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E. T. Contis et al. (Editors) Food Flavors: Formation, Analysis and Packaging Influences © 1998 Elsevier Science B.V. All rights reserved

689

Effect of a - t o c o p h e r o l ( v i t a m i n E) o n t h e r e t e n t i o n of e s s e n t i a l oil, color a n d t e x t u r e of Chios m a s t i c r e s i n d u r i n g s t o r a g e . D. Papanicolaou, M. Melanitou and K. Katsaboxakis National Agricultural Research Foundation, Institute of Technology Agricultural Products, 1 Sof Venizelou, Lycovrissi, 141 23, Greece

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Abstract Vitamin E was added to Chios mastic resin packed in laminated packages. Product hardness, color and essential oil concentration were measured during storage for 450 days at 25 and 37°C. It was found that although sample weight remained constant, essential oil decreased gradually during storage as determined by clevenger distillation. This indicates that essential oil was transformed into other, nonvolatile, products. The presence of vitamin E retarded the essential oil loss significantly at both storage temperatures in comparison to control samples. Vitamin E retarded resin hardening and also Umited changes in color in comparison to control samples.

1. INTRODUCTION

The Chios mastic, is extracted from a tree, Pistacia lentiscus var. chia, which grows spontaneously on the Greek island of Chios. It is used primarily as a natural chewing gum and its essential oil, obtained by steam distillation, is used for the preparation of Hquors, as a flavor agent in food products (sweets, icecreams), in cosmetics e.t.c. The major problems of mastic resin during storage are the loss of essential oils and the simultaneous increase of its hardness which results in a product of inferior quality and of low commercial value (1). Previous studies suggest that the mastic gum could be considered as a system of a resin and a solvent and that the loss of the later by evaporation is the principal cause for the hardening of the product (2, 3). However, the effect of evaporation on the texture has not been studied. Furthermore, mechanisms other than evaporation have not been investigated whether they are involved in the loss of essential oil and/or the increase in hardness of the raw material. The known positive effect of antioxidants on oil preservation led the authors to study the effect of the addition of vitamin E on the stabiUty of the essential oil and the mastic texture during storage. Color measurement is a quahty parameter of the product and changes in color were also expected to be influenced by the addition of vitamin E.

690 Specifically, the main objective of this research were to study the effect of evaporation and of vitamin E (a-tocopherol), a natural antioxidant, on the loss of essential oil, the increase of hardness and the changes in color during storage.

2. MATERIALS AND METHODS The mastic was harvested according to the method developed recently (4). The essential oil concentration was four times higher than that of the traditionally harvested product and was semi-fluid in texture. In order to remove foreign impurities from the raw material, the harvested resin was slightly heated to liquefy it and then it was passed through a centrifugation filter. The filtered mastic was divided in two lots; Vitamin E (a-tocopherol) was added to the first at a concentration of 0.02% (w/w) and thoroughly mixed so that it was well distributed into the mass of the resin. No addition was made to the second lot. Samples of 5 g from each lot were packed in heat sealed plastic laminated bags, so that the loss of essential oils by evaporation was prevented. The packages were stored at two different temperatures: 25°C and 37°C. In addition, samples were stored at 37°C in uncovered petri dishes in order to study the loss of essential oil by evaporation through the surface of the samples. The weight loss of the samples, the texture, the color and the retention of essential oil were determined during storage. The retention of essential oils was determined by clevenger distillation. The texture (hardness) was measured by means of a Food Technology Corporation apparatus using a probe of 5 mm diameter which was submerged to a depth of 2 cm and with a speed of 5 cm/sec. The sample was placed on a hive of 7 mm diameter. The color of the samples was measured with a Minolta portable color difference meter.

3. RESULTS AND DISCUSSION

3.1. The influence of vitamin E in the loss of essential oil. Figure 1 presents weight loss and percent loss of essential oil during storage of mastic samples at 37°C in uncovered petri dishes. As shown in this figure, although the sample weight remained practically stable during storage, a loss of about 70 % of essential oil occurred within the first 60 days. This loss reached about 90 % after 90 days and increased sUghtly until the end of the storage period. This loss of essential oil as determined by clevenger distillation is higher than that which could be explained by the loss of weight of the samples (as a result of

691

0 0 -1

90

WEIGHT OF SAMPLE T LOO (% LOSS) •

ESSENTIAL OILS (% LOSS)

80 70

t=37 ^C

60 50

-p

40

-•-ess.oil

30

-^—weight 1

0.50

20 10 0 i, 0 0 0

or—^^—Q—1

50

100

1

1

150 200 TIME (DAYS)

