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Research Summary
November
26, 1997
Manuel Jimenez, Elata Trejo and Marita Cantwell
University of California Cooperative Extension, Tulare County and
Department of Vegetable Crops, University of California, Davis
in cooperation with growers and
Joe Santellano, Sunnyside Packing, Selma
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Slide 1. Shows the translucent darkened areas on the beans cv. Jade obtained at Sunnyside Packing on May 27. Beans were held 2 days at 7.5°C (45°F) before the photo was taken. Slide 2. Typical injury symptoms on beans cv. Jade after rough handling (finger pressure to bruise and scraping against wood and plastic). Photos were taken immediately after injury. Slide 3. Examples of beans with injuries as removed from the box brought back to the Mann lab. Beans were held 2 days at 7.5° C (45° F) before the photo was taken. Slide 4. Three maturities of green beans: Slightly immature, mature and over mature. The slightly immature and mature can be marketed; the over mature beans should not be marketed. Slide 5. Visualization of shrivel symptoms on green beans: no shrivel, slight shrivel and moderate shrivel (top). Moderate shrivel receives a score of 3 on the 1 to 5 rating scale. Slide 6. Typical chilling injury symptoms on green beans (cv. Xera) stored at 0°C (32°F). A general darkening of the beans if the most common symptom. Photo was taken immediately after removal from storage. Slide
7. Typical chilling injury symptoms on green beans (cv. Xera) stored
at 5°C (41°F). Rusty brown discolored spots or lesions are the
most common symptom. Photo was taken immediately after removal from
storage. |
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Objectives |
1) Document development of chilling
injury symptoms in green beans
2) Document weight loss changes and effect on green bean quality
3) Evaluate the postharvest quality of different varieties
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Introduction |
These tests were initiated because
of a postharvest problem observed at Sunnyside in May 1997. There were large
translucent areas on the beans which later decayed readily. Samples of recently
harvested beans were brought back to the Mann Lab 1 30-lb box (harvested 1
day and stored at 7.5°C). We were unable to reproduce this problem during storage,
but very rough manual handling and bruising against hard sides of plastic
or wooden containers did appear to cause some translucent damaged areas similar
to those observed at Sunnyside. Because weight loss, cooling and storage temperatures
were discussed at the time of the damage problem, some tests were done at
the Mann Lab to document postharvest quality changes of beans stored under
a wide range of conditions.
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Experimental |
Three tests were conducted.
Test 1: Continuous storage at several temperatures
The cultivar "Jade" (harvested
from older plants, held overnight at 7.5°C) was stored at 0, 2.5, 5, 7.5,
10 and 12.5°C (32, 36, 41, 45, 50 and 55°F) to determine if it can
reproduce the translucent areas. Also obtained from another grower (young
plants, first harvest) was a box of another cultivar, Xera (excellent quality,
picked up in the field). These were set up at the same temperatures. Three
replicates of 20 selected beans were used at each temperature for each evaluation.
Beans were placed on paper plates, with a paper towel on top and stored inside
small plastic bags overlapped at the ends (therefore moisture loss was reduced,
but the atmosphere was not modified). Beans were evaluated for overall visual
quality, decay, and discolored or translucent areas.
Test 2: Time to cause visible chilling symptoms:
Beans of cv. Zera were harvested on May 27 about 5 p.m. into waxed 30-lb cartons; transported in an air-conditioned car to Davis and held overnight in the 5°C (41°F) room. Samples (defects removed) of 20 medium size beans were stored on paper plates inside plastic bags and placed at 0, 2.5, 5, and 7.5°C (32, 36, 41, and 45°F) for 0, 3.5, 7, 10.5 and 14 days after which they were transferred to 15°C (59°F) for 12 hours and then evaluated for overall visual quality, decay, and discolored or translucent areas.
Test 3: Weight loss and appearance
Beans cv. Xera were placed on
paper plates in open white plastic trays and placed on racks in controlled
temperature rooms at 0, 5, 10, 15, 20 and 25°C. Beans were weighed daily
for several days and evaluated for appearance of shrivel or limpness.
Evaluations
Overall visual quality was scored
on a 9 to 1 scale, where 9=excellent, 7=good, 5=fair, 3=poor and 1=unuseable.
A score of 6 indicates the limit for wholesale marketability. Chilling injury
was evaluated as the development of various specific symptoms including internal
and external discoloration and pitting. These symptoms and decay were all
scored on a scale of 1 to 5, where 1=none, 2=slight, 3=moderate, 4=moderately
severe, and 5=severe. Shrivel and limpness were evaluated on a scale of 1
to 5, where 1=none, 2=slight, 3=moderate, 4=moderately severe, 5=severe. Initial
and final weights were recorded and % weight loss was calculated. Data are
the means from 20 beans per evaluation per treatment + standard deviation.
