Evaluation of Postharvest Quality of Three Commercial Blueberry (Vaccinium corymbosum) Cultivars Stored in Two Different Types of Clamshells

Abstract

Three commercial blueberry cultivars, ‘Duke’, ‘Bluecrop’, and ‘Elliott’ were hand-harvested and stored at 0–4°C and in room temperatures using two types of clamshells: commercial or moisture efficient. The changes in post-harvest berry quality attributes, such as fruit weight, firmness, and soluble solids, were analyzed during storage. As expected, blueberries stored at 0–4°C had a longer shelf-life and better quality compared to those stored at room temperature. The blueberries stored in the moisture efficient clamshells had significantly less weight loss for all three blueberry cultivars and longer shelf-life than the commercial clamshells. Thus, the moisture efficient clamshell is recommended as a better storage/packaging method for prolonging fresh blueberry's shelf-life and maintaining good berry quality.

Keywords:

• moisture efficient
• opening ratio
• firmness
• shelf-life

INTRODUCTION

Blueberries are a rapidly growing global commodity with almost half its production intended for the fresh markets (Strik, 2005). With a steady increase in demand for fresh blueberries worldwide, blueberries with good fresh-keeping quality are desired to expand domestic and international markets. Good quality attributes, such as color, size, soluble solids, and firmness, are very important especially under various cold storage conditions.

The shelf-life of small fruits, such as blueberry, is limited by physiological senescence and deterioration caused by fungal disease during storage (Ballinger and Kushman, 1970). Low temperatures as close to 0°C with minimal evaporative losses were reported to be important factors in prolonging shelf-life of perishable fruits, such as blueberries (Richardson, 1988; Robinson et al., 1975). In addition, the humidity inside the storage room directly affects berry quality. If the humidity is too low, wilting or shriveling of berries may occur (Hardenburg et al., 1986). However, humidity greater than 95% within a closed container may encourage mold development.

Several studies have investigated the effect of packaging materials on the post-harvest quality of berry crops (Dekazos and Smit, 1976; Miller et al., 1984; Miller et al., 1993). Dekazos and Smit (1976) reported that good quality berries and appropriate packaging are necessary to maximize the benefits of refrigeration in preserving fruit quality. They also reported that refrigeration of blueberries at 1°C and 95% relative humidity seems to be optimal for extending storage life after harvest. In a study conducted by Miller et al. (1988), they found that fruit weight was reduced significantly in rabbiteye blueberry, which was packed in molded pulp fiberboard baskets as compared to experimental styrofoam packages, although there were no cultivar differences in fruit firmness or weight loss due to packaging.

Commercial clamshells used for storing berries and other fresh fruit have opening ratios of 3–10% according to a market survey (personal communication with Dr. Jinhe Bai of USDA). Cherries and strawberries packed in such clamshells lose marketability quickly. A moisture efficient clamshell with a 0.24% opening ratio was found to decrease water loss of cherries and strawberries significantly and extended shelf-life by two-fold at 1, 10, and 21°C (personal communication with Dr. Jinhe Bai of USDA) in comparison to the commercial clamshells.

In this article, we studied the effect of the commercial and moisture efficient clamshells on the post-harvest quality of three blueberry cultivars at two storage temperatures. The post-harvest berry qualities evaluated included changes in fresh berry weight, firmness, and soluble solids.

MATERIALS AND METHODS

Three commercial blueberry cultivars, ‘Duke’, ‘Bluecrop’, and ‘Elliott’ were harvested at maturity in 2007 from a research plot at the North Willamette Research and Extension Center, Aurora, OR. After pre-cooling at 1°C overnight, defect-free fruits were randomly packed in one-pound clamshells and weighed to 454 g. Two types of clamshells with different opening ratios were used: a commercial clamshell with a 3.39% opening ratio and a moisture efficient clamshell with a 0.12% opening ratio. Fruits in the clamshells were stored at 1 or 21°C room temperature until they were deemed unmarketable when at least one berry was moldy in a clamshell. The following quality parameters were measured every 3–4 days during storage: fruit weight, berry size, firmness, and soluble solids. The container berry weight was determined immediately after being taken out of cold storage and/or at room temperature. Weight difference for each cultivar was calculated by subtracting the weight after storage from the initial weight. Berry firmness and berry size were measured with 25 berries using the FirmTech II firmness tester (BioWorks Inc., Stillworks, OK, USA). Soluble solids were determined by using an automatic pocket Refractometer Pal-1 (Atago Co., Japan; distributed by Novatech-USA).

