HISTORIA DE LOS REFRACTARIOS EN LA PRODUCCIÓN DE CEMENTO.

1.1 Introduction

Appearance, flavour, texture and nutritional value are the main attributes considered by consumers in evaluating food quality when making food choices. Appearance may be influenced by a number of physical attributes of a food as well as a series of psychological perceptions. Appearance, which is significantly impacted by colour, is one of the first attributes considered in consumers’ preference.

Colour in food may be influenced by naturally occurring pigments such as chlorophylls, carotenoids and anthocyanins, or by pigments resulting from both enzymatic and non- enzymatic browning reactions. Non enzymatic browning is an important chemical reaction occurring in food, which causes the colour to change to a golden brown. The reaction is very complex, and normally happens in food during high temperature processing. The Maillard reaction is the main reaction for non-enzymatic browning (Ames & Benjamin, 2003). It has been important to the food industry because the Maillard reaction impacts not only on the colour change, but also the aroma, taste and the nutritional values of foods, particularly in traditional processes such as the roasting of coffee, nuts and cocoa beans (Saklar et al., 2001; Demir et al., 2002; Kahyaoglu & Kaya, 2006 and Heyd et al., 2007), the baking of bakery products (Shibukawa et al., 1989; Zanoni et al., 1995; Broyart et al., 1998; Zhang & Datta, 2006; Purlis & Salvadori, 2007, 2009 and Purlis, 2010) and the cooking of meat products (Goňi & Salvadori, 2011 and Matsuda et al., 2013).

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If controlled appropriately, non enzymatic browning is seen as being positive by consumers and is considered a good indicator of quality in food products. Specifically, colour changes into a golden colour are a pleasant characteristic occurring in optimal process conditions. But, then, on the other extreme, unpleasant burnt brown colours can form during overcooking (Labuza et al., 1994; Ilo & Berghofer, 1999; Demir et al., 2002; Nourian & Ramaswamy, 2003; Charissou et al., 2007 and Purlis & Salvadori, 2009). Therefore, monitoring and controlling the browning reaction is a key process for obtaining good product quality (Sakla et al., 2001).

Kinetic models describing the Maillard browning reaction have been developed for use to predict and control the process. The kinetics of the reaction must be characterised under a range of conditions. These include the chemical composition (type and availability of reactant), the pH, and water activity and processing factors such as temperature and time of heating (Ellis, 1959; Lopez & Pique, 1997; Ames, 1998; Fayle & Gerrard, 2002 and Ames & Benjamin, 2003). The reaction is also affected by metals, oxygen, and the presence of inhibitory agents, e.g., sulphite (Ames, 1990; deMan, 1999; Martins et al., 2001; Fayle & Gerrard, 2002). However, the key factors that highly affect the kinetics of the reaction are the combined effect of temperature and time of the cooking process (Maillard, 1912; Labuza et al., 1994; Lopez et al., 1997; Martins et al., 2001; Lee et al., 2007 and Ajandouz et al., 2008).

Most literature studies developed the browning kinetics by assuming isothermal conditions and by measuring the change of brown colour on the food surface. There are many methods for measuring the brownness on the surface of food. The newest technology of image analysis is the most popular and is generally used nowadays. The browning colour is generally measured and reported as an average value of the data for an area of the food sample. However, in real products such as crackers, cookies, biscuits or toasted bread, the extent of browning is not uniform. Browning is dependent on the temperature of the surface undergoing change. The distribution of colour on the food surface could be explained by different surface temperature histories across the surface of the product. Edges or blisters for example may heat more quickly than other parts of a product surface. Hence, assuming an average colour on the food surface as the browning index is unlikely to be useful.

Chapter 1: Introduction 3 Based on this it should be possible to explain the product colour distribution by coupling the local temperature and browning reaction through consideration of the product surface. In many cases it is difficult to measure the surface temperature during experimentation and most often, the surface is simply assumed to be at the cooking medium temperature. However, in food processes, phenomena such as water evaporation in baking or frying processes cause a thermal gradiant across the food surface. This makes using these systems unsuitable for isothermal studies of browning kinetics.

To evaluate the kinetics of browning, ideally the surface temperature would be constant (isothermal) throughout the experiment. This would result in an even colour distribution on the food surface. To ensure that the heat transfer on the food surface would be constant, a uniform shape and a smooth surface of the model food would be preferred. Therefore, a food and cooking system that resulted in uniform and constant conditions at the surface of the product should be developed so that accurate browning kinetics can be measured.

Consequently, the experimental conditions for the assumed conditions of the kinetic model will be achieved so the browning reaction in a range of foods can be explained. This model will provide a quantitative tool to predict high temperature browning which will be of benefit in food processing and product quality control.

1.2 Research aim

This research project aimed to study the kinetic reaction of Maillard browning that occurs during high temperature cooking including the effect of the water content of the sample, temperature and time of the cooking process on the reaction. This knowledge would lead to the development of an accurate mathematical model that could predict the reaction in a commercial food process.

1.3 Research objectives

To achieve the research aim, the specific objectives of this work were to:

1.3.1 Develop an appropriate model food, a cooking system design and a browning assessment method (an image analysis system) that would be applicable to use for

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the study of the non-enzymatic browning reaction. These systems would provide reliable experimental data to be able to develop a kinetic model (Chapter 3). 1.3.2 Investigate the effects of the water activity of the food sample, the temperature

and time of the cooking process on the kinetic rate of the non-enzymatic browning reaction (Chapter 4).

1.3.3 Develop a kinetic model for foods to predict non-enzymatic browning rates as a function of temperature and time (Chapters 4 and 5).

1.3.4 Combine the surface temperature and kinetic models to predict colour in browning processes and generalise the modelling methodology to be applicable for a range of foods (Chapters 6 and 7).

1.3.5 Design an experimental study and cooking conditions to investigate the best method for accurate estimation of the kinetic parameters for the model (Chapter 8).

Chapter 2