Optimizing Cherry Stem Quality
Sweet cherry is a crop of significant commercial value, especially for top quality fruit. Stem color is an important quality index, and brown stems often result in rejection of cherries by produce buyers. Consumers associate brown and shriveled stems with produce that is not fresh.
Cherry stems are extremely vulnerable to injury due to their physical structure. They are long and thin, with a large surface area to volume ratio. They can dry out very quickly, lack components to resist sunlight, and are easily damaged through handling.
Because cherry stems are delicate and susceptible to injury, measures must be taken to preserve freshness. Considerations taken to preserve stem quality also promote superior fruit quality. When cherry supply is high or when selling cherries to high-end markets, it is important to produce cherries with green stems to remain competitive.
Materials and Methods
Three experiments were performed to assess the effects of light, temperature, heat, and maturity on cherry stem quality. Experiments 1 and 2 evaluated the use of reflective tarps to retain cherry stem quality and experiment 3 evaluated the effects of temperature and maturity on cherry stem bruising.
Experiment 1. Sunlight, low relative humidity, and heat can affect cherry stems adversely, especially after harvest. We covered commercial bins of Lapins cherries with reflective Mylar tarps during harvest and transport to determine whether this material could reduce temperature and prevent stem deterioration. One side of the tarp was reflective aluminum in color and the other side was white. The tarp was placed so that the white side faced up.
Fruit and ambient air temperature, as well as ambient humidity and the humidity in the immediate space next to the cherry fruit, were monitored using HOBO brand data loggers.
Fruit samples were sealed in PD-941 bags (Cryovac), stored at 1 °C, and evaluated over a 4- to 6-week period. Stems were assessed for browning using a four point scale: 4 = 0 to 25% brown, 3 = 25% to 50% brown, 2 = 50% to 75% brown and 1 = 75% to 100% brown.
To determine water content, the stems were weighed and dried in a vacuum oven for 24 hours and then re-weighed. Membrane leakiness was calculated as a relative leakage rate (RLR) and was determined by dividing the absorbance at 280 nm before freezing by the absorbance at 280 nm after freeze-killing the stem tissue.
Fruit were evaluated for presence of decay and pitting. Statistical analyses were performed using SAS software, using the general linear method (GLM) procedure.
Experiment 2. To determine how tarps preserve quality and whether low relative humidity (RH), high temperature, or sunlight is most detrimental to stem quality, an experiment was conducted with Lambert and Lapins cherries inside a sealed chamber in the lab.
Some cherries were exposed to different humidities (35%, 55%, 75%, or 95% RH) in the dark at 20 °C for 8 hours. Other cherries were exposed to temperatures of 20 °C, 30 °C, or 40°C in the dark at 100% RH for 8 hours.
Metal halide lamps and fluorescent bulbs were then used to simulate the effect of sunlight. Light intensity was 541 µE/m2/s for 8 hours at 20 °C. Temperature and humidity were monitored by HOBO data loggers and a VWR brand digital hygrometer. The fruit samples were sealed inside modified atmosphere packaging (MAP) bags and stored at 1 °C for 48 hours and evaluated for browning and RLR in a similar manner as experiment 1.
Control cherries were freshly harvested fruit that was placed directly in MAP bags, stored at 1 °C, and evaluated for browning and RLR after 48 hours.
Experiment 3. Effects of temperature and maturity on stem bruising were studied on Van, Bing, and Lapins cherries. Fruit were evaluated at two stages of maturity (4 [red] and 6 [mahogany] on the CTIFL Color Chart). Cherries were harvested fresh each morning and held at 0 °C, 10 °C, 20 °C, and 30 °C. After 4 hours, a 50 g weight was dropped from a height of 8.7 cm onto the stems. Samples were then stored at 1 °C for 48 hours and evaluated for stem browning. The portion of the stem that was in direct contact with the bruising apparatus (wooden dowel) was scored for degree of browning.
Results and Discussion
Experiment 1. Reflective tarps significantly reduced stem browning, decay, and pitting in cherries following storage. Cherries that were left open to the ambient air scored 2 or less (> 50% browning) on the 4-point browning scale, through 6 weeks of storage. Cherries that were covered scored 3 or higher (< 50% browning). Cherry stems that score less than 2.5 would be unmarketable.
After 6 weeks, more than 20% of the fruit in the open bins showed decay, whereas less than 10% decay occurred in the covered fruit. After 2 weeks of storage, 60% of the covered fruit showed pitting while 80% of the fruit that was exposed to air showed some pitting.
