Inhibition of polyphenoloxidase in mango puree with 4-hexylresorcinol, cysteine and ascorbic acid
Introduction
Enzymatic browning in fruit products is mainly caused by polyphenol oxidases (Walker, 1995). PPO (oxygen oxidoreductase), also called tyrosinase, phenol oxidase, monophenol oxidase or cresolase, can promote enzymatic browning by catalyzing the oxidation of mono- and diphenols to o-quinones. These quinones polymerize to produce brown pigments (melanins) (McEvily, Iyengar, & Otwell, 1992). PPO exhibits optimum activity at pH between 5 and 7, however, its activity may be inhibited by heat or removal of one of its necessary components: O2, enzyme, Cu2+ or substrate (Lambrecht, 1995; Richardson & Hyslop, 1985).
Reducing agents, antioxidants, and enzymatic inhibitors prevent browning by chemically reducing the o-quinones to colorless diphenols (McEvily, Iyengar, & Otwell, 1992). Acidulants, such as citric, malic, or phosphoric acid, can inhibit PPO activity by reducing pH and/or chelating copper in a food product (Richardson & Hyslop, 1985).
Sulfites and analogues are restricted in the USA due to their harmful effects on health. AA is commonly used to prevent enzyme discoloration of fruits by reducing the colorless o-quinones to diphenols (McEvily, Iyengar, & Otwell, 1992). Unfortunately, once the AA is oxidized completely, the o-quinines are no longer reduced and darkening may occur due to formation of melanins (Walker, 1995). Cysteine is suggested as an alternative reducing agent. It reduces PPO activity by delaying discoloration due to formation of the colorless Cys-quinone-adducts (Dudley & Hotchkiss, 1989). The substance 4-HR interacts with PPO to render an inactive complex incapable of catalyzing the browning reaction (Lambrecht, 1995). This substance is generally recognized as safe (GRAS) (Luo & Barbosa-Cánovas, 1995) for shrimp melanosis (McEvily, Iyengar, & Otwell 1991). Due to its nontoxic, nonmutagenic and noncarcinogenic properties (McEvily, Iyengar, & Otwell, 1992), 4-HR exhibits potential use in the food industry. Mixtures of 4-HR and AA for inhibiting the browning of vacuum-packaged apple slices (Luo & Barbosa-Cánovas, 1997) and apple slices preserved by combined methods (Monsalve-González, Barbosa-Cánovas, Cavalieri, McEvily, & Iyengar, 1993) have been used. The combination of both 4-HR and AA significantly inhibited the browning of apple.
The objective of this study was to evaluate the anti-browning effect of 4-HR, Cys or AA alone or in combination on mango puree stored at 3 °C.
Section snippets
Materials
Mature mangoes (Mangifera indica) purchased at a local market were washed, held in ice water (1–2 °C), peeled, and de-seeded. Mango pieces were blended in a domestic blender to obtain a puree and held in glass beakers (around 10–20 min) surrounded with ice water to slow down discoloration. The puree was adjusted to desired pH (3.5, 4.0, or 4.4) with phosphoric acid (50 g/100 g) immediately after preparation. Puree (50 g) was weighed and mixed with 4-hexylresorcinol (Sigma-Aldrich, St. Louis, MO), L
PPO activity
Table 2 presents PPO activity in mango puree at selected pH and concentrations of anti-browning agents. The PPO activity decreased as the pH of the mango puree decreased. Since the optimal pH for PPO activity is between 5 and 7 (Lambrecht, 1995), acidification to low pH may inhibit, prevent, or minimize PPO activity (Richardson & Hyslop, 1985). McEvily, Iyengar, and Otwell (1992) pointed out that PPO was completely inhibited when the pH of fruit products was lowered to below 3. However, fruit
Conclusions
PPO activity in mango purees was reduced at pH 3.5, however, the higher the concentration of Cys or AA, the smaller the PPO activity. Mango purees at pH 3.5 changed from bright lemon-yellow to a less yellow color more slowly, in contrast with colors of mango purees at pH 4.0 and 4.4. A yellow color at pH 3.5 was still acceptable after 35 days of storage. Purees at pH 4.0 and 4.4 changed from bright lemon-yellow to yellow-green or yellow-brown more rapidly than mango purees at pH 3.5 during
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