PREDICTING SHELF LIFE AND QUALITY OF RASPBERRIES UNDER DIFFERENT STORAGE TEMPERATURES

Red raspberries (Rubus idaeus ‘Killarney’) were harvested twice at the full ripe stage and held for 7 days at 0, 5, 10, 15, or 20 °C. The objectives of this work were 1) to obtain quality curves for raspberries stored at different temperatures; 2) to identify, for each temperature, which quality factor(s) limits raspberry marketability; and 3) to compare the quality curves and shelf-life of raspberries based on quality evaluations with those predicted by respiration rates reported in the literature. Raspberry weight loss, instrumental color (L*a*b*), visual color, firmness, shriveling, decay, taste and aroma were evaluated every day for a 7-day storage period. Darkening of the color was the primary limiting factor at 0, 5, 10 and 15 °C for raspberries from the first harvest, while darkening of the color, loss of firmness and objectionable aroma were the primary limiting factors at 20 °C. For raspberries from the second harvest, darkening of the color, objectionable taste or aroma was the primary limiting factor for fruit stored at 0 or 5 °C. Development of off-flavor was the primary limiting factor for raspberries stored at 10 or 15 °C, and objectionable aroma was the primary limiting factor for those fruit stored at 20 °C. For each temperature, the shelf life of raspberries predicted based on the Q10 calculated from reported respiration rates was on average 1 to 2 days longer at 0 °C, the same number of days at 10 and 20 °C and less than 1 day shorter at 5 and 15 °C when compared with the shelf life of raspberries obtained from quality evaluations. The results showed that a single quality attribute cannot be used to express loss of quality of raspberries over the normal physiological range of temperatures and that raspberry shelf life is closely correlated with respiration rate. INTRODUCTION The physiological behavior of a particular crop is very much dependent on the postharvest handling temperatures. The use of an optimum temperature during handling and storage of fresh horticultural crops is the major factor that determines the quality of the fresh product. Several studies report the effect of temperature on quality attributes of small fruits (Salunkhe and Desai, 1984; Nunes et al., 1998; Robins and Moore, 1990). Raspberries are a highly perishable fruit, the shelf life of which can be greatly reduced by storage temperatures above 0 °C (Salunkhe and Desai, 1984). Even when stored at the optimum temperature, the shelf life of raspberries can be as short as 2 to 3 days (Hardenburg et al., 1986). During storage at 0 °C, color of ‘Meeker’ raspberries darkens, and pH, total anthocyanin concentration and decay increase while titratable acidity decreases (Sjulin and Robbins, 1987; Robbins et al., 1989). Robbins and Moore (1990) stored several raspberry cultivars at 0, 4.5 or 20 °C for 16 days and also noticed that they became darker, less red and bluer during storage. Although some studies refer to the quality changes of red raspberries during storage no information was found regarding precise quality curves for raspberries stored at different temperatures or regarding which quality factor(s) are the most important to determine the limits of marketability. The objectives of this work were 1) obtain quality curves for raspberries stored at different temperatures; 2) to identify, for each temperature, which quality factor(s) limits raspberry marketability; and 3) to compare the quality curves and shelf-life of raspberries Proc. XXVI IHC – Issues and Advances in Postharvest Hort. Ed. R.K. Prange Acta Hort. 628, ISHS 2003 Publication supported by Can. Int. Dev. Agency (CIDA) 600 based on quality evaluations with those predicted by respiration rates reported in the literature. MATERIAL AND METHODS ‘Killarney’ red raspberries were obtained from a commercial field near Quebec City, Canada. A total of two harvests/experiments were conducted during the 2001 summer season. Commercially harvested fruit without calyxes attached were packed in fiberboard flats containing twelve plastic cups of red raspberries, removed from the field with minimal delay after harvest and transported to the laboratory in Quebec City within approximately 30 minutes. A total of 330 raspberries from four flats were selected for uniformity of color and freedom from defects, weighed and carefully placed in plastic clamshells to avoid injury to the fruit. Each individual clamshell was placed inside an open plastic bag to maintain a relative humidity level of about 95 to 100%. Twenty-two clamshells containing 15 raspberries each were then distributed among five temperaturecontrolled rooms at 0.5°C ± 0.5 °C, 5 °C ± 0.2 °C, 10 °C ± 0.4 °C, 15 °C ± 0.2 °C and 20 °C ± 0.2 °C for a 7-day storage period. The same trained person assessed the quality of fruit throughout storage. Quality Evaluation 1. Weight Loss. Weight loss was calculated from the initial weight of three individual replicated samples of 15 raspberries each and after every storage day. 2. Color. Instrumental surface color measurements (L*, a*, b*) of each individual raspberry were taken with a reflectance colorimeter on the side of a slightly flattened whole fruit (Robbins and Moore, 1990). Numerical values of a* and b* were converted into hue angle (Ho = tanb*/a*) and chroma (Chroma = (a + b) ) (Francis, 1980). Color of each individual raspberry was also assessed using a 1 to 5 visual rating scale (Perkins-Veazie and Nonnecke, 1992). 3. Firmness. Firmness of each individual raspberry was assessed using a 1 to 5 visual rating scale where 1 = very firm and turgid, 2 = firm, 3 = moderately firm, raspberry is more soft than firm, 4 = soft and leaky, 5 = very soft and deteriorated. 4. Shriveling. Shriveling of each individual raspberry was assessed using a 1 to 5 visual rating scale where 1 = none, field-fresh, no signs of shriveling, 2 = slight, minor signs of shriveling, not objectionable, 3 = moderate, shriveling evident, becoming objectionable, 4 = severe shriveling, definitely objectionable, 5 = extremely wilted and dry, not acceptable under normal conditions. 5. Decay. Decay of each individual raspberry was assessed using a 1 to 5 modified visual rating scale from Horsfall and Barratt (1945) where 1 = 0%, no decay, 2 = 1-25% decay, probable decay (brownish/grayish sunken minor spots), 3 = 26-50% decay, slight to moderate decay (spots with decay and some mycelium growth), 4 = 51-75% decay, moderate to severe decay, 5 = 76-100% decay, severe to extreme decay 6. Aroma and Taste. Aroma and taste of three replicated samples of 15 raspberries each was assessed using a 1 to 5 rating scale where 1 = excellent, fresh and pleasant characteristic aroma and taste 3= acceptable and 5 =unacceptable, unpleasant taste and odor. Limiting Factor For each temperature a limiting quality factor(s) was established considering the rating value of 3 as the maximum acceptable quality before the product is considered unmarketable. More specifically, for each temperature the factor(s) that limit the product marketability were identified from the quality curves. Statistical Analysis Data analysis was performed using the Statistical Analysis System computer package (SAS Institute, Inc., 1982). Statistical analysis of data showed a significant difference between harvests; therefore, results from the two harvests were subsequently