Quality of irradiated and nonirradiated black sapote (Diospyros digyna jacq.) after storage and ripening (1998)

Black sapote (Diospyros digyna Jacq.) were irradiated at 0.15 or 0.3 kGy and held at 25C until ripe. Nonirradiated fruit were placed in storage at 1,5,10,15,20, or 25C for up to 7,10, or 15d and fruit that remained unripe were then held at 25°C un til ripe. Nonirradiated fruit held at 15,20, or 25°C for 7,10, or 15 d ripened normally when transferred to 25C. Fruit stored at 10°C for 7 d then transferred to 25°C ripened normally, but some fruit held at 10C for 10 or 15 d had abnormal ripening. Most fruit stored at 1 or 5°C did not ripen normally or failed to ripen regardless of storage duration. Fruit exposed to 0.15 kGy ripened normally, but some fruit treated at 0.3 kGy had random areas of pulp that failed to fully soften and ripen. Ripening of irradiated fruit was delayed about 4 d compared with controls. Based on this preliminary study, ripening may be satisfactorily delayed by storage at 15 or 20°C prior to ripening at 25C. Ad ditionally, black sapote will tolerate irradiation at 0.15 kGy, but abnormal ripening will likely occur with some fruit treated at 0.3 kGy. Black sapote (Diospyros digyna Jacq.:syn. D. obtusifolia Humb. & Bonpl. ex Willd.) is native to Mexico (Campbell et al., 1988; Popenoe, 1924; Ruehle, 1953; Sturrock, 1959), how ever, this minor tropical fruit is also grown in Dade County, Fla., mostly as a backyard crop. Most sales of black sapote are through roadside stands with minor quantities marketed through commercial wholesalers. In the future, fruit most likely will be shipped to markets that require extended stor age or that have quarantine restrictions for pests such as the Caribbean fruit fly (CFF) (Anastrepha suspensa Loew.). Black sapote is considered a minor commercial crop in Dade Coun ty and production is expected to slowly increase (personal communications, R. Knight). This fruit is climacteric and is usually harvested at full maturity (fully shapened, green/ Mention of a trademark, warranty, proprietary product, or vendor does not constitute a guarantee by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable. slight yellowish peel) and is very soft when ripened (edible stage) at ambient conditions. Mature fruit will usually ripen within 10 d (Morton, 1987) after harvest. Black sapote will rip en very rapidly at maturity and within 1 to 2 d of reaching full ripe and will disintegrate rapidly. Delaying the ripening of mature fruit may be useful to growers in their attempt to gain some control over the duration and timing of the ripening process so that more orderly marketing can occur. However, no information is available on postharvest storage tempera ture/duration and subsequent ripening information for the emerging black sapote industry. Delaying postharvest ripening of certain climacteric fruits is achievable by methods such as storage at temperatures low er than optimum for ripening (Hardenburg, et al., 1986), prestorage heat treatments (Paull, 1990), or irradiation (Akamine and Moy, 1983; Moy, 1977). Low-dose irradiation also has potential for quarantine treatment purposes (Nation and Burditt, 1994). Fresh fruits and vegetables can be treated with irradiation at doses to 1.0 kGy (Anonymous, 1986) and rec ommended doses for sterility of fruit flies is about 0.3 kGy (Burditt, 1994). Therefore, the purpose of this investigation was to determine the postharvest physiological response of black sapote to low-dose irradiation, storage and ripening, and to determine ripening response of fruit after storage, at various temperatures. Materials and Methods Source of fruit. Black sapote were harvested from a planting of seedlings that are part of the USDA national germplasm re pository at the USDA Subtropical Horticultural Research Lab oratory, Miami, Fla. Harvests were on two different dates for the two separate experiments of tolerance to irradiation, and storage potential prior to ripening. At harvest, fruit were de termined mature as indicated by size, color and being well shapened. Fruit preparation. For the irradiation experiment fruit were harvested on 24 February, wiped clean of surface residue and irradiated at doses of 0.15 or 0.3 kGy applied at the rate of 0.0933 kGy-min1 with a Gammacell (Co60) irradiator located at the U.S. Subtropical Horticultural Research Station. Con trol (nonirradiated) and irradiated fruit were carefully packed 23 fruit each into six 4/5 bu. commercial fiberboard citrus boxes. The boxes of fruit were shipped overnight, arriv ing