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Organic produce and food sales have increased dramatically in the last several years. This demand for organic fruits and vegetables also includes the demand for organic potatoes from year-round storage. In the past, supply of organic potatoes has been limited to a few months following harvest during the period of natural dormancy. Techniques such as temperature control and cultivar selection can be utilized to influence the natural dormancy period and to delay sprouting. There are limitations relying solely upon those two techniques. What if you are storing potatoes destined to be processed? Cold storage temperatures may not be an option due to the potential for unacceptable fry color. At cooler storage temperatures, there is an increase in reducing sugars that causes a darkened fried product upon cooking. Another concern is whether you have the option to grow and store a long-dormancy variety. There may be production issues with that variety for your area, or your negotiated market contract stipulates production of a particular variety. If you need an additional tool besides cooler storage temperatures and variety selection, another option for controlling sprouting in organic potatoes is the application of an essential oil to the potatoes in storage.

Research at the University of Idaho Potato Storage has shown essential oils such as spearmint, peppermint, muna and clove may be used for potato sprout control. This research was undertaken to evaluate organically certified methods of potato sprout control with funding support from the Idaho Potato Commission. The mode of action of essential oils is completely different from conventional sprout inhibitors, as it physically damages the sensitive sprouting tissue. Since only the exposed sprout is damaged, new sprout development will not be inhibited. If long-term sprout control is desired, repeated applications within the storage environment will be required. Judicious monitoring of sprout development is necessary when relying upon these essential oils for sprout suppression.

Although all these essential oils have a similar mode of action, the active ingredient associated with the oil varies. Clove oil is distilled directly from the evergreen plant Syzygium aromaticum. The active ingredient of clove oil is eugenol and other eugenol-based components in the distillate product. Due to the chemistry and volatility of clove oil, it can be applied with a thermal applicator and distributed throughout the storage similar to applications of chlorpropham (CIPC). Spearmint oil is derived from the mint plant Mentha spicata. An active ingredient of carvone and peppermint oil comes from the Mentha piperita plant, with an active ingredient of menthol. A wick application of these oils gave better sprout control compared to thermal or cold aerosol applications. A wicking, or forced evaporation, method entails saturating the volatile oil on absorbent material and placing the material near the air circulation fans for distribution in bulk storage. A “low-tech” wick method can also be as simple as placing a small piece of blotter paper saturated with mint oil in the box of potatoes. Both peppermint and spearmint oil must be applied in a manner to allow sufficient concentration of the volatilized oil in the atmosphere of the storage to damage the sprouts. Muna oil has been used for centuries in South America for both sprout and insect suppression. It is derived from a mint plant called Minthostachys mollis, and the active ingredient is considered to be comprised of various monoterpenes. We have collaborated with the Papa Andina Initiative from the International Potato Center (CIP) on the potential use of muna oil in storage. Our preliminary research efforts have shown successful thermal aerosol applications of muna oil to have good sprout suppression properties.

The two most common questions we receive regarding the use of essential oils are (a) how effective is the sprout control, and (b) does it alter the taste of the potato or processed product? The first question has a complex answer, dependent upon essential oil application rate, timing and method of application, frequency of applications, cultivar, storage temperature and management.

Our research has shown applications of an essential oil should be made when sprouts are first visible or at the “peeping” stage. Depending upon cultivar, not all eyes on a potato sprout at the same time, so carefully watch the sprouting behavior and time the application accordingly. One benefit of using an essential oil for sprout control in storage is the ability to capitalize on the inherent dormancy of the cultivar. Applications are not made until the potato actually begins to sprout, which is dependent upon cultivar, storage temperature and growing season. For example, Russet Burbank stored at 42° F typically won’t break dormancy for approximately 175 days, compared to 135 days at 48° F.

Research results indicate clove oil rates between 50 to 100 ppm applied when sprout development occurs, or approximately at three to six week intervals, will provide adequate sprout control. For example, nine thermal aerosol applications of clove oil (applied at three-week intervals; 100 ppm rate) reduced sprout weight in Russet Burbank by 70 percent compared to the untreated potatoes. Multiple applications of spearmint or peppermint oils minimized sprout development to acceptable levels compared to untreated tubers. Spearmint and peppermint oil applications should be approximately 100 ppm per month with reapplication necessary every two to three weeks or applied on a daily basis. That is equivalent to 10 pounds of oil per 1,000 cwt. potatoes (1 lb oil/5 ton) per month in applications of 50 ppm every two weeks, 75 ppm every three weeks or a daily application of 4 ppm. In large potato storages, it will be necessary to close fresh air inlets and recirculate the storage air for one to two days. Frequent, repeated applications are necessary for adequate long-term sprout control. These rate recommendations for both clove and mint oils will vary with stage of sprout growth, cultivar and storage facility. Delay in subsequent applications after initial treatment may result in greater sprout growth than if no product was applied.

To answer the second most common question, professional taste test trials were conducted on Russet Burbank baked potatoes previously treated in storage with either clove oil or CIPC. Potatoes were exposed to clove oil six times, at rates ranging from 30 to 90 ppm prior to the taste test. No differences in baked potato color, texture, flavor or overall taste were detected between the CIPC-treated and clove oil-treated potatoes. Similar results were obtained when potatoes were exposed to multiple applications of spearmint and peppermint oils, although tubers treated with spearmint oil did show some culinary and palatability concerns. We would recommend the use of peppermint oil due to the potential off-taste issues with potatoes treated with spearmint oil.

Application methods and rates of these essential oils will need to be fine-tuned for individual growers, storage facilities, seasons and cultivars. Some cultivars that sprout rapidly and vigorously, such as Russet Norkotah, may not respond as well to these alternative sprout control methods. Cultivars need to be assessed on an individual basis for proper timing and frequency of application. Certified organic growers should always check with their organic certification agency and the National Organic Standards for current organic production regulations prior to utilizing these described organic and alternative sprout control products.

Originally posted Monday, Jul. 23, 2007

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