Location: Vegetable Crops Research2012 Annual Report
1a. Objectives (from AD-416):
Pressure bruise is a primary concern for all market classes of potato. Pressure bruise limits the duration of storage or reduces the grade of stored potatoes. Typical losses caused by pressure bruise are 20 to 30% for potatoes stored from September/October through May. Multiple factors contribute to pressure bruise including variety, hydration status of tubers at harvest, pile height, temperature and humidity in storage and storage duration. Pressure bruise is a complex process that involves physical damage to cells on the surface of the tuber and to discoloration of internal tissues. Previous work has shown that tubers with pressure bruise have flattened or compressed areas caused by the weight of the potato pile. These flattened areas may have crushed periderm and damaged underlying cells. Tissues beneath pressure flattened areas turned black to gray within 4 to 5 days after removal from storage. The black coloration resulted from increased levels of melanin within the tissue, a defect commonly associated with blackspot bruise. Our preliminary data suggest that formation of black bruises may be linked to an increased susceptibility of pressure-flattened areas to further tissue damage by impacts that occur during handling and washing after removal from storage. It is generally agreed that the severity of pressure flattening and the development of pressure bruise increase with decreased tuber hydration. Potato tubers prior to harvest are susceptible to water loss. Stolons decay on vines that have been desiccated with herbicide, senesced as a result of disease or reached maturity and tubers are no longer attached to the root system. Water loss occurs by evaporation from the periderm, and rates of water loss can be rapid before skins are fully suberized. Rehydration of tubers in soil possible only when free water is available. As skins mature suberization of the periderm dramatically restricts the potential for water uptake by tubers and may prevent appreciable rehydration late in the year. Data documenting the influence of pre- and postharvest practices on tuber hydration and associated cellular turgor pressure, however, are lacking. Data are needed to define relationships between tuber hydration status and pressure bruise incidence and methods are needed to quantify the effect of pre harvest and post harvest management practices on tuber hydration and turgor pressure. The long term goals of this research are to (1) understand physiological processes influencing tuber hydration status and turgor pressure and their relationships with shrink and sensitivity to pressure bruise and (2) to develop techniques to simulate, characterize and predict the incidence of pressure bruise. 1) Determine the influence of pre-harvest management practices on shrink and the occurrence of pressure flattening and pressure bruise. 2) Determine the influence of storage management practices on shrink and the occurrence of pressure flattening and pressure bruise. 3) Determine the sensitivity of pressure-flattened and non-flattened tubers to impact-generated internal bruising.
1b. Approach (from AD-416):
Objective 1: We will quantify the effect of pre-harvest water management practices on the development of pressure flattening and pressure bruise. Standard cultivars will be used for trials in CO and WI. Research plots from the time of vine kill to harvest will either be irrigated to maintain soil available water at >75%, irrigated by rainfall only which is the grower standard, or covered with tarps during rain events to prevent irrigation. For each treatment, tuber water content will be determined at the time tubers are placed into storage. The water required to rehydrate cut tubers to an osmotic potential typical for fully hydrated tubers will also be determined. These two measures of tuber water status will be compared to observed incidence of pressure flattening and bruise. Pressure bruise evaluations will be conducted in two ways. Small-scale evaluations will occur in custom build pressure bruise-chambers holding approximately 30 tubers. The effect of pre-harvest management will also be determined by burying bagged samples of each treatment within potato storages at WI and CO. These will be evaluated for incidence of flattening and bruise at the time that bins are unloaded. Objective 2: Post-harvest management will evaluate storage temperature and ventilation management for fresh market russets on a commercial scale. Three commercial-scale storage bins will be utilized to demonstrate the effects of temperature and ventilation rate on shrink, pressure flattening, and pressure bruise as well as energy costs associated with heating, cooling and ventilation for each treatment. For the reference bin, tubers will be preconditioned at 55°F and cooled to a holding temperature of 38°F at a rate of 0.5°F/d. Ventilation rates will be managed by maintaining a bottom-to-top of pile temperature differential of 1.5°F. Supplemental humidification will be used to maintain relative humidity at 95%. Management of a second bin will mirror the reference bin except that the final temperature will be 42°F and the humidity will be adjusted so that volumetric water content of the bins is equal. A third bin will be the same as the reference, except that ventilation will be increased to maintain a pile temperature differential of 0.5°F. Objective 3: The hypothesis to be tested is that handling tubers upon removal from storage leads to development of impact bruises under pressure flattened areas. Tubers will be obtained with severe pressure flattening and no pressure flattening directly from commercial storage and from research scale bins. Flattened and normal tubers from each storage facility will be subjected to a factorial treatment design. Treatment factors will include pulp temperature before handling (4 or 10°C), washed or unwashed, and blunt force trauma or no trauma on pressure flattened and non-flattened areas. Data collection will include shrink, pressure flattening, internal bruise, and storage conditions. Samples subjected to impact will be examined under the microscope to more precisely characterize the nature of cellular damage.
