Can raspberries be picked pink for fresh markets?

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.