Research noteA study on the degradation kinetics of visual green colour in spinach (Spinacea oleracea L.) and the effect of salt therein
Introduction
Colour plays an important role in visual recognition and assessment of the surface and the subsurface properties of the object. It has a great influence on the appearance, processing and acceptance of food materials. The degree of greenness, attributed to chlorophyll pigments, is important in determining the final quality of thermally processed green vegetables. Thermally processed green vegetables exhibit poor colour quality as compared to the fresh ones. The colour change from bright green to olive brown occurring during the processing is mainly due to the degradation of chlorophyll to pheophytin by the replacement of magnesium in the chlorophyll by hydrogen and further formation of the degradation products such as pheophorbides and chlorins (Canjura, Schwartz, & Nunes, 1991; Schwartz & Lorenzo, 1991; Schwartz & Von Elbe, 1983; White, Jones, & Gibbes, 1963). More over the conversion of chlorophyll to pheophytin is regarded as an index of the severity of the processing (Woolfe, 1979).
Factors such as pH, temperature, presence of salts, enzymes and surface-active ions influence the stability of chlorophyll. Chlorophyll has better stability at higher pH than at lower pH (Ryan-Stoneham & Tong, 2000; Sweeney & Martin, 1961). Salts of magnesium, calcium, sodium, ammonium and some surface-active agents are known to have some stabilizing effect on chlorophyll degradation (Eheart & Odland, 1973; Haisman & Clarke, 1975; Woolfe, 1979).
Accurate knowledge of the kinetic parameters, rate constant and activation energy is essential to predict the quality changes that occur during thermal processing. Numerous researchers have evaluated the kinetics of pigments and colour degradation in fruits and vegetables such as broccoli (Weemas, Ooms, Van Loey, & Hendrickx, 1999), peas (Ryan-Stoneham & Tong, 2000; Steet & Tong, 1996), spinach (Canjura et al., 1991; Gupte, El-Bisi, & Francis, 1963; Schwartz & Von Elbe, 1983) during thermal processing and found it to follow first order reaction kinetics.
It is believed that colour vision is a complex phenomenon and its measurement is much more complex than the absorption by the stimuli pigments at specific wavelengths (Govindarajan, Rajalakshmi, & Chand, 1998). Furthermore, the spectrophotometric methods have some limitations for the colour measurement of thermally processed foods. Thermally decomposed pigments like pheophytins in the case of chlorophyll, cause interference during pigment concentration measurements (Eheart & Gott, 1965). There are several reports on the measurement of colour using instruments. Such instruments are usually based on the principle that a colour can be mathematically described as a combination of the three primary colour intensities (Clydesdale, 1978). Tristimulus colourimeter has been widely accepted as a rapid and simple instrumental method for measuring visual perception of the food products (Clydesdale, 1991; Gnanasekharan, Shewflet, & Chinnan, 1992; Hayakawa, 1977; Ozkan, Yemenicioglu, Asefi, & Cemeroglu, 2002). The colour of any food product can be represented in terms of L, a, and b values or combination of these three depending upon the nature of the pigment present in the food material. A study conducted by Weemas et al. (1999) on the kinetics of chlorophyll degradation and colour loss in heated broccoli juice shows the advantage of objective colour measurements for studying the chlorophyll degradation as compared to conventional chemical analysis and from the point of view of the consumers the green colour is more important than the residual chlorophyll content.
Spinach is one of the important leafy vegetables consumed all over the world. It is consumed as fresh, pureed or processed. During thermal processing, colour changes from bright green to olive green due to the conversion of chlorophyll to pheophytin and pyropheophytin. It is a common belief that common salt (sodium chloride) has a stabilizing effect on green colour (Haisman & Clarke, 1975; Hudson, Sharples, & Gregory, 1974). However, the stabilization effect of salt is not well established and no information is available on the effect of salt on colour degradation of these products, especially in terms of visual colour.
Therefore, present study was undertaken: (1) To find out the effect of salt (1% and 2% by weight) on colour degradation in spinach (Spinacea oleracea L.), a model food system, over a temperature range of 50–120 °C (steady state temperature) and to determine kinetic parameters for colour degradation in spinach (S. oleracea L.). (2) To study the degradation kinetics of colour in different cooking methods (unsteady state process). (3) To develop a mathematical model relating the calculated kinetic data from the steady state temperature and the time–temperature profiles of different cooking methods (unsteady state process). (4) To apply this model to predict the green colour degradation for unsteady state heating process, from the time–temperature data of the unsteady state heating process and comparing it with the actual degradation values, which could then be used to assess the changes in the colour quality as a function of method of cooking.
Section snippets
Materials and methods
Fresh green spinach leaves were obtained locally, washed thoroughly and drained.
Effect of salt and temperature on visual green colour of spinach puree
Table 1 shows the effect of temperature and addition of salt on the Hunter L, a and b values of spinach puree. As can be seen, there is a consistent decrease in L and ‘−a’ values with an increase in the treatment time and temperature. But there was no consistent change in b values. The change in L and b values may be due to pheophytin–pyripheophytin conversion or due to degradation/reaction of other components present in the spinach puree (Weemas et al., 1999). Since the greenness is indicated
Conclusions
A stabilization effect of 2% sodium chloride on green colour degradation resulting from heat treatment is confirmed from the kinetic studies, although, the exact mechanism of stabilization is not known. Slow cookers, as exemplified by EcoCooker, show no significant difference in the magnitude of retention as compared to normal open pan and pressure-cooking. Based on the colour retention and fuel savings, an overall judgment in favor of the slow EcoCooker is suggested.
Acknowledgements
The authors gratefully acknowledge the financial support provided by Land Research Institute, Mumbai in carrying out this work.
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