Original article Stability of vacuolar betaxanthin pigments in juices from Moroccan yellow Opuntia ficus indica fruits

Summary Pigment stability of yellow juices prepared from Moroccan cactus pear (Opuntia ficus indica) was determinedas a function of temperature and pH. The experiments were carried out at temperatures ranging from 80 to100 C with juices at pH 3.5, 5 and 6.5. The degree of pigment retention decreased when the temperatureincreased. The degradation constant rates were determined for thermal degradation rates of pseudo-firstorder. The Arrhenius plot obtained for the degradation of betaxanthin from the yellow fruits was not linear.Regardless of the temperature treatment, the lowest degradation was obtained for pH 5. Where somestabilizers were tested for the protection of pigments, the results showed that ascorbic acid was a betterprotective agent at pH 3.5, increasing the protection by 40%.Keywords Acidity, betalains, cactus, juice, Opuntia ficus indica, stability, temperature, vacuole. Introduction Since the World War II, the consumption of fresh fooddecreased while that of processed foods increased.Despite the care taken during processing, the naturalcontent in pigments may be altered and⁄or destroyed,resulting in the decrease of the commercial and⁄ornutritional value of the product. It is thus necessary toincorporate pigments to restore the original colour.Similarly, the preparation of fortified products requiresthe addition of pigments to the products (Smith, 1991;Castro et al., 2005). Therefore, food colour has becomea crucial issue to food manufacturers (Jackman S Schoefs, 2002, 2003). On the other hand,the restrictions on artificial food colorants have risenbecause they may confer the product allergenic and⁄orintolerance properties (e.g., Hendry & Houghton, 1996;Wissgott & Bortlik, 1996). As a consequence, thedemand for natural pigments has increased as comparedto synthetic colorants (Pszczola, 1998; Joppen, 2003).However, the demand for natural colorants cannot bealways satisfied because of limited supply of rawmaterials, and pigment production using conventionalplant cultivation methods is strongly influenced byclimatic conditions, plant cultivars and varieties(Rodriguez-Amaya, 2000). Consequently, a part ofplant pigment research is today oriented towards findingnew sources of pigments and⁄or food products; amongthose is the cactus pear. The cactus pear is a fruit of thegenus Opuntia ficus indica (Cactaceae). It is a berryconsisting of a thick pericarp and a number of clefts ofsmall prickles, reddish, purple, yellow or white in colour,with a luscious sweet pulp intermixed with numeroussmall seeds. From the pigment point of view, cactus pearpulp contains water-soluble betalain pigments that areaccumulated into the vacuole. Betalain is a generic termdesignating red and yellow families of pigments calledbetacyanins and betaxanthins, respectively. Pigmentanalyses have indicated that the main betacyanin andbetaxanthin in cactus pear are betanin and indicaxan-thin. Modifications in the proportion of these pigmentsmay reflect in the pulp colour (Forni et al., 1992;Fernandez-Lopez & Almela, 2001; Stintzing et al., 2002;Castellar et al., 2003). Beside the colouring property,both betanin and indicaxanthin present antioxidantactivities (Kanner et al., 2001; Pedreno & Escribano,2001; Butera et al., 2002).The use of cactus pears is of particular interest to theeconomy of arid regions (Dominguez Lopez, 1995;Munoz de Chavez et al., 1995) because this plantconstitutes a very important food source, presenting