By Penelope Perkins-Veazie, Plants for Human Health
Institute, North Carolina State University and Gina Fernandez, Department of
Horticultural Science NC State University
Raspberries are the most perishable of the temperate fruit
crops. If you set them on your kitchen counter, you can watch the mold grow
within 24 hours. This fruit’s delicate nature is due to its fragile structure,
where drupelets are connected together by only a few trichomes (fruit hairs),
no cuticle is present, and gray mold (Botrytis cinerea) can set up spores during
bloom and produce fuzzy gray fruit as the berries are ripening.
While raspberry fruit mostly produce ethylene from the fruit
calyx (the part of the fruit that remains on the plant), there is a small
amount of ethylene, the fruit ripening hormone, present in many varieties. This
actually can pose an advantage for fruit growers producing raspberries in the
warmer parts of the season. Fruit at the pink or even pink-yellow stage will often
detach from the calyx with minimal tugging.
We initiated a small test in 2010 to investigate the ability
of raspberries to attain full ripeness if harvested unripe. These fruit were
harvested in August and September from plants grown in high tunnels at the
Upper Mountain Research Station, Laurel Springs, NC. Temperatures within the
tunnels were above 85°F for approximately four hours per day. Unripe and ripe
raspberries were picked at weekly intervals for the tests, over a three-week
period, and one to two clamshells per cultivar and ripeness were used for the
study.
Raspberries were picked into halfpint clamshells and transported at 5°C
in refrigerated ice chests (Kooltron) to Kannapolis, and held at 39 °F for six
days. Subsamples were removed at day 0 to check firmness, color, sugars, and
acidity. Subjective ratings were taken after storage by checking each berry for
softness, leak, and mold. The overall color of the fruit within the clamshell
was determined subjectively as 0 (light red) to 3 (dark purple red). Percent
saleable fruit was determined by using the relationship of color to percent
(where rating of 0 was 100% saleable to 3 was 0% saleable).
Surprisingly, even fruit picked considerably unripe
(yellow-pink) achieved full color, soluble solids content, acidity, and flavor
(tasted at random) after six days storage (Table 1). The biggest disadvantage
of picking unripe berries was a depression in berry size of 4% to 20%,
depending on variety and relative ripeness at harvest. What was clear from ratings
was that fruit picked pink was much firmer and less leaky than berries picked
at the normal commercial fresh market ripe stage (Table 2). The amount of moldy
berries was slight (less than 10%), due to a rigorous fungicide spray program
and the protective effect of the tunnels from moisture and wind.
We hoped that berries varieties known to turn dark red after
storage, such as Joan J, would be less fully red if picked pink prior to
storage. In fact, we found that color could not be slowed enough, with fruit
reaching full color as soon as 2 days at 39° F after harvest. Figure 1
illustrates the change in color of ‘Culivar’ in ripe and unripe berries at 0, 5
and 10 days after harvest.
Flavonoids are compounds are compounds that are associated with health benefits,
and higher levels in fruit are good. Flavonoids in raspberry include the
anthocyanins that give raspberries much of their red color, along with other
colorless phenolic compounds. In raspberries picked before full ripeness, flavonoid
content was decreased by 5-15% after storage. The slight loss in flavonoids in
the less ripe fruit was madeup in the better appearance and firmness of the
raspberries.
Harvesting raspberries at the pink stage is possible. We did
not observe significant problems with composition and flavor, and early picking
improves the number of marketable fruit. However, harvesting less ripe fruit is
likely dependent on air temperature (detaching raspberries is difficult in cool
weather), and will require more attention and training of pickers during
harvest than pulling off fully ripe berries. Although we did not determine
optimal temperatures for picking unripe berries in this study, the ability of
raspberries to fully color up and soften may depend greatly on having a production
environment where temperatures are at 75° F for at least four hours.
Table 1.
Comparison of raspberry fruit harvested unripe (pink) or ripe (red)
|
before and after storage at 4C, averaged for Joan
J, Nantahala, Caroline
|
Variant
|
Days
|
Unripe
|
Ripe
|
Mean
|
Total phenolics
|
0
|
2858
|
2866
|
2862
|
(mg/kg gallic acid equivalents)
|
6
|
3090
|
3144
|
3117*
|
Total anthocyanin
|
0
|
508
|
530
|
519
|
(mg/kg cyan-3-glucoside equivalents)
|
6
|
510
|
589
|
550*
|
FRAP (Ferric reducing antioxidant potential)
|
0
|
28.6
|
26.8
|
27.7
|
(umol/g trolox equivalents)
|
6
|
28.5
|
28.8
|
28.6
|
Soluble solids content (%)
|
0
|
11.2
|
10.9
|
11.1
|
|
6
|
11.9
|
11.7
|
11.8
|
Titratable acidity (%)
|
0
|
1.23
|
1.26
|
1.24
|
|
6
|
1.05*
|
1.19*
|
1.12*
|
Means separated within column and days 0,6 using
student's t-test, P<5%.
|
Table 2.
Comparison of raspberries picked unripe (pink) or ripe (red) after
storage at 4C for 6 days
|
|
%Leaky berries
|
%Soft
|
|
%Saleable
|
color
|
Variety
|
Unripe
|
Ripe
|
Unripe
|
Ripe
|
Unripe
|
Ripe
|
Autumn
Britten
|
1a
|
42b
|
48ab
|
93a
|
43b
|
7b
|
Caroline
|
8a
|
19a
|
61a
|
86ab
|
47b
|
33ab
|
Nantahala
|
2a
|
37b
|
38b
|
75b
|
67a
|
47a
|
Mean
|
3a
|
33*
|
49
|
82*
|
52
|
29*
|
Means separated within column among cultivars, by letter
(P<5%) using REGWQ.
|
The original version of this article (including photos) appears in Autumn 2011 issue of "The Bramble." To access this article, you must be a member of the North American Raspberry and Blackberry Association. To become a member to: http://www.raspberryblackberry.com/index.cfm and click on Membership in the left purple panel.