3. Progress Report:
This study evaluated dry matter change, penetration resistance as a surrogate to turgor pressure, shrink in storage, and pressure flattening and bruise upon removal of tubers from storage in response to irrigation following vine desiccation. A pressure bruise simulation was set-up to facilitate measurement at multiple storage times. Varieties evaluated include Russet Burbank, FL1879, Norkotah, and Goldrush. Post vinekill irrigation treatments included standard irrigation, no irrigation, and no irrigation or rain. Soil moisture data demonstrated a rapid rate of soil moisture change post vinekill. Evapotranspiration was minimal in September post vinekill due to a lack of canopy and green leaves, but the moisture loss due to evaporation and drainage resulted in drying of soils to levels below the recommended allowable depletion level (<75% of field capacity) within 5 days. The differences observed in soil moisture confirmed the importance of irrigation for maintaining soil moisture content during sunny dry weather after vine kill to minimize potential effect on potato tubers. Dry matter relates to tuber moisture content (an inverse relationship) and differed over time within each variety. The dry matter prior to harvest did not show differences between treatments across all varieties. The consistent decrease in dry matter content between vinekill and harvest suggests tubers take in water post vinekill. Dry matter content of tubers at storage removal was lowest for the cover treatment across all of the varieties except for Gold Rush per the cultivar x treatment interaction as assessed by analysis of variance. Dry matter decreased slightly in Russet Burbank and FL1879 from harvest until potatoes were removed from storage. In contrast, dry matter of Norkotah and Goldrush increased slightly from time of harvest until potatoes were removed from storage. Shrink in storage was greatest in the irrigated treatment across all varieties. Shrink was lowest in the Russet Burbank compared to the other varieties. The greater shrink in irrigated potatoes is likely due to the tubers having greater water content at harvest, which was lost through evaporation during storage. Bruise development in storage did not differ across treatments or across varieties. The simulation had differences from bulk storage, possibly due to a constant air velocity and a higher temperature. The way tubers are arranged in a small-scale storage container may produce different forces on them when compared to bulk storage, even though the simulation attempted to mimic commercial storage. Further research is necessary to determine if the simulation is appropriate for evaluating pressure flattening and bruise. This research also evaluated dry matter, shrink, penetration resistance, and pressure flattening and bruise incidence in relation to storage management. Varieties evaluated included Russet Burbank, FL1879, Russet Norkotah, and Goldrush. Storage treatments included a set point of 5.6°C with a 0.83°C top-to bottom of pile temperature differential (delta T), a set point of 3.3°C with 0.3°C delta T, and a set point of 3.3°C and 0.83°C delta T. Differences in flat spot rate and shrink were observed between varieties. The greatest penetration resistance was observed in the 5.6°C, 0.83° delta T across all varieties. Shrink and flattening rate was highest for Goldrush and FL1879 in the 5.6°C set point with a 0.83°C delta T storage treatment. Bruise rate was greatest for Goldrush in the 3.3°C set point with a 0.3°C delta T, though no statistical differences were detected in bruise between varieties and treatments. Ventilation management did not influence shrink. The inability to identify the impact of storage ventilation management on shrink may have been due to late establishment of ventilation treatments, 4 months after tubers were placed in storage. This research relates to Objective 4, Characterize molecular, physiological and environmental parameters that are determinants of potato quality, especially seed vigor and tuber processing quality.