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Results |
We were unable to reproduce the symptoms observed on the beans obtained at Sunnyside Packing (Slide 1). Rough physical handling in the laboratory caused translucent areas and also a lot of other visible injury (Slide 2). However, it did not exactly duplicate the damage observed at the commercial storage room. During the storage experiments, none of the beans of cv. Jade or cv. Xera showed similar translucency problems observed in the samples at the Sunnyside warehouse.
The beans of cv. Jade were produced by Hmong growers and tended to be oven-mature and were generally of fair-poor quality (decay from tips touching soil, physical damage, tough, etc). They also had considerable physical injury at the time of arrival at Sunnyside (Slide 3).
Test 1: Continuous storage at several temperatures
Because postharvest quality changes
can be related to maturity at harvest, beans used in this test were slightly
immature or mature (Slide 4). Beans of cv.
Jade tended to be more mature than those of cv. Xera. The beans were evaluated
immediately after removing from storage (Figure 1).
Very good quality was maintained for about 8 days at all temperatures except
0°C (32°F). Beans of cv. Xera lost visual quality more rapidly than
those of cv. Jade, primarily due to low temperature induced discoloration
(Slide 5). A moderate level of generalized discoloration
developed by 7 days at 0°C (Slide 6).
Discoloration at 5°C appeared more as discreet rusty-brown lesions (Slide
7). Less discoloration was observed in beans stored at 2.5°C (36°F)
than at 0°C or 5°C. No discoloration was observed on beans stored
at 10 or 12.5°C (50 or 55°F). Some decay was observed in beans stored
at 7.5, 10 or 12.5°C (45, 50 or 55°F).
Test 2: Time to cause visible chilling symptoms
For this test beans of cv. Xera
were evaluated after the storage period plus an additional 12 hours at 15°C
(59°F). This transfer period was used to aggravate chill-induced injuries
and present a more typical commercial handling scenario. The beans were stored
on plates inside plastic bags as in Test 1. During
the first week of storage, beans stored at 7.5°C (45°F) showed more
quality loss than beans stored at 0, 2.5 or 5°C (32, 36, or 41°F)
(Figure 2). Between day 10 and 14, however, chill-induced
damage resulted in a great loss in quality in beans stored at the three lower
temperatures. Most of the quality loss at the low temperatures was due to
discoloration. The transfer period accentuated quality loss at 7.5°C (45°F)
but did not seem to aggravate injury observed at the lower temperatures. Perhaps
the 12 hour period was too short to permit visual development of all chill-induced
injury.
Test 3: Weight loss (transpiration) and appearance
Weight loss and visible signs
of dehydration are practical concerns during postharvest handling of green
beans. For this test the beans were stored on plates, and on open trays at
the indicated temperatures, representing a worse case scenario for water loss
at each temperature. The average relative humidity at each temperature (measured
twice) was 93, 93, 78, 74, 63, and 47% at 0, 5, 10, 15, 20 and 25°C (32,
41, 50, 59, 68 and 77°F), respectively.
Weight loss is cumulative and generally linear with time in storage (Figure 3). The increase in scores for limpness and shrivel generally followed the weight loss of the beans. With storage at this broad range of temperatures, quality loss of unprotected beans was closely related to water loss. At 0 and 5°C (32 and 41°F) with 93% RH limpness and shrivel were not observed until day 6.
Scores of 3 for shrivel and limpness
indicate beans that are still marketable. The number of days required to reach
a shrivel score of 3 were estimated to be >12, 10, 8, 6, 3 and <2 for 0,
5, 10, 15, 20 and 25°C (32, 41, 50, 59, 68 and 77°F), respectively.
A score of 3 (moderate) for limpness and shrivel represented on average a
weight loss of about 10 and 15%, respectively (Figure 4).
A visual quality score of 6 (minimum score for marketability) also corresponded
to a total weight loss of about 15%.
Figure 1. Quality changes in green beans of cv. Xera and cv. Jade stored at 6 different temperatures for 14 days. Beans were evaluated immediately after removal from storage. Data are the averages of 3 reps of 20 beans each.
Figure 2. Quality changes in green beans cv. Xera stored 14 days at
4 temperatures. Beans were evaluated after transferring to 15°C (59°F)
for 12 hours. Data are means of 3 reps of 20 beans each.
Figure 3. Changes in visual quality, weight loss, limpness and shrivel
in green beans cv. Xera stored at 6 different temperatures. Data are means
of 3 reps of 20 beans each.
Figure 4. The relationships between visual quality, limpness and shrivel
scores and the % weight loss of green beans cv. Xera.
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