250

1

—

°—i 0.00 300

F i ^ r e 1. Essential oil and weight loss of Chios mastic resin stored at 37 °C in uncovered petri dishes. % Loss= (Essential oil at time 0 - Essential oil at time T) X 100 Essential oil at time 0

evaporation mechanisms). This impUes that mechanisms other than evaporation contribute to the decrease of essential oil. This loss of essential oil is also demonstrated in figure 2 where the samples of mastic resin were stored at 25 and 37°C in sealed laminated packages preventing any weight loss by evaporation. As shown in this figure, the essential oil loss reached about 80 % at both temperatures within the first 100 days and above 90 % at the end of storage period (450 days). The addition of vitamin E limited the loss of essential oil, particularly for samples stored at 25°C. It was remarkable that at 25°C the presence of vitamin E reduced by half the loss of essential oil within the first 100 days. The level then remained almost constant even after 450 days of storage. The decrease in the essential oil content of mastic suggests that some of the components were transformed into nonvolatile substances which were no longer recoverable by clevenger distillation. The mechanism of that transformation could be polymerization and/or oxidation of those substances. That could also explain the increase in hardness of the resin as well as the modification of the chemical composition of the essential oil taking place throughout the storage of the product, the most important being the decrease in the myrcene content and the increase in a-pinene (5). Vitamin E reduces the rate of loss of essential oil. This indicates that vitamin E blocks the mechanism of polymerization and/or oxidation.

692

100 90

ESSENTIAL OILS (% LOSS)

80 70 60 50 + 40 30 20 10 0 200 300 TIME (DAYS)

500

F i ^ r e 2. Effect of a-tocopherol on essential oil loss of Chios mastic resin stored at 25 and 37 °C in sealed laminated packages

3.2. The influence of vitamin E in the texture (hardness) of mastic during storage. As mentioned above, the progressive hardening of mastic during storage is the most serious problem in the quality and the commercial value of mastic resin. As can be observed in figure 3, the addition of vitamin E retarded this undesirable change in texture significantly for 60 days at both temperatures (25 and 37 °C) in comparison with the control samples. The presence of vitamin E minimized also the negative effect of the high temperature (37°C) for 40 days while a higher increase rate of hardening was observed thereafter. This was probably due to the destruction of vitamin E at this temperature. The increase in hardness of the mastic during storage was attributed to the polymerization reactions of the constitutents of the resin (3). It seems, therefore, that vitamin E acts protectively by blocking or retarding this mechanism of polymerization and/or oxidation, as in the case of the essential oil.

693

160 140 120 100 80 + 60 40 20

25 oC+vitE 25 oC 37 oC+vitE 37 oC

HARDNESS (Kg)

30 TIME (DAYS)

Figure 3. Effect of a-tocopherol on hardness of Chios mastic resin stored at 25 and 37 °C in sealed laminated packages.

3.3. The influence of vitamin E on the color of mastic during storage. Figure 4 presents changes in L value and b/a ratio of color values measured with a Minolta Colorimeter during storage. A gradual loss of L value which represents the lightness can be observed in all samples with a higher loss rate at 25 °C than at 37 °C. The addition of vitamin E did not influence significantly these changes in L value. L VALUE

b/a VALUE

^-25oC B—37oC+vitE e—37oC L VALUE •—25oC+vitE •-25oC A—37oC+vitE

100

150

200

250

300

•—37oC

TIME (DAYS)

Figure 4. Effect of a-tocopherol on color of Chios mastic resin stored at 25 and 37 °C in sealed laminated packages. An increase in b/a ratio indicates the development of a yellowish color in the product which is an undesirable alteration. As shown in figure 4, the addition of

694 vitamin E retarded this increase in comparison to controls at both temperatures. The alteration of color is observed even in the partial absence of oxygen as the samples used for measurements were stored in laminated packages of aluminum foil in the absence of atmospheric oxygen.

4. R E F E R E N C E S 1. M. Melanitou, D. Papanicolaou, K. Katsaboxakis and K. Stamoula. Comparison of some physicochemical characteristics between solid and fluid Chios mastic resin. Dev. Fd. Sci. 37B, Proceedings of the 8th International Flavor Conference, Cos, Greece, July 1994, 1937. 2. M. Codounis. Application du froid au conditionnement et a la conservation du mastic de Chios. Rev. Gen. Froid (1979) 11, 657. 3. A. Kehayoglou, G. Doxastakis, V. Kiosseoglou and M. Mikedis. Compressional properties of Chios Mastiche (Pistacia lentiscus var. Chia) (1993). In: Food Flavors Ingredients and Composition Ed. G. Charalambous Elsevier Publishers p.429. 4. D. Papanicolaou, M. Melanitou, K. Katsaboxakis, D. Bogis and K. Stamoula. A new method for harvesting of Chios mastic resin in fluid form. Dev. Fd. Sci. 37A, Proceedings of the 8th International Flavor Conference, Cos, Greece, July 1994,311. 5. D. Papanicolaou, M. Melanitou and K. Katsaboxakis. Changes in chemical composition of the essential oil of Chios mastic resin from Pistacia lentiscus var. Chia tree during solidification and storage. Dev. Fd. Sci. 37A, Proceedings of the 8th International Flavor Conference, Cos, Greece, July 1994, 303.