Location: Vegetable Crops Research2013 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 project was renumbered from 3655-21000-049-21S to 3655-21220-002-07S. This is the final report, project terminated 8/31/2013. Long-term storage of potato tubers increases financial risks to growers and processors, which are often attributed to shrink and quality loss. Shrink is a decrease in marketable tuber weight that occurs after harvest. In most years, losses caused by shrink are 5-10% of the harvest crop. To minimize shrink resulting from loss of water by tubers while they are in storage and to maintain high tuber processing quality, producers must closely monitor the condition of the crop during storage and make necessary adjustments to management plans. Evaluation procedures for assessing crop quality in storage are time consuming and labor intensive. We have begun to develop additional management tools to improve real time storage decisions and ultimately marketing strategies for stored potato crops. Ten potato cultivars were grown in Central Wisconsin in 2012 and placed into temperature-controlled, humidified storage. Fresh market varieties were stored at 38°F and included Russet Burbank, Russet Norkotah CO8, Russet Norkotah TX296, Freedom Russet, Millennium Russet, Silverton, and Goldrush. Chip and fry processing varieties were stored at 55, 48, and 45°F and included Snowden, Lamoka, Nicolet, and Russet Burbank. Nicolet, Russet Norkotah CO8, Russet Norkotah TX298, and Russet Burbank are being evaluated to see how vine desiccation method used prior to harvest and storage set-point temperature affect weight loss and tuber processing quality during long-term storage. The four desiccation treatments included application of chemical desiccant at a time consistent with grower practice or application two weeks earlier, mowing without chemical desiccant, or natural vine senescence. All varieties are being evaluated to see how genotype affects total weight loss during long-term storage. Additionally, relationships between potato tuber respiration rates and future processing quality across treatments are being evaluated. Six months after harvest, total weight loss across all cultivars and set-point temperatures ranged from 2.96% to 6.86%. Changes in dry matter content were also evaluated and a maximum increase of 2.96% in dry matter content was observed six months post-harvest. Respiration rates are being quantified on the same set of materials to assess relationships between tuber weight loss, dry matter content and processing quality. By developing post-harvest modeling tools, producers and user of potato products will be able to make efficient assessments of crop storage status and predict changes in crop quality based upon variety and storage conditions. This research directly addresses Objective 1, Determine the influence of pre-harvest management practices on shrink and the occurrence of pressure flattening and pressure bruise, by varying pre-harvest irrigation management and vine desiccation methods. Results to date indicate that genotype has a greater effect on water loss and sensitivity to pressure bruise than pre-harvest management of the crop. This research also directly addresses Objective 2, Determine the influence of storage management practices on shrink and the occurrence of pressure flattening and pressure bruise, by evaluating the rate of shrink throughout the storage period for several cultivars under varied storage temperature conditions. Weight loss was found to be greater at 38F than at 45F.