compare the quality curves and shelf-life of raspberries based on quality ... range of temperatures and that raspberry shelf life is closely correlated with.
Predicting Shelf Life and Quality of Raspberries Under Different Storage Temperatures M.C.N. Nunes and J.P. Émond Air Cargo Transportation Research Group University Laval, FSAA-SGA Quebec City, Quebec Canada
J.K. Brecht Horticultural Sciences Department University of Florida, IFAS Gainesville, FL USA
Keywords: Rubus idaeus, weight loss, color, visual quality, aroma, taste, quality curves Abstract 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)
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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 = tan-1b*/a*) and chroma (Chroma = (a*2 + b*2) ½) (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 600
analyzed separately. In order to determine the primary limiting factor(s), quality attributes for each temperature were compared using the LSD 5% (Least Significant Difference at 95%). RESULTS AND DISCUSSION According to the literature, the maximum weight loss acceptable before raspberries become non-saleable is 6% (Robinson et al., 1975). After 5, 4 and 3 days at 10, 15 and 20 °C, respectively, raspberries from the first harvest had lost less than 6% of their initial weight. Raspberries stored at 0 or 5 °C reached the maximum acceptable weight loss after approximately 6 days. Weight loss of raspberries from the second harvest remained below the maximum limit of acceptability for all temperatures (data not shown). In these experiments, the L* value decreased regardless of the storage temperature, meaning that the raspberries lost their bright red color during storage, becoming darker (data not shown). Raspberries from first harvest were more red (higher a* value) at the time of harvest than those from the second harvest (data not shown). During storage, the color changed from a bright reddish-orange to a darker red (decrease in L* and hue angle). At the end of storage, the raspberries were more purplish-red than red when compared with freshly harvest fruit. Again, fruit from first harvest had a larger hue angle at the time of harvest compared with second harvest, which corresponded to a more reddish-orange fruit. The hue angle of fruit stored at higher temperatures decreased faster than those stored at 0 °C. Chroma of raspberries decreased during storage, but the decrease was more evident in raspberries stored at temperatures above 5 °C (data not shown). During storage, the color of the fruit became less vivid than at the time of harvest (lower chroma). Overall, the lower temperatures tended to better maintain the red color of the fruit. Previous studies showed that color of raspberries changes from an orange-red to a red-bluish color with increasing storage time and temperature (Robbins and Moore, 1990; Perkins-Veazie and Nonnecke, 1992). As in the present study, Robbins and Moore (1990) reported that L*, a* and b* values of several raspberry cultivars decreased with increasing storage time. During storage at 0, 4.5 or 20°C, the fruit become darker, less red and bluer. Results from the present study agree with those from Robbins and Moore (1990) as the raspberries stored at 20 °C darkened, became less red and more blue through 3 days of storage. Raspberries stored at 5, 10, 15 or 20 °C attained a maximum acceptable color rating faster than those stored at 0 °C (Fig. 1-5). After 1 day, raspberries stored at 20 °C had reached the maximum acceptable rating for color (Fig. 5). Raspberries from the second harvest stored at 0 °C maintained an acceptable color up to 5 days (Fig. 1). Firmness of the fruit decreased during storage, regardless of the storage temperature (Fig. 1-5). However, fruit from both harvests stored at 20 °C were considered unacceptable after 1 to 2 days. After 2 days, raspberries stored at 15 °C were soft and leaky. Raspberries stored at 5 °C maintained an acceptable firmness for 3 days. Firmness of the fruit stored at 0 °C was considered unacceptable after 4 and 6 days for the first and second harvests, respectively. As for weight loss, visual signs of shriveling was not considered a critical quality factor since they did not increase above a maximum acceptable rating of 3 for fruit stored at 10, 15 or 20 °C and remained below a rating of 3 for raspberries stored at 0 or 5 °C (Fig. 1-5). One of the reasons for little dryness and shriveling can be explained by the high humidity inside the package, which was maintained around 95 to 100% throughout the experiment. Raspberries stored at 0, 5 or 10 °C showed slight signs of decay such as minor brown grayish spots (visual rating of 2) that did not develop beyond that initial stage (Fig. 1-3). Decay was more evident in fruits stored at 15 or 20 °C, which attained a moderate to severe stage (visual rating of 4), mostly in fruit from the first harvest (Fig. 4 and 5). Loss of aroma and taste was very fast in the raspberries stored at 15 or 20 °C (Fig. 4 and 5). After 1 to 2 days, the aroma of the raspberries stored at 15 or 20 °C was considered unacceptable due to off-flavors. Fruit stored at 0 °C maintained an acceptable 601
aroma for 5 to 6 days (Fig. 1). However, loss of taste was faster than loss of aroma in fruit stored at lower temperatures. In fruit from the second harvest stored at 0 and 5 °C, the taste was no longer acceptable after 4 days while raspberries from the first harvest stored at 0 °C maintained an acceptable taste for 7 days. Limiting Quality Factor(s) for Each Temperature 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 (Fig. 1-5). 