The experiment was conducted as a completely randomized design with five replications. The treatment factors included two clamshell types, two storage temperatures, and three cultivars and were analyzed as a complete factorial experiment. All data analyses were carried out with SAS 9.1 (SAS Institute, Gary, NC, USA). To determine the overall effects of clamshell and temperature on the measured parameters for each of the three blueberry cultivars, the analysis of variance was performed using the general linear model (GLM) procedure. Simple t-test was used to determine the mean differences between the post-harvest parameters used in the study.

RESULTS

There were significant interactions among cultivar, clamshell type, and storage temperature for mean weight, weight differences, firmness, and soluble solids (Table 1).

TABLE 1 Analysis of Variance of Weight Change, Berry Size, Firmness, and Soluble Solids Affected by Cultivars, Storage Temperatures, and Clamshell Types in a Complete Factorial Design

Effects	DF	Mean weight	Weight difference	Berry weight	Berry size	Firmness	Soluble solids
Cultivar	2	ns	*	*	ns	*	*
Clamshell	1	*	*	*	*	*	*
Temperature	1	*	*	ns	ns	*	ns
Cultivar * Clamshell	2	ns	ns	*	*	ns	ns
Cultivar * Temperature	2	*	*	*	*	ns	ns
Clamshell * Temperature	1	*	*	ns	ns	ns	ns
Cultivar * Clamshell * Temperature	2	*	*	ns	ns	*	*
*Significant at p < 0.05; ns: not significant.

No interactions were observed between clamshell type and storage temperature for any of the parameters measured except weight difference. While individual berry weight was not affected by clamshell type in ‘Elliott’, it was significantly lower for both ‘Duke’ and ‘Bluecrop’ stored in commercial clamshells (Table 2). There was a significant interaction between berry size, cultivar, and storage conditions with ‘Duke’ being smaller in the commercial clamshell and ‘Bluecrop’ being smaller under room temperature storage (Table 2). Soluble solids were significantly lower in ‘Bluecrop’ stored in moisture efficient clamshell and in ‘Duke’ and ‘Bluecrop’ stored at room temperature. Fruit firmness differences were observed after 3 weeks of storage for ‘Duke’ and ‘Bluecrop’, but not for ‘Elliott’ under cold temperatures (Fig. 1).

FIGURE 1 Berry firmness of ‘Duke, ‘Bluecrop’, and ‘Elliott’ blueberries affected by commercial and moisture efficient clamshells over every 3–5 days sampling. Graphs in the left column indicate cold temperature storage, while graphs in the right column indicate room temperature storage. Error bars indicate the standard error of means (color figure available online).

There were significant weight differences for all three cultivars during storage, with the moisture efficient clamshell retaining the most weight over time (Fig. 2).

FIGURE 2 Weight difference of ‘Duke, ‘Bluecrop’, and ‘Elliott’ blueberries affected by commercial and moisture efficient clamshells over every 3–5 days sampling. Graphs in the left column indicate cold temperature storage, while graphs in the right column indicate room temperature storage. Error bars indicate the standard error of means (color figure available online).

TABLE 2 Mean Fruit Weight, Berry Size, and Soluble Solids of ‘Duke’, ‘Bluecrop’, and ‘Elliott’ Affected by Commercial vs. Moisture Efficient Clamshells Stored at cold and Root Temperatures

