Location: Vegetable Crops Research2011 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. Theses 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
We designed, built, and validated a ventilated container and tank system that allows us to induce pressure flattening on tubers enabling us to do a replicative study in smaller scale. The design relies on ventilated container of potatoes holding experimental samples maintained between layers of other potatoes. Pile pressure is applied through a water tank mounted on a crib holding tubers. Additional studies may alter the amount of water stored in the water tank to supply different pressures and therefore simulate different commercial potato pile heights or pressures. This may allow for improved development of cultivar specific storage pile height recommendations. Temperature, relative humidity, storage duration, and contact between potatoes can be incorporated into an experimental design. Four popular Colorado cultivars (Canela Russet, Centennial Russet, Rio Grande Russet, and Russet Norkotah) were selected for pressure bruise study using ventilated container and tank system. We subjected potato tubers with two kinds of moisture loss treatments in order to have tubers with different moisture levels. When all treatments and cultivars were averaged together, there was a linear increase in the number of bruised areas per tuber until approximately the fifth month of storage. However, the flattened area per tuber continued to show a linear rate of increase beyond the fifth month of measurement. Differences among four potato cultivars in development of pressure flattening and moisture loss were observed. An increase in pressure flattening within a cultivar that had been subjected to high rates of moisture loss was also observed. Tubers with irrigation after vine kill performed better when compared with tubers that received no water in a 3, 6, and 7 month storage study. Research results were presented at the International Potato Processing and Storage Convention (IPPSC2011) in Denver in June, 2011. This project is monitored though in person discussions, phone calls, and e-mail exchanges.