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 objectionable taste was the primary limiting factor for raspberries stored at 10 °C. Objectionable taste and aroma were the primary limiting factors for fruit stored at 15 °C. Development of objectionable aroma was the primary limiting factor for raspberries stored at 20 °C. Shriveling and decay were more evident in raspberries stored at 15 or 20 °C compared with stored at 0, 5 or 10 °C. After 3 days at 20 °C, the development of decay became objectionable, particularly in the fruit from the first harvest. Based on the limiting factor(s) for each temperature, a general quality graph was re-plotted considering only the curves obtained for limiting factors (Fig. 6). For any temperature, whenever two or more limiting factors were observed, new curves were established using the limiting factors average. Quality Curves from Respiration Rate As the temperature of a fruit or vegetable increases, the rate of chemical reactions increases. The changes in the rates of the reactions due to temperature are usually characterized using a measure called the Q10, which is the ratio of the rate of a specific reaction at one temperature (T1) versus the rate at that temperature + 10 °C (rate at T1 + 10 °C/rat at T1). The Q10 values are often used in reference to respiration since respiration gives a very general estimate of the effect of temperature on the overall metabolic rate of the tissue (Kays, 1991). For each temperature, the shelf life of the fruit was predicted based on the Q10 calculated from the respiration rates for raspberries found in the literature (Hardenburg et al., 1986). The reference from the quality evaluations for calculating the relative shelf life at each temperature based on respiration rate was the maximum shelf life for raspberries (5 days) at optimum temperature (0 °C). The predicted shelf life of raspberries based on Q10 values was 5 days at 0 °C, 3.3 days at 5 °C, 2 days at 10 °C, 1.3 days at 15 °C and approximately 1 day at 20 °C (Fig. 7). Overall, the shelf life of raspberries from experimental quality evaluations was 3 or 4 days at 0 °C, 3 or 2.5 days at 5 °C, 2 days at 10 °C, and 2 or 1 days at 15 °C for the first and second harvests, respectively (Fig. 6). The difference in the shelf life of raspberries from the two harvests was presumably due to the occurrence of rainfall during the first harvest, which reduced the shelf life of the raspberries, as the maturity of the fruit was very similar. The shelf life of raspberries based on Q10 values was 1 to 2 days longer at 0 °C, 0.3 to 0.8 day longer at 5 °C, the same at 10 °C and 0.2 day shorter at 15 °C, when compared with the shelf life of raspberries obtained from quality evaluations. Some of the reasons for the difference between the results from this study and those from respiration rates found in the literature, although quite small, might be related to uncertain information in previous reports about the type of raspberry cultivar used, the quality of the raspberries at harvest or at the beginning of the data collection, harvest season and temperatures as well as other environmental factors during harvest, transport or storage. In the present study, all of the previous variables were well known and the quality of the raspberries was excellent at the beginning of the experiment. In fact, one of the factors that has a great influence on the shelf life of any fruit or vegetable is the quality at harvest. If the initial quality of a product is considered merely good or acceptable instead of excellent, the shelf life of the product will be obviously reduced. That might explain the slightly longer shelf life of the raspberries stored at 15 °C, when 602
compared with data collected from the literature (Q10). Another reason for the difference between the values for raspberry shelf life from experimental quality curves and respiration rate curves was most likely due to the fact that when the respiration rate of a particular fruit or vegetable is measured the only aspect considered is the O2 consumed and CO2 released and not the visual quality. That means that respiration measurements may be continued beyond a point that could be considered unacceptable in terms of sensorial quality. CONCLUSIONS Storage of red raspberry fruit at 0 °C is recommended for maximum storage life. The quality curves obtained from quality evaluations for each temperature showed that a single quality factor cannot be used to express loss of quality of raspberries over the normal physiological range of temperatures. In addition, prediction of raspberry shelf life calculated from data on the respiration rates from the literature is not precise unless the characteristics of the fruit, the environmental factors involved as well as the quality are well known. Nevertheless, this kind of calculation can produce a good overall estimate of raspberry shelf life. ACKNOWLEDGEMENTS Thanks to ETR Group, Sweden (http://www.envirotainer.com/) for funding this research and N. Béland for technical support. Literature Cited Francis, F.J. 1980. Color quality evaluation of horticultural crops. HortScience 15:58-59. Hardenburg, R.E., Watada A.E. and Wang, C.Y. 1986. The Commercial Storage of Fruits, Vegetables and Florist and Nursery Stocks. Agr. Handbook No. 66. U.S. Dept. Agr., Washington, D.C. Horsfall, J.G. and Barratt, R.W. 1945. An improved grading system for measuring plant disease. Phytopathol. 35: 654. (Abstr.). Kays, S.J. (Ed.). 1991. Postharvest Physiology of Perishable Plant Products. AVI, Van Nostrand Reinhold, New York. Nunes, M.C.N.N., Brecht, J.K., Morais, A.M.M.B. and Sargent, S.A. 1998. Controlling temperature and water loss to maintain ascorbic acid levels in strawberries during postharvest handling. J. Food Sci. 63:1033-1036. Perkins-Veazie, P. and Nonnecke, G. 1992. Physiological changes during ripening of raspberry fruit. HortScience 27:331-333. Robbins, J., Sjulin, T.M. and Patterson, M. 1989. Postharvest storage characteristics and respiration rates in five cultivars of red raspberries. HortScience 24:980-982. Robbins, J.A. and Moore, P. 1990. Color changes in fresh red raspberry fruit stored at 0, 4.5 or 20 °C. HortScience 25:1623-1624. Robinson, J.E., Browne, K.M. and Burton, W.G. 1975. Storage characteristics of some vegetables and soft fruits. Ann. Appl. Biol. 81:399-408. Salunkhe, D.K. and Desai, B.B. 1984. Small fruits-berries. Postharvest biotechnology of fruits. Vol I. CRC, Boca Raton, Florida., USA. Sjulin, T.M. and Robbins, J. 1987. Effects of maturity, harvest date, and storage time on postharvest quality of red raspberry fruit. J. Amer. Soc. Hort. Sci.112:481-487.
