Sweet Potato Harvest, Curing and Storage

Published: September 4, 2020

Sweet potato [Ipomoea batatas (L.)] harvest is approaching, so this article will review practices and recommendations for sweet potato harvest, curing and storage for produce growers in Missouri. Sweet potato is a tropical, warm-season perennial crop originated in South America (known as batata or camote), but grown as an annual in the U.S. It is a member of the morning glory family and its commercial part is the enlarged storage roots. It is not a "yam". This term is used mainly for marketing purposes. The true "yam" (Dioscorea spp.) is a completely different plant species from a different plant classification family.

Southern states and the west coast produce most of the sweet potato in the U.S., but there were 147,500 acres planted in 2019, up from 115,700 acres in 2013. This corresponded to a total production of almost 32 million cwt (100lb) valued at $588 million (USDA-NASS. 2020, Vegetables 2019 Summary). Demand for fresh and processed sweet potatoes grown in the U.S. extends throughout the U.S. up to Canada and Europe. Primary supply is fresh market, but processed sweet potato has increased recently. Fresh market demands attractive medium size roots of uniform shape that are free from blemishes (U.S. No.1). This class brings the highest price; however, there has been a recent increase of bagged small storage roots sold as fingerlings or nuggets in the fresh market.

When to harvest?

Sweet potato storage roots grow as long as conditions (temperature and moisture) are favorable, so they never really "mature" or "ripen". Therefore, the highest proportion of U.S. No.1 size (table 1) usually determines time of harvest because of the fresh market value. Small growers with direct sale market, however, can sell all sizes as fresh sweet potato, so they may delay harvest to get the highest yield, but when soil temperatures fall below 60°F, sweet potato stops growing. Furthermore, sweet potatoes cannot tolerate freezing, so harvest them before the first freeze for best quality.

Table 1 Excerpt from the U.S. Standards for Grades of Sweet Potatoes.

Pre-harvest

Sweet potato storage roots are prone to skinning that render unappealing roots for the consumer (figure 1). In addition, excessive skinning and bruising will shorten the roots shelf life since may cause roots to spoil or shrivel in storage. Therefore, growers take every effort to minimize skinning and bruising. The first cultural practice seven or more days prior to harvest is to remove vines and foliage, which promotes skin set and reduce skinning at harvest. Small growers use grass scythe, hedge trimmer or weed eater to remove the foliage and a disc coulter to cut vines. Large growers use a raised row shaped flail mower or a vine snatcher with coulters to cut the vines. Harvesting sweet potatoes when the soil is too dry increases skinning incidence, so harvest when soil separated easily without large clods. Good soil organic matter may help loosen the soil and reduce skinning.

Figure 1 Sweet potato storage roots with sunken leathery scars from skinning because of inappropriate healing conditions.

Harvest

There are mainly two ways to dig the storage roots in commercial operations. With a chain digger that picks the storage roots with soil into a moving chain where the soil falls through while the storage roots continue and fall behind on top of the soil (figure 2). With a disc plow that turns the soil exposing the storage roots. Then, a crew picks and grades the storage roots by hand and put them in boxes accordingly. The crew uses gloves to minimize skinning. Large growers use more mechanized harvesters, in which the chain digger takes the storage roots up to a platform where the crew select the roots by size and put them in boxes. Another type of harvester includes rollers spaced according to the standard sizes to separate the storage roots and drop them into large boxes or bins.

Figure 2 Chain digger dropping sweetpotato storage roots after sieving the soil through the chain.

Curing

Postharvest conditioning is necessary to enhance fast healing and reduce losses to decay and moisture loss because of injuries as well as to improve culinary attributes (sweetness, flavors, etc.). Cure sweet potatoes immediately after harvesting by placing them in a room at 85°F and 85% to 90% relative humidity for 5 to 7 days. It is important that the curing/storage rooms have fans for uniform distribution of the warm/humid air and air vents to maintain appropriate oxygen levels. Curing helps to speed up the healing of wounds that occur during harvest, preventing shriveling and reducing the risk of rot during storage. Curing also makes the sweet potato more palatable by converting starches to sugars and improving aroma and texture. The aesthetic appearance of storage roots depends on how fast the roots are put in curing conditions to generate a new skin similar to the original (figure 3). A delay in curing may cause the wound to dry out leaving unappealing scars.

Figure 3 Wound healing and generation of new skin: sunken dry wound scars (left) and new skin generated when curing immediately after harvest (right).

Curing rooms are usually the same storage rooms, but at 85°F for the curing. The size depend on the volume and length of the harvest period. The rooms should be large enough to hold the volume of 3-4 days of harvest and then close it to complete the curing period. After curing, stop heating to allow room and storage roots to cool down, but never below 58-60°F because sweetpotato is chilling sensitive and physiological disorders such as "Hard Core" may occur.

Storage

Under the right storage conditions (58-60°F, 80%-90% humidity), properly cured sweetpotatoes can be stored for over six months. Roots should be kept in a dark, cool place after curing. If roots are stored above 60°F for extended periods, sprouting may start. Some growers reduce the relative humidity to promote skin set and toughness before washing and packing for delivery to markets. Reducing the humidity too early promotes moisture (weight) loss.

Yield

Variety, location and management influence sweet potato yield and proportion of root sizes. With acceptable management practices, yield may range from 150 to 350 bushels (50lb) per acre of U.S. No.1 roots, the preferred size for fresh market (table 1). A 50% to 60% of the harvest should correspond to U.S.No.1, so total harvest could reach 300 to 700 bushels per acre. In general, growers separate and classify the rest into small roots as canners (diameter between 1-1/2 and 2 inches) and large ones as jumbos (diameter above 3-1/2 inches). Under exceptionally good conditions and irrigation, total yields of 800 to 1,000 bushels per acre are possible in southern states (Figure 4).

Figure 4 Harvest of sweet potato 'O'Henry' (white flesh); total yield over 800 bushel per acre.