Effect of hot acidic fructose solution on caramelisation intermediates including colour, hydroxymethylfurfural and antioxidative activity changes
Introduction
Browning reaction occurs during food processing and storage, and is an important factor on food quality. The causes of food browning can be classified as enzymatic and non-enzymatic browning. Non-enzymatic browning is mainly caused by Maillard reaction, caramelisation, and ascorbic acid degradation, and is favoured by heat treatment (Manzocco, Calligaris, Mastrocola, Nicoli, & Lerici, 2001). Although, Maillard reaction is documented predominantly in most of the studies concerning non-enzymatic browning reactions, caramelisation is nevertheless critical in the quality of some food and requires systematic exploration. Because sugars are the reactants of both Maillard reaction and caramelisation, caramelisation and Maillard reaction proceed simultaneously (Ajandouz and Puigserver, 1999, Buera et al., 1987). That enhances the development of brown colour.
The reactions of 1,2-enolisation, sugar dehydration to furfural, and sugar fission have been known to proceed during caramelisation (Hodge, 1953). The reaction of enolisation occurred in both acid and alkaline solution, although it proceeded more easily with increasing alkalinity. The reactions subsequent to enolisation, including the formation of furfural, reductone, pyruvaldehyde, and hydroxypyruvaldehyde, are also dependent upon acidity. According to Berl and Feazel (1954), formation of furfural was favoured in acidic solution, whereas formation of reductone, pyruvaldehyde and hydroxypyruvaldehyde were favoured in neutral and basic solution. In addition to colour, browning reaction can increase the antioxidative activity of foods. In the studies conducted by Rhee and Kim, 1975, Benjakul et al., 2005, Phongkanpai et al., 2006, caramelisation products (CPs) were found active in antioxidativity. Rhee and Kim (1975) indicated that the acetone-extracted CPs of glucose could inhibit the oxidation of soybean oil. Benjakul et al. (2005) prepared CPs from glucose, fructose, ribose and xylose by heating at 100 °C, pH 7 and 10, and found that fructose CPs exhibited the highest DPPH radical scavenging activity, reducing power, and ferrous chelating potential. Phongkanpai et al. (2006) indicated that CPs prepared by heating fructose or glucose solutions under alkaline conditions could develop antioxidative activity. They also found that CPs from fructose had higher antioxidative activity for DPPH radical scavenging activity and reducing power than that from glucose.
Among the CPs, HMF (Hydroxymethylfurfural) is an intermediate product important both in aromatic and antioxidative properties (Pous et al., 1991, Shaw et al., 1967, Lee and Nagy, 1988). HMF was used as an indicator of caramelisation of foods (Berry and Tatum, 1965, Pous et al., 1991, Tatum et al., 1967). The existence of HMF in juice concentrates may indicate over-heating or long heating process in concentration. Although overheating or long heating is a deteriorative factor for fruit juice, it is required in the preparation of bainiku-ekisu. Bainiku-ekisu, a dark brown pasty product produced by Japanese apricot juice concentrated, is prepared at 100 °C for long time, and the final weight of the product is 50th of the raw fruit (Chuda et al., 1999, Nakamura, 1995). Bainiku-ekisu is functional in increasing antioxidative activity (Utsunomiya et al., 2002), anti-bacterial activity (Nakajima et al., 2006, Fujita et al., 2002), anti-cancer property (Jeong, Moon, Park, & Shin, 2006), and improving blood fluidity (Chuda et al., 1999, Utsunomiya et al., 2002), and is one of the favourite healthy foods in the area of Japan and Taiwan. HMF in bainiku-ekisu reacts with compounds like citric acid to produce compounds having the above functions (Chuda et al., 1999, Utsunomiya et al., 2002). Thus, the formation of HMF in the caramelisation of fruit concentration becomes one of the concerns worthy of investigation.
However, most of the researches concerning antioxidative activities of CPs were achieved by preparation in neutral or alkaline conditions. These researches obviously were not consistent enough with all the real conditions in food processing so far. A common case happened in the concentrating processing of the majority of fruit juices, which was done at acidic conditions, e.g. in the preparation of bainiku-ekisu.
Consequently, it becomes our concerns how we optimise the processing condition of caramelisation in fruit concentration preparation to obtain product having optimal health functionality. To explore such concerns, the nearest way is begin with model system. Through model system, we shall resolve questions such as whether the product of caramelisation is effective enough to contribute the antioxidative activity to the products like bainiku-ekisu, and how the processing conditions like sugar concentration and temperature affect the antioxidative activity of CPs.
Purposes of this study are trying to figure out the effect of fructose, which exists widely in fruit, and temperature on the antioxidative activity of caramelized product via the application of model systems. In addition, the changes of colour, pH, and hydroxymethylfurfural (HMF) formation in the model solutions were also investigated.
Section snippets
Fructose model solutions
All chemicals were purchased from Sigma Chemical Co. (St. Louis, MO, USA) and solvents from Merk Co. (Darmstadt, Germany). Model systems were prepared by dissolving α-d(+)-fructose in de-ionized water to make fructose concentration 0.14, 0.28, 0.56, and 1.11 M separately. System pH was adjusted to 3.0 with 1.0 N HCl solution. The solutions were dispensed into screw-capped glass tubes and incubated at 75, 85, or 95 °C immediately. These solutions were sampled at intervals for colour, pH, HMF, and
Colour change
In the beginning of incubation, absorbance at wavelength 420 nm in model systems stayed unchanged for a period over 30, 50, and 80 h at 95, 85, and 75 °C, respectively. This was the lag phase of browning. After lag phase, the absorbance increased linearly (Fig. 1). Similar trend of browning was also reported by Phongkanpai et al., 2006, Ajandouz et al., 2001. They used fructose and glucose as the reactants of caramelisation at pH 7–12 and 4–12, respectively. The lag phases of their studies were
Conclusion
This study illustrated the possibility of caramelisation in the processing of acid foods, which was common in fruit juice concentration, at temperature as low as 75 °C. The values of Q10 and activation energy of colour development were 1.87 and 66 kJ/mol, respectively. The contents of HMF were increased and the values of Q10 and activation energy were 4.48 and 160 kJ/mol, respectively. This study illustrated that CPs could not only brown the colour, but also act as naturally produced antioxidants
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