the next morning at the USDA U.S. Horticultural Re search Laboratory (USHRL) in Orlando. On arrival, fruit were immediately unpacked and immature and bruised fruit were removed. The sound fruit were randomized into three 10 fruit samples for each of the three dosages (0, 0.15, 0.3 kGy). Fruit were inspected initially before storage at 25°C (RH 90% ± 5%) and on reaching the soft ripe stage. To determine storage potential, fruit were harvested on 6 March, boxed and shipped to the Orlando USHRL as stated above. On arrival, fruit (all nonirradiated) were unpacked and immature, ripe, or bruised fruit were culled and remain ing fruit were randomized into 18, 5-fruit samples. Three, 5fruit samples were placed in each of six storage temperatures, Proc. Fla. State Hort. Soc. 110: 1997. 215 1,5,10,15, 20, or 25°C. Fruit placed at 25°C were control fruit since 25°C is considered the best temperature for ripening. One 5-fruit sample was removed from each storage tempera ture at the end of 7, 10, and 15 d. After 15 d any unripe fruit were placed at 25°C to ripen. Fruit were inspected initially be fore storage, at the end of 7,10, or 15 d storage, and on reach ing soft ripe. Quality and condition evaluations. Before storage, fruit weights were measured using a Mettler balance. Fruit firm ness was initially determined objectively on a 5-fruit sample with an Instron Food Firmness Texture Machine (model 1142, Canton, Mass.). The Instron was calibrated to record the force (N) for an 11 mm diam. round faced cylinder to penetrate 3 mm into the fruit cheek at 25 cm-min1 travel speed. Since black sapote are extremely soft when ripe only subjective firmness ratings were used to determine fruit firm ness at the soft ripe stage (terminal inspection). Subjective firmness ratings were; 1 = firm (no yield to moderately ap plied finger pressure (MAFP)), 2 = fairly firm, (slight yield to MAFP), 3 = fairly soft, (moderate yield to MAFP), 4 = soft (ripe stage, little resistance to MAFP), and 5 = very soft (over ripe, 'slip skin' with watery mushy pulp). Peel color was sub jectively rated; 1 = 100% green (mature), 2 = mostly green/ slight yellow, 3 = moderately yellow/green, 4 = olive green/ brown (ripe), and 5 = mostly brown 'slip skin' (over ripe). Ca lyx color was rated 1 = green, 2 = green/yellow, 3 = green/ brown, 4 = yellow/brown, and 5 = brown/black. Pitting was rated 1 = no disorder, 2 = 50% of surface area affected. When ripe, pulp color was rated 1 = dark brown, 2 = light brown, or 3 = mixed color (dark/light brown). Pulp consistency at the soft ripe stage was rated 1 = mostly very moist and viscous, 2 = viscous with some dry fibrous tissue, and 3 = mostly fibrous and dry. Initially, surface scarring and blossom end cracking were not ed and percentage of fruit affected was calculated. Percentage of decayed fruit (not specifically identified) or fruit with superficial mold growth on lesions or scarred tissue was also calculated. Informal flavor and mastication texture sensory evaluations were made as each fruit reached the soft ripe stage, and ratings were indicated as acceptable or unac ceptable. Fruits from the irradiation experiment were analyzed for soluble solids concentration (SSC) by refractometer (Jones and Scott, 1984), titratable acidity (TA) with NaOH Qones and Scott, 1984), and individual sugars by HPLC (Baldwin et al., 1991). Fruit homogenate (40g) was extracted in 70 mL of 80% ethanol. The filtered extract was injected onto a 20 iL sample loop and on a Perkin Elmer 410 HPLC system equipped with a Waters sugar pak column at 90°C with a mo bile phase of 100 m ethylenediaminetetraacetic acid disodium-calcium salt (CaEDTA) and a flow rate of 0.5 mL-min1. A Waters 410 Differential Refractometer was used to measure sugars and filtered analytical grade reagents were used for standard preparation to establish HPLC retention times and calibration. Experimental design. The irradiation experiment was a com pletely randomized design consisting of three replicated 15fruit samples for each of three treatments. Mean separations among treatment combinations for each attribute were con ducted using ANOVA procedures or General Linear Model procedures for the chemical assays (SAS, Cary, N.C.) at P <. 0.05. For the storage experiment, the design was completely randomized with a single 5-fruit sample for each storage tem216 perature/time combination, hence replications were on fruit for mean separations. Treatment means were analyzed for separations among storage temperatures at each storage du ration (i.e., 7, 10, or 15 d) separately.