Potato shrink results from loss of moisture and carbohydrate by tubers once placed in storage.
Conditions that promote the dehydration of potatoes in the field and after placement in storage can potentially contribute to losses due to shrink. Furthermore, conditions that promote increased respiration or stress can also promote shrink in harvested potatoes.
This harvest season posed a number of diverse challenges across the country with regard to placing high-quality potatoes into storage.
Early to mid-September was quite warm, especially in the Midwest, which led to elevated pulp temperatures at harvest and limitations in availability of cooling air.
Many potatoes were harvested in late September and early October when conditions were typically near freezing, resulting in pulp temperatures near 40 degrees at harvest, which can promote black spot bruise.
A substantial portion of the crop was damaged by field frost across the Northern regions.
COOL THOSE POTATOES
Potatoes harvested with pulp temperatures warmer than 60 degrees must be cooled to near 55 degrees to allow for preconditioning with minimal risk of disease development. This fall potatoes harvested in mid-September had pulp temperature of 68 degrees and took two weeks to cool to 55 degrees (Figure 1).
We ramp potatoes at 0.1 F/3 hr to limit refrigeration and fresh air. The goal is to maintain higher levels of humidity in the storage by using slower ramp.
At the Hancock Ag Research Station in Hancock, Wis., we have the benefit of refrigeration for maintaining cooling ramp even when no cooling air is available.
The largest struggle with harvesting tubers with high pulp temperatures is accessibility to cooling air for managing pile temperatures during warm conditions.
As a result, storages with no refrigeration must be more aggressive in cooling potatoes as opposed to gradual ramp, when cooling air is available.
In addition, the ventilation system in the Storage Research Facility at Hancock has CO2 sensors that forces the outside doors open when threshold of 2000 ppm is reached.
Potatoes harvested in early October had tuber pulp temperatures of 45 degrees, which increased potential for black spot bruise.
The potato pile had to be warmed to 55 degrees to allow for preconditioning of the chipping potatoes. As with cooling, we warmed the pile at 0.1 F/3 hr to minimize potential for free water on tuber surfaces by maintaining ?t of 1.5 degrees during the warming process.
OPTIMAL STORAGE
Pulp temperatures at harvest have had substantial affects on shrink in MegaChip. MegaChip was harvested and placed into storage on the same day, but had pulp temperatures of 58 and 68 degrees due to different times of harvest during a sunny day.
The increased pulp temperature resulted in 1 percent higher shrink during the first four months of storage (20 cwt loss in 2,000 cwt storage) (Figure 2).
Potatoes were sold for chipping due to concerns on chip quality in early February. This year, the same variety of potato was harvested on two different dates with 20 degrees differences in pulp temperatures, but we anticipate similar differences in shrink across the different pulp temperatures.
We will quantify pressure bruise and black spot bruise out of storage this year to determine effects of pulp temperatures on both.
Of course, identifying tuber pulp temperature at harvest for optimal storage is relatively easy compared to managing the entire potato harvest. Time constraints require harvesting potatoes that have less than ideal pulp temperatures.
The concern of delaying harvest until late fall is risk of field frost. We have some field frost in harvested potatoes in Wisconsin, as do many other states.
Tuber temperatures less than 28 degrees can result in freezing of tissue and ultimately tissue death.
The key to storing potatoes with field frost is to promote formation of closing layer at boundary of healthy and dead tissue. We promoted this in the storage at HARS by maintaining a pile temperature of 50 degrees for two weeks.
We set CO2 maximum threshold of 1500 ppm to ensure abundant fresh air is brought into the pile to promote development of the closing layer.
Finally, we established ?t of 1.5 F to facilitate dehydration of frosted tissues and damaged tubers.
Shrink is a multifaceted issue and the 2009 crop has posed a number of challenges for new storage season without even mentioning issues of pink rot, leak and other tuber diseases.
However, new tools are available that provide new opportunities for managing shrink in stored potatoes. Variable frequency drive fans that can maintain targeted ?t for managing free water and drying up damaged or infected tubers has proven extremely effective in commercial storage trials at HARS for storing less than ideal potatoes.
In addition, CO2 sensors allow for new approaches for managing air quality for promoting wound healing and improving fry color of chipping and processing potatoes.