	Mean weight (g/clamshell)	Berry weight (g/ea.)	Berry size (mm)	Soluble solids (%)
Cultivar * Clamshell-type
Duke
Moisture efficient ^z	429.59 a	1.39 a	17.37 a	16.22 a
Commercial	418.30 b	1.32 b	13.86 b	16.50 a
Bluecrop
Moisture efficient	417.67 a	1.75 a	18.62 a	14.70 a
Commercial	411.04 b	1.63 b	16.82 a	15.26 b
Elliott
Moisture efficient	441.07 a	1.63 a	15.28 a	14.20 a
Commercial	435.89 b	1.65 a	15.23 a	14.70 a
Cultivar * Temperature
Duke
Cold ^z	420.76 a	1.46 a	15.82 a	16.49 a
Room	431.90 b	1.04 b	15.00 a	15.99 b
Bluecrop
Cold	406.63 a	1.78 a	18.77 a	15.18 a
Room	433.68 b	1.43 b	14.57 b	14.38 b
Elliott
Cold	437.63 a	1.64 a	15.14 a	14.29 a
Room	440.17 b	1.65 a	15.53 a	14.85 a
^zSame lower case letters between clamshell types and storage temperatures with a column are not significantly different determined by simple t-test (p < 0.05).

DISCUSSION

Weight Difference

As expected, there was a significant weight difference or moisture loss in all cultivars when stored under both cold and room temperatures using a commercial clamshell. The mean fruit weight was always higher for all the cultivars in the moisture efficient clamshell as reflected by significantly lower weight differentials. The fewer holes and covering of the moisture efficient clamshell apparently reduced the loss of water from fruits and, thus, maintained the freshness of the blueberries longer. This observation agreed with findings that the shriveling of blueberries was a result of fruit moisture loss (Dekazos and Smit, 1976). For all three cultivars, ‘Duke’, ‘Bluecrop’, and ‘Elliott’, the mean fruit weight in clamshells was higher at room than cold temperature, whereas fruit weight losses were also higher at room than at cold temperature. This discrepancy can be attributed to longer sampling times resulting in less berries remaining in the clamshells stored under cold temperatures.

It was previously reported that machine harvested berries held at 22°C had a weight loss of 2.6% per day as compared to 0.2% per day at 1°C during the first 2 days of storage (Nunez-Barrios et al., 2005). It was also shown that the berries had a longer fresh life at the cold temperature than room temperature. In our study, the first visible mold appeared approximately 6–11 days at room temperature as compared to more than 2 weeks under low temperatures for all three cultivars. As reported in many other studies, low temperature is the most important factor in prolonging shelf-life of blueberries (Robinson et al., 1975; Richardson, 1988). In addition, low temperature is recommended not only because it retards undesirable metabolic changes and respiratory heat production, but it also reduces moisture loss and spoilage caused by invasion of fungi and other rot causing organisms (Ben-Yehoshua, 1985; Hardenburg et al., 1986; Richardson, 1988).

Berry Weight and Size, Firmness, and Soluble Solids

The changes in berry weight in ‘Duke’ and ‘Bluecrop’ compared to 'Elliott” can be attributed to greater moisture loss. In the case of berry firmness, it is interesting to note that after 3 weeks of cold storage, berry firmness tended to increase in ‘Duke’ and ‘Bluecrop’ stored in both types of clamshells. The increase in berry firmness may be partially due to berry moisture loss, although it was reported that cell wall thickening and change of firmness at different temperatures are related to an increase in fruit enzymatic activity and higher respiration rates (Trinchero et al., 1999). A study also showed that firmness of blueberry declines rapidly at temperatures above 20°C (NeSmith et al., 2005). A small but significant reduction of soluble solids in ‘Duke’ and ‘Bluecrop’ at room temperature may be a reflection of the downshifts in hemicellulose molecular size, which is the main change observed during the ripening process in ‘Bluetta’ blueberries (Proctor and Peng, 1989). Because blueberries are non-climacteric fruits, the post-harvest maturation process does not have an autocatalytic ethylene rise that would have had an effect on soluble solids levels during longer storage time (Ban et al., 2007). As shown in some studies, the loss of blueberry quality due to softening and decay increases when fruit are stored at temperatures above 1°C, and when the storage temperature rises above 20°C, 15–20% of the berries will have unacceptable quality within 1 to 5 days (Ballinger et al., 1978; Cappellini et al., 1982). Our results agree with the effectiveness of reduced temperatures on preserving berry quality and showed that by reducing the percentage of openings in the clamshells, higher berry weights can be retained to achieve better storage quality of fresh blueberries.

ACKNOWLEDGMENTS

We appreciate the thoughtful discussions with Dr. Jinhe Bai of USDA-ARS before conducting this experiment. Partial funding was provided by a grant from the Oregon Blueberry Commission.