding author. E-mail address: rquevedo@ulagos.cl (R. Quevedo). Contents lists available at ScienceDirect LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt http://dx.doi.org/10.1016/j.lwt.2015.08.052 0023-6438/© 2015 Elsevier Ltd. All rights reserved. LWT - Food Science and Technology 65 (2016) 406e413