Effect of heat-treatment on the antioxidant and pro-oxidant activity of milk
Introduction
Milk is a very complex food whose consumption confers a number of nutritional benefits (Walzem, Dillard, & German, 2002). In particular, beyond the presence of valuable macro- and micro-nutrients, milk also contains antioxidant factors. These are represented by naturally occurring vitamins (i.e. E and C), beta-carotene and enzymatic systems, mainly represented by superoxide dismutase, catalase and glutathione peroxidase (Landmark-Månsson & Åkesson, 2000). In addition, possible antioxidant activity of milk whey has also been suggested. It would include chelation of transition metals by serum albumin and lactoferrin, an iron-binding glycoprotein (Gutteridge, Paterson, Segal, & Halliwell, 1981; Meucci, Mordente, & Martorana, 1991; Shinmoto, Dosako, & Nakajima, 1992) as well as free radical scavenging activity by amino acids, such as tyrosine and cysteine (Taylor & Richardson, 1980; Wayner, Burton, Ingold, Barclay, & Locke, 1987; Colbert & Decker, 1991; Ostdal, Daneshvar, & Skibsten, 1996; Tong, Sasaki, McClements, & Decker, 2000).
Heat-based stabilisation processes, generally required to achieve milk safety and stability, can be responsible for different effects on the quality attributes of the product, including development of undesired colour and flavour, enzyme inactivation, modification of nutrient bioavailability and natural antioxidant depletion (Renner, 1988; Olano & Martinez-Castro, 1989; Schaafsma, 1989; Pearce, 1989). Furthermore, it has been reported that milk antioxidant activity may increase as a consequence of thermal treatments, due to protein unfolding and exposure of thiol groups, potentially acting as hydrogen donors (Taylor & Richardson, 1980; Walstra & Jenness, 1984; Tong et al., 2000).
Most of the detrimental consequences of milk heating are related to the development of the Maillard reaction (O’Brien & Morrissey, 1989; van Boekel, 1998). It must be pointed out that under the processing conditions usually carried out at industrial level to produce pasteurised/sterilised milk, only the early phase of the Maillard reaction takes place. At this stage, no colour nor flavour changes arise, while biologically unavailable lysine-sugar derivatives are formed (Erbersdobler & Dehn-Müller, 1989; Pellegrino, de Noni, & Resmini, 1995). On the contrary, desired or undesired flavour and browning products (i.e. melanoidins) are often obtained when milk or dairy derivatives are treated at higher temperatures. Depending on the intensity of the thermal treatment applied, pro-oxidant or antioxidant molecules are expected to be produced (Nicoli, Anese, & Parpinel, 1999). In particular, highly reactive radicals are formed in the early phases of the Maillard reaction, just prior to the Amadori rearrangement, while strong antiradical properties are attributable to the high molecular weight compounds which are formed in the advanced phases of the reaction (Lingnert & Eriksson, 1980; Namiki & Hayashi, 1983; Hayase, Hirashima, Okamoto, & Kato, 1989; Namiki, 1990; Pischetsrieder, Rinaldi, Gross, & Severin, 1998; Hofmann, Bors, & Stettmaier (1999a), Hofmann, Bors, & Stettmaier (1999b)). It is likely that also during milk heating, as observed for other foods (e.g. tomato and pasta) (Nicoli, Anese, Manzocco, & Lerici, 1997; Anese, Nicoli, Massini, & Lerici, 1999), the formation of such compounds could be responsible for different, and sometimes opposite, effects on the overall antioxidant activity. Such effects could contribute to the observed increase in antioxidant activity of milk upon heating (Taylor & Richardson, 1980; Walstra & Jenness, 1984; Tong et al., 2000).
The net result of loss of natural antioxidants, protein unfolding and formation of heat-induced antioxidants and/or pro-oxidants in milk products greatly depends on the processing conditions. Current knowledge is not sufficient to identify the technological conditions which promote or inhibit the formation of pro-oxidants and/or antioxidants in milk products. For these reasons, it would be valuable to determine the processing conditions that improve antioxidant potential and minimise oxidative reactions responsible for a decrease of milk quality attributes. This knowledge could provide information not only on the overall health protecting potential of milk products but also on the stability of complex foods containing milk. For instance, the use of milk ingredients with different antioxidant/pro-oxidant potential is likely to dramatically affect the stability of formulations rich in oxidable compounds (e.g. lipids, pigments and vitamins).
This study addressed the influence of different heat treatments on the antioxidant and pro-oxidant properties of milk. The time–temperature combinations were selected in order to simulate low- and high-intensity heat treatments. In particular, much more severe heating conditions than those applied in manufacture of milk products were considered to further investigate the causal relationships between processing conditions and antioxidant/pro-oxidant activity.
Section snippets
Sample preparation
Pasteurised skim milk (d.m. 9.8% w/w, fat 0.1% w/w, pH 6.75) was purchased from a local market. 10 mL of milk was added to 20 mL vials and hermetically sealed with butyl septa and metallic caps. Samples were heated in an oil bath at 80°C, 90°C and 120°C for up to 24 h. After heating, samples were rapidly cooled in cold running water and analysed.
Total solid content
Total solid content determinations were carried out according to AOAC (1980) methods.
Colour
Colour analyses were carried out using a tristimulus colorimeter
Results and discussion
The redox potential of milk heated at 80°C, 90°C and 120°C was assessed to estimate the changes in the overall reducing properties of the system (Table 1). It can be noted that milk heated at 80°C showed a progressive enhancement in redox potential, while at 90°C an initial increase was followed by a decrease. The redox potential of milk heated at 120°C slightly decreased during the first 15-min of heating but no significant changes were observed upon further heating. Since the redox potential
Conclusions
In light of these findings, it is evident that conclusions about the oxidative state of foods can only be achievable by concomitant use of various methodologies, each providing different information about sample reducing properties and the kinetic capability to act as anti- and/or pro-oxidants. In the case of milk, its reducing properties were shown to be strongly affected by heating. In particular, heat treatment of milk can promote an increase in its pro-oxidant activity, probably as a
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