Chapter 3: Effect of Drying Temperature and Sugar Concentration on the Drying Kinetics
3.3 Results and Discussion
3.3.5 Colour properties of marula fruit leathers
3.3.5.3 Browning Index (BI)
The browning index is a vital parameter indicating the occurrence and purity of browning in products containing sugar (Buera et al., 1986) and in quantifying the quality of dried fruits (Akoy, 2014). Moreover, evaluation of browning in dried products helps with the selection of appropriate drying technique so as to minimise the products quality in terms of colour (Pathare et al., 2013).
The changes of the browning index of marula fruit leather as affected by both drying temperature and sugar levels are depicted in Table 3.3.5.1. According to the results the browning index of marula fruit leather increased with an increase in temperature. On the other hand, the browning index of marula fruit leathers decreased with increasing sugar concentration at a specific temperature.
It has been stated that colour change during drying is mainly due to a number of many chemical and biochemical reactions which are affected by drying temperature and additive ingredients such as sugar, pectin, glucose and corn syrup, meltodextrin etc. (Man and Sin, 1997, Maskan et al., 2002b, Maskan et al., 2002a, Perera, 2005, Diamante et al., 2014). Thus, the formation of browning is a phenomenon likely to occur during the processing of any dried produce due to thermal effects of drying air (Cernîşev, 2010) and it prevails where processes of enzymatic and non-enzymatic reaction are taking place (Pathare et al., 2013, Cernîşev, 2010, Kotwaliwale et al., 2007).
Enzymatic browning results from an oxidation of phenolic compounds from the polyphenol oxidase to 0-quinones, which further polymerise to form dark pigmentation where as non-
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enzymatic browning (Maillard reaction) results from the degradation of carbohydrate and amino acids associated with caramelisation (which occurs as a result of direct heating of carbohydrates such as sucrose and reducing sugars) (Perera, 2005, Pathare et al., 2013), enzymes denature reactions (Friedman, 1996) and ascorbic acid browning reactions (Vámos‐
Vigyázó and Haard, 1981, Tomás‐Barberán and Espin, 2001, Perera and Baldwin, 2001, Perera, 2005). Maillard browning and caramelisation are most prominent at higher temperatures (>100 ⁰C) (Vega-Gálvez et al., 2009, Damasceno et al., 2008). Therefore, in the present study, the reaction rates of these two reactions could have been very low because the marula pulp was dried between 50-70 ⁰C.
Previous researchers have also demonstrated the occurrence of browning associated with heat treatment. For instance, hot air dried pepper was reported to reflect browning pigments due to increased temperature and this was attributed to the increase in kinetic reaction rate which showed higher values of non-enzymatic browning (Vega-Gálvez et al., 2009). During the drying of mango slices, Akoy (2014) reported the formation of browning in mango slices was due to a decrease in L* values, which was attributed to high heat temperatures and prolong drying times. In another study, where orange pulp and peels were hot-air dried (at 30 °C to 90
°C), Garau et al. (2007) reported orange pulp dried at 30 °C had the highest browning formation as compared to pulp dried at 70-90 °C and attributed this observation to prolonged drying time associated at the lower temperatures. In a comparison study of cabinet (at 65 °C for 6 hours) vs. solar (36.7 °C for 3 days)and convective oven (at 50 °C for 18 hours) dried jackfruit leathers, Okilya et al. (2010) reported that all fruit leathers changed from yellow-orange and turned brown during solar and oven drying. It was suggested that this was probably due to longer drying times that the fruit leathers underwent under those heat treatments. The change in yellow-orange colour to brown was suggested to occur because the temperature used was above 22 °C (Krokida et al., 1998, Okilya et al., 2010).
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According to Friedman (1996), Rahman and Perera (2007) and Cernîşev (2010), the browning of fruit products could also result from non-enzymatic reactions of vitamin C oxidation of polyphenols. In the present research, naturally marula fruit pulp is yellow in colour and high in vitamin C content which is three times than that found in oranges (Mdluli, 2005, Dlamini and Dube, 2008). The fruits pulp is also characterised by high content of polyphenolics including flavonoids and condensed tannins (Mdluli, 2005, Borochov-Neori et al., 2008, Mdluli and Owusu-Apenten, 2003) . So the formation of browning which resulted in these fruit leathers, was probably both due to enzymatic browning of polyphenols present in the fruits pulp and non-enzymatic browning of vitamin C oxidation and pigment degradation contributed in the formation of browning. However, since sodium metabisulphite was used in the preparation of the pulp, enzymatic browning was not expected. The oxidation of ascorbic acid (vitamin C) was reported to contribute in the formation of non-enzymatic browning of processed food products by López et al. (2010) during hot air drying of blueberries and Cernîşev (2010) for tomato drying. Thus, in the present research, it could be suggested that the browning was non- enzymatic, and it resulted from the oxidation of the polyphenols by the vitamin C that maybe present in high levels in the pulp.
Heat treatment can also result into sugar being caramelised to produce brown pigments during drying of products (Perera, 2005, Pathare et al., 2013). However, sugar and other additives such as pectin, glucose syrups, etc. are reported to minimise browning during drying of fruits (Perera, 2005, Diamante et al., 2014). Minimised browning was reported by Krokida et al.
(2001) during drying of apple and banana slices after they have being sugar infused. In another study, Ruiz et al. (2012) reported reduced browning index for apple leather treated with sugar and citric acid as compared to the untreated leathers. During hot air drying of litchi, Mahayothee et al. (2009) reported that the addition of 0.5% Trehalose and 0.5% Glycerol reduced browning of the dried product. This again shows that adding or introducing additives
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during drying of produce will minimise browning (Figure 3.3.5.3.1 below). In the present study, the added sugar is a non-reducing sugar (sucrose), it would not be expected to react with the amino acids through the Maillard browning. The sugar therefore probably reduces the browning by inhibiting the vitamin C catalysed oxidation of the polyphenols to brown pigments.
So the formation of browning which resulted in these fruit leathers, was probably both due to enzymatic browning of polyphenols present in the fruits pulp and non-enzymatic browning of vitamin C oxidation and pigment degradation contributed in the formation of browning.
However, since sodium metabisulphite was used in the preparation of the pulp, enzymatic browning was not expected. The oxidation of ascorbic acid (vitamin C) was reported to contribute in the formation of non-enzymatic browning of processed food products by López et al. (2010) during hot air drying of blueberries and Cernîşev (2010) for tomato drying.
The results from the verification of the occurrence of non-enzymatic browning through extraction from the leathers dried at 50 °C, with 0, 5 and 10% w/w sugar are shown in Figure 3.3.5.3.1 below. The non-enzymatic browning index decreased with increasing sugar content.
Thus, in the present research, it could be concluded that the browning was due to non- enzymatic browning and that probably resulted from the oxidation of the polyphenols by the vitamin C.
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Figure 3.3.5.3.1: Effect of added sugar concentration of the non-enzymatic browning index of marula fruit leathers dried at 50 ℃, with 0, 5 and 10%. At least three replications were
done.Error bars are standard deviations of means. Non-enzymatic browning index unit A is absorbance.
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