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. 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 is the final report for the project 3655-21000-049-21S terminating in August 2011. We designed, built, and validated small-scale pressure bruise-chambers holding approximately 30 tubers each. Multiple chambers have been fabricated and this allows for replication of experimental treatments and varieties. We have demonstrated time-dependent pressure flattening and pressure bruise development using these chambers. We implemented post vinekill irrigation treatments to vary tuber hydration and maturity at harvest. Post vinekill irrigation treatments included standard irrigation, no irrigation, and no irrigation or rain. Tuber penetration resistance increased while dry matter content decreased post vinekill. The data suggest this to be part of the tuber maturation process. Soils with inadequate soil moisture content could prevent water uptake and reduce tuber hydration. Tubers in drier soils had lower penetration resistance and lower dry matter content at harvest. Tubers from these plots were placed into storage for season-long evaluations of pressure flattening and pressure bruise development. Tubers in the pressure bruise chambers showed an increase in shrink and a decrease in penetration resistance over time. Three research scale bulk storage bins containing 100 tons of Russet Norkotah each were evaluated; with bagged tuber samples of other varieties from field treatments buried at 2, 4, and 6 m pile heights. The following three ventilation rates and temperatures were used to assess the affect of airflow rate, temperature and top-to-bottom pile temperature differential (deltaT) on pressure flattening and pressure bruise development: a set point of 5.6°C with a deltaT of 0.83°C, a set point of 3.3°C with 0.3° C deltaT, and a set point of 3.3°C and 0.83°C deltaT. The greatest penetration resistance was observed in tubers from the 5.6°C, 0.83° deltaT across all varieties. Shrink and flattening rate were highest for two of four varieties in the 5.6°C set point with a 0.83°C deltaT storage treatment. Bruise rate was greatest for a fresh market russet variety at the 3.3°C set point with a 0.3°C deltaT. Statistically supported differences between varieties and treatments were not observed. The project was monitored through in person discussions, phone calls, and e-mail exchanges.