Changes in phenolic composition of avocado cultivars during harvesting time

Abstract

Avocado, a tropical fruit native to Central America, is cultivated in tropical climates throughout the world. Avocado is also grown in Turkey, and plantations have rapidly expanded during the past decade. There are numerous cultivated varieties of avocados in the world [1]. Avocado has high oil content (8–32%) relative to other fruits and vegetables and, thus, has a relatively high-energy value [2]. Avocado is rich in vitamin E, ascorbic acid, vitamin B6, -carotene, and potassium [3]. The composition of avocado depends on ecotype, cultivar, degree of maturity, and growing conditions [1]. Fruit maturity and picking time are determined according to external markers (color and size), or by measuring dry matter and oil content in the flesh [4, 5]. The minimum oil content necessary for marketing avocado fruit is 8% [4, 6]. The phenolic content of fruits is also affected by the degree of maturity. Spanos and Wrolstad [7] determined that the phenolic content of pear depends primarily on variety and the level of maturity. For example, the phenolic content in apple and pear increases in the first three months and decreases later [8]. Avocado fruits have a long harvesting period depending on cultivars. In previous study, although the phenolic compounds were characterized according to chemical structures, there was no information regarding the amount of the compounds in avocado. The phenolics were determined to be p-hydroxybenzoic, protocatechuic, -resorcyclic, -resorcyclic, -resorcyclic, gallic, isovanillic, vanillic, syringic, o-coumaric, m-coumaric, p-coumaric, caffeic, ferulic, and sinapic acids [9]. These compounds can contribute to the quality of the fruit in many ways, such as its color and aroma [10]. Thus, in fruits such as avocados, the phenolics contribute to the formation of a brown color, which manifests itself after these fruits are diced or stored for a long time [9, 11]. The most important quality parameter of avocado fruit and products (puree) is color [12]. The color of the product changes according to enzymatic activity and phenolic contents of the fruit. With enzymatic oxidation of the phenolic compounds catalyzed by polyphenoloxidase enzyme, they eventually turn into quinones, a polymerized structure responsible for the brown colour. In the present study, the phenolic compound contents of four common avocado cultivars (Bacon, Zutano, Fuerte, Hass) grown in Turkey are analyzed to determine whether they change between cultivars depending on harvesting time. The total amount of phenolic as well as individual phenolic compounds in three harvesting period of four different avocado cultivars (12 samples) were determined by UV-VIS spectroscopy plus analytical RP-HPLC, and the results are shown in Tables 1 and 2. The avocado cultivars studied showed small differences in total phenolic content of the edible portion (wet weight). However, this differences also had statistical significances at p<0.05 level. On the basis of wet weight, Zutano and Fuerte cultivars had the highest phenolic content, followed by Bacon and Hass cultivars (in descending order) (Table 1). The total phenolic content was significantly affected by harvesting time in all cultivars. The lowest phenolic content was determined on the third harvesting time except for the Zutano cultivar. In addition, the total quantity of phenolic compounds determined by RP-HPLC showed similar trends as that determined by the spectrophotometric method. The decrease of total phenol amount is a well-known phenomenon during ripening. Torres et al. [9] determined the total phenolic content of the Hass and Fuerte cultivars as 1.8 mg/g and 1.1 mg/g respectively. Soong and Barlow [13] obtained 1.3 mg/g based on fresh weight (gallic acid equivalent). These values were similar to our result. Slightly differences might be due to differences in harvesting time (maturity degree), agricultural practices, and climatic factors.