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Figurese Color
Firmness
Shriveling
Color
Firmness
Shriveling
Taste
Decay
Aroma
Taste
Decay
Aroma
5.0 B
A
4.0
Quality rating (1-5)
Quality rating (1-5)
5.0 LSD0.05=0.128
3.0 2.0 1.0
LSD0.05=0.159
4.0 3.0 2.0 1.0
0
1
2
3
4
5
6
7
0
1
2
Time (days)
3 4 Time (days)
5
6
7
Fig. 1. Quality characteristics of raspberries stored at 0 ºC. Darkening of the color was the limiting factor for the first harvest (A); darkening of the color and development of objectionable taste were the limiting factors for the second (B) harvest. Color
Firmness
Shriveling
Color
Firmness
Shriveling
Taste
Decay
Aroma
Taste
Decay
Aroma
5.0
5.0 B
Quality rating (1-5)
Quality rating (1-5)
A LSD0.05=0.042
4.0 3.0 2.0 1.0
LSD0.05=0.067
4.0 3.0 2.0 1.0
0
1
2
3 4 Time (days)
5
6
7
0
1
2
3 4 Time (days)
5
6
7
Fig. 2. Quality characteristics of raspberries stored at 5 ºC. Darkening of the color was the limiting factor for the first harvest (A); darkening of the color and development of objectionable aroma were the limiting factors for the second (B) harvest. Color
Firmness
Shriveling
Color
Firmness
Shriveling
Taste
Decay
Aroma
Taste
Decay
Aroma
5.0 A
Quality rating (1-5)
Quality rating (1-5)
5.0 LSD0.05=0.051
4.0 3.0 2.0 1.0
B LSD0.05=0.042
4.0 3.0 2.0 1.0
0
1
2
3 4 Time (days)
5
6
7
0
1
2
3 4 Time (days)
5
6
7
Fig. 3. Quality characteristics of raspberries stored at 10 ºC. Darkening of the color was the limiting factor for the first harvest (A); development of objectionable taste was the limiting factor for the second (B) harvest.
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Color
Firmness
Shriveling
Color
Firmness
Shriveling
Taste
Decay
Aroma
Taste
Decay
Aroma
5.0
5.0
B LSD0.05=0.079
Quality rating (1-5)
Quality rating (1-5)
A LSD0.05=0.065
4.0 3.0 2.0
4.0 3.0 2.0 1.0
1.0 0
1
2
3 4 Time (days)
5
6
0
7
1
2
3
4
5
6
7
Time (days)
Fig. 4. Quality characteristics of raspberries stored at 15 ºC. Darkening of the color was the limiting factor for the first harvest (A); development of objectionable taste and aroma were the limiting factors for the second (B) harvest.
Color
Firmness
Shriveling
Taste
Decay
Aroma
Color Taste
5.0
Shriveling Aroma
5.0
Quality rating (1-5)
A
Quality rating (1-5)
Firmness Decay
4.0 3.0 2.0
B
4.0 3.0 2.0 LSD0.05=0.079
LSD0.05=0.075
1.0
1.0 0
1
2
3 4 Time (days)
5
6
0
7
1
2
3 4 Time (days)
5
6
7
Fig. 5. Quality characteristics of raspberries stored at 20 ºC. Darkening of the color, loss of firmness and development of objectionable aroma were the limiting factors for the first harvest (A); development of objectionable aroma was the limiting factor for the second (B) harvest.
0C
5C
10C
15C
20C
0C
5C
10C
15C
20C
5.0 Quality rating (1-5)
Quality rating (1-5)
5.0
4.0 3.0
2.0
4.0 3.0 2.0 B
A
1.0
1.0 0
1
2
3 4 Time (days)
5
6
7
0
1
2
3 4 Time (days)
5
6
7
Fig. 6. Quality curves for raspberries from the first (A) and second (B) harvests stored at different temperatures.
605
0C
5C
10C
15C
20C
Quality rating (1-5)
5.0 4.0 3.0 2.0 1.0 0
1
2
3 4 Time (days)
5
6
7
Fig. 7. Quality curves based on Q10 values for raspberries stored at different temperatures.
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