Maintaining water content may be an important component in preserving green stems. More water (~10%) was retained in stems that were under tarps. Electrolyte leakage across plant cell membranes always correlated with stem browning (Pearson Correlation Coefficient = - 0.92). This would indicate that browning is related to at least some structural damage to the plant tissue.
The theory that injury or water loss is occurring in the orchard was supported by temperature-humidity data (Table 1). The tarps kept the fruit cooler, and the humidity within the covered bin air space was much higher than in the control. Data were recorded with HOBO temperature/humidity data loggers. These values represent averages recorded over 4 hours postharvest at the orchard site.
Table 1. Average temperatures and relative humidities for ambient air and air within the bins.
|Location||Temperature (°C)||Relative Humidity (%)|
|Open bin (fruit temperature)||23.6||53.0|
|Tarp bin (fruit temperature)||21.9||76.0|
Experiment 2. Light had no effect at all on stem browning; however, temperature and humidity did. As relative humidity decreased, and/or as the temperature increased, browning increased. For example, at 20°C, 35% RH, and darkness, the cherries were visually assessed at 3.5 on the browning scale compared with 3.9 for the control (data not shown).
Much larger differences were observed under combined temperature, humidity, and light (typical data shown in Table 2). At 30°C, 25% RH in the light, Lapins scored 3.22 on the browning scale. These conditions are not atypical of the conditions that would exist in the orchard on a warm sunny day.
On some occasions, cherry bins are kept overnight in large warehouses (with high airflow rates produced by cooling fans), awaiting delivery or shipping to other markets. The large fans have a tremendous drying effect on the fruit. Reflective tarps could be beneficial under these circumstances to reduce or eliminate evaporation of water from the fruit and create an immediate surrounding airspace with optimal humidity for preserving quality in cherry stems.
Table 2. The combined effects of temperature, humidity, and light on stem browning in two cultivars of sweet cherries.
(% of fresh weight)
|x Browning scale: 4 = 0 to 25% brown, 3 = 25% to 50% brown,|
2 = 50% to 75% brown, 1 = 75% to 100% brown.
yWC is the mean of the water content of 5 replicates (n = 25 stems). Means within a column followed by the same letter are not significantly different at P < 0.05.
z RLR is relative leakage rates of the cellular membranes (see methods).
Experiment 3. There is a strong effect of cultivar on the resistance of cherry stems to bruising. Van cherries always scored better than Bing or Lapins regardless of temperature or maturity. Van stems are short and compact, and stem browning is not considered to be a problem (Figure 1). Lapins stems showed the highest amount of bruising regardless of temperature. Lapins stems are long and thin and tend to grow in tightly held clusters, with little or no light penetration during growth. For this reason they may be more susceptible to physical and physiological injury. In addition, Lapins may be more vulnerable to high temperature and low humidity immediately postharvest.
Less browning occurred in more mature Lapins and Bing (no. 6 CTIFL) than in immature Lapins and Bing (no. 4 CTIFL). Cherries bruised more easily at 0 °C, 20 °C, and 30 °C than at 10 °C. Based on these results, the optimum harvest time for Lapins and Bing for stem quality was CTIFL color stage no. 6 (mahogany), and the best handling temperature was 10 °C.
Figure 1. Stem Browning in three cultivars of sweet cherries at different temperatures and maturities. Color stage 4 = red and color stage 6 = mahogany on the CTIFL Color Chart.
Reflective tarps are a definite asset in cherry production. They reduce stem browning significantly when used during harvest and transport. Tarps reduce stem browning by reducing field heat exposure and by increasing relative humidity within the bins.
The level of light used in this experiment was only about 20% of bright sunlight, so the results were inconclusive regarding the effects of sunlight directly on fruit and stem quality. All of these parameters have been cited as potentially damaging to highly susceptible fresh commodities, such as cherries.
With respect to handling, both maturity and temperature should be considered. Cherries that are harvested too early may be more susceptible to handling injury. The optimum temperature for least amount of stem damage was 10 °C.
This project was supported by Sun Fresh Co-operative, Okanagan-Similkimeen Co-operative, and the Agriculture and Agri-Food Canada Matching Investment Initiative. We would like to thank Sabina Stan for her technical assistance.
Joanne L. Schick and Peter M. A. Toivonen
Agriculture and Agri-Food Canada
Pacific Agri-Food Research Centre
CANADA V0H 1Z0
16th Annual Postharvest Conference, Yakima, WA
March 14-15, 2000