CARACTERIZACIÓN DE LOS LADRILLOS.

The method for determining the Maillard browning reaction can be divided into two main categories: direct and indirect methods. The direct method involves aiming to measure the chemical substances relating to the brown pigment formation or the consumption of the reactants. Conversely, the indirect method intends to measure the variation of colour produced by the Maillard reaction.

2.4.1 Direct or chemical method

Normally, the Maillard reaction is able to be measured by chemical analysis. Intermediate substances are employed as indicators for measuring the reaction kinetics in each of the three stages. In the early stage, after enzymatic breakdown of the protein, the Amadori product can be measured by amino acid analysis because the Amadori and Heyns product cannot be detected by UV absorption (Labuza, 1990 and Van Boekel, 1998). The Amadori product is hydrolysed in acid conditions giving an artificial amino acid which is furosine (Van Boekel, 1998). Furosine can also easily be monitored by reversed-phase high-performance liquid chromatography (HPLC) (Resmini et al., 1990) and ion-exchange chromatography (Hartkopf & Erbersdobler, 1993), in addition it is possible to apply capillary electrophoresis (Tirelli & Pellegrino, 1995). The gas chromatography-mass spectroscopy (GC-MS) method for measuring the lysine, furosine and carboxymethyllysine (CML) in cereal products is reported by Charissou et al. (2007). However, there is some problem with the furosine method because the content

Chapter 2: Literature review 15 of the Amadori is calculated from furosine content by using a conversion factor. This method possibly shows an error in calculation, since only 30-40% of the Amadori product is converted into furosine (Furth, 1988 and Labuza, 1990). Another method for estimating the amount of Amadori product is the application of HPLC to evaluate carboxymethyllysine (CML), which is oxidized by periodic acid and subsequent acid hydrolysis of the protein (Badoud et al., 1991 and Van Boekel, 1998).

The formation of hydroxymethyfurfural (HMF) from the Amadori compound in stage two is usually observed by UV spectrophotometer (Yeo & Shibamoto, 1991). This method is another indirect way to measure the Maillard reaction rate. HMF is also evaluated by the HPLC method (Van Boekel, 1998). Meydav et al. (1977) used spectroscopy in the visible wavelength range at 420 nm to evaluate HFM. However there were some drawbacks. The disadvantage for this method is that it is time consuming because this measurement requires filtration and centrifugation steps in the purification process. Such processes may influence the concentration of the pigments and cannot be used on-line (Amiot et al., 1992). Fluorescence spectroscopy was first used to assess the progress of browning in food by Adhikari & Tappel (1973). Since then there have been many studies in food research which have applied fluorescence to monitor the browning reaction, such as Matiacevich & Buera (2006) who focused on the kinetics of fluorescence and pigment development for the Maillard reaction in a liquid model system. Schamberger & Labuza (2006) and Zhu et al. (2009) applied fluorescence spectroscopy to assess the Maillard reaction in milk.

During the final stage of the Maillard reaction, the formation of brown colour pigments (melanoidins) from reactive compounds occurs, which can be monitored by the absorbance at a chosen wavelength: typically 420-460 nm (Van Boekel, 1998).

The chemical method is a conventional method, which has been generally used in liquid food systems such as apple juice concentrates during storage (Toribio et al., 1984), milk heating (Van Boekel, 1998), thermal treatment of apple puree (Ibarz et al., 2000), apple juice concentrate heating (Vaikousi et al., 2008), cashew apple juice (Damasceno et al., 2008), honey heating (Vaikousi et al., 2009) and blackberry juice heating (Jiménez et al., 2012). The application of the chemical method of measuring the browning reaction for some solid foods, such as dried skim milk (Franzen et al., 1990) and fried potato (Sahin, 2000) has also been used.

16 Chapter 2: Literature review

Some of the problems of the classical colour measurement method, including colouric observation at 420 or 490 nm, measurement of UV and IR (infared) spectrum analysis, are destructive of the sample, laborious and time consuming. Furthermore, these chemical methods are not so useful in following the history of colour development during the process (Purlis & Salvadori, 2007 and Puris, 2010).

2.4.2 Indirect or physical method

The obvious appearance of brown colour in the last stage can be measured by a colorimeter in tristimulus CIE L*a*b* or Hunter Lab colour systems (Zhu et al., 2009). The application of a colorimeter has been widely used to identify the colour of food surfaces that have undergone the Maillard browning reaction for several kinds of food and cooking types.

Examples of the colorimeter method in food studies are cookie baking (Shibukawa et al., 1989), pea puree heating (Shin & Bhowmik, 1995), bread baking (Zanoni et al., 1995 and Mohd Jusoh et al., 2009), pastry baking in a microwave oven (Zuckerman & Miltz, 1997), maize grits in extrusion (Ilo& Berghofer, 1999), soy milk heating (Kwok

et al., 1999), peach puree heating (Ávila & Silva, 1999), hazelnut roasting (Özdemir & Devres, 2000; Saklar et al., 2001; Demir et al., 2002 and Alamprese et al., 2009), kiwifruit drying (Maskan, 2001), potato frying (Krokida et al., 2001 and Nourian & Ramaswamy, 2003), cracker baking (Broyart & Trystram, 2002), wheat germ heating (Ibanoglu, 2002), tofu frying (Baik & Mittal., 2003), sesame seed roasting (Kahyaoglu & Kaya, 2006), gulabjamun indian dough ball frying (Jayendra Kumar et al., 2006), dehydrated potato (Acevedo et al., 2008), high temperature treatment of cashew apple (Lima et al., 2010) beef roasting (Goňi & Salvadori, 2011) and fish grilling (Matsuda et al., 2013).

However the main drawback of the reflectance measurement is that only a reduced section of the food or a small area of the surface material is analysed. The diameter of the measurement head of the spectrophotometer is normally between 6 to 8 mm. This is quite small compared with the area of the objective sample. Large surface correlations cannot be observed and the robustness of its applicability in non-homogeneous samples is obviously limited (Gökmen et al., 2008). For all the disadvantages of the chemical

Chapter 2: Literature review 17 and colorimeter methods, a new method of computer vision analysis system has been developed to apply in measuring the colour change of food during the browning process.

Computer vision system has been used in many research projects in the agriculture and food technology area (Wang & Sun, 2001). Computer vision analysis has important advantages over the classical colorimeter by analyzing the colour distribution over the whole surface and monitoring it during processing. It is cost effective, consistent and has high speed and accuracy (Locht et al., 1997; Brosnan & Sun, 2004; Yam & Papadakis, 2004 and Gökmen et al., 2008). Moreover, the computer vision does not come into contact with the sample during measurement and the images taken from this system can be saved to be re-analyzed at a later time (Purlis & Salvadori, 2007).