Producing and maintaining a good looking tuber skin during long-term storage is vital to high profit margins in the potato industry, as the modern trade is dominated by washed and packaged potatoes. Poor or uneven color and skin condition is a significant and unacceptably costly problem for the industry as a reason for not purchasing or downgrading the quality of potatoes. Of course, there are other skin problems associated with the manifestation of a number of diseases and physiological disorders (netting, greening, overgrown lentils, cracks, mechanical damage), but this article will only deal directly with the natural skin and the possibilities of improving its condition.
In specialized literature, the skin or outer tissues of the potato tuber are collectively referred to as the periderm. The periderm is a protective layer of cells that minimizes water loss from the underlying parenchyma cells and provides protection from soil pathogens. The periderm consists of three types of cells: phellem (cork), phellogen (cork cambium), and phelloderm (Fig. 1). The term "rind" is sometimes used to refer to the entire periderm, and sometimes to the phellem only.
Phellem or cork is the outermost periderm tissue that resists water loss, has mechanical strength and acts as an effective barrier to pathogenic bacteria and fungi. The phellem cells are approximately "brick" in shape, tightly adjacent to each other without intercellular spaces. A typical potato periderm in different varieties is 7-18 cell layers with a total thickness of 100-200 microns. By fluorescence and staining with dyes such as berberine, the phellem is easily shown to be rich in suberin, and this clearly distinguishes the phellem cells from the underlying cell layers. Suberin is a hydrophobic polymer composed of phenolic and aliphatic compounds cross-linked with glycerol and is localized between the primary wall and the plasmalemma. The suberated cells are filled with air and therefore provide thermal insulation, the suberated walls prevent the invasion of microorganisms (mechanically and chemically), and the wax deposits that are embedded in the suberin prevent the internal tissues from drying out.
In addition to suberin, potato tuber periderm contains many other protective chemicals with antioxidant, antibacterial, and insecticidal properties. These substances may be intermediates in suberin biosynthesis or independent protective metabolites. Metabolites include non-polar waxes, saturated and unsaturated fatty acids, saturated dicarboxylic acids, monoacylglycerols, 1-alkanols, n-alkanes, sterols and polyphenols, quinic acid, phenolicamines, phenolic acids, flavonoid glycoalkaloids (solanine, chaconine, leptin, solanidine, solatriose and others), saponins, polyamines (putrescine, spermine and spermidine derivatives), as well as methylprotodioscin and protodioscin.
The formation of a natural (native) potato peel takes place in three stages: 1- periderm initiation - cambial phellogen is formed by differentiation of subepidermal cells; 2-development of immature periderm - active phellogen adds more skin layers to the expanding tuber; fissile phellogen is fragile and prone to breakage, which can lead to separation of the skin from the underlying tuber pulp and to the costly production problem of skin damage; 3- maturation of the periderm - the tuber stops growing at the end of the growing season, new skin cells are not required, and the phellogen becomes inactive. As a result, the layers of the periderm adhere strongly to the pulp of the tuber (parenchyma) in a process referred to as setting, maturation, peel stabilization (Fig. 2).
The potato tuber is a modified stem that begins to differentiate as a swollen internode near the apical bud of the stolon. The outer layer of the stolon is the epidermis, which has widely scattered stomata. While the tuber is still very young, the epidermis is already replaced by the periderm, which begins at the end of the stem of the developing tuber and soon spreads over the entire surface. The periderm becomes full when the tuber reaches the size of a pea. As the periderm develops, the cells directly below the location of the stomata actively divide and form lenticels. During tuber growth and periderm development, phellogen is the active lateral meristem. The phellogen cells divide and the new cells located on the outside of the tuber become the phellome cells. The production of phellem cells by phellogen and the loss of phellem cells by exfoliation at the tuber surface are roughly in balance as the tuber grows. Phelloderma is also derived from phellogen.
Cross sections were stained with hematoxylin and viewed under a light microscope (left panel) and an ultraviolet microscope (right panel, black background) to study the morphology of tissue and cell nuclei, as well as autofluorescence of suberized cell walls, respectively. (A) Periderm initiation—Subepidermal cells undergo dedifferentiation to form phellogen (Phg) initials (circled), which successively produce fellemcelles (white cells). (B) Immature epidermal development - phellogen remains active and adds more cells (Ph) to the expanding tuber. The enlarged image (2,5 times magnification) shows the split cells between two cells (red arrows). The cell membrane is prone to destruction, which leads to the separation of the immature peel from the surface of the tuber. (C) Periderm maturation—after leaf removal or plant senescence, tuber growth stops, cell phellogen stops dividing, and a stabilization process is induced. The phellogen layer is not detected at the maturation stage. Scale rulers: 200 µm.
With incomplete formation of the potato peel, it is damaged (separated) by mechanical contact with the working bodies of machines, stones, lumps, falling tubers, etc. These injuries heal due to the formation of wound periderm (photo 3). Native and wound periderms are similar in terms of tissue origin, structure and morphology, but differ in the process of saturation and the composition of pectin and anthocyanin. In addition, the suberin of the wound periderm is enriched in waxy alkyl ferulates and is more permeable to water. Within 1-3 days, a covering layer is formed in the damage zone, in which the walls of open cells of the tuber parenchyma undergo lignification/suberization. On the 3rd day, the rudiments of phellogen become visible, and columns of new phellema cells are clearly visible under the covering layer. From the 4th day, the newly formed phellem undergoes suberization from the outer layers inwards, and on the 8th day, the suberized layers of the phellem become flattened and compacted, which indicates the maturation of the wound periderm.
A transient increase in auxin and lipid hydroxyperoxide levels 20-30 minutes after injury initiates cytological events that lead to the formation of wound periderm. The levels of abscisic acid, ethylene, and jasmonic acid also temporarily increase shortly after injury and before periderm formation begins. Wound-induced periderm formation occurs most rapidly at 20-25°C, delayed at lower temperatures (10-15°C), inhibited at temperatures above 35°C, at O2 less than 1% and temperature 15°C or higher. The combinations of temperature, oxygen concentration and relative humidity must be optimized for the physiological state of the tubers in order to seal exposed internal tissues as quickly as possible and prevent pathogen penetration and water loss.
Skin development failure resulting in browning of smooth-skinned varieties (Photo 3B) is most often due to sub-optimal growing conditions. This physiological disorder is not caused by pathogens. The reddish brown color may be a genetic trait, such as in the well-known American variety Russet Burbank. Tubers with a reddish-brown skin have a thicker fellem layer than potatoes with a smooth skin, and for technical varieties this is a useful feature, since the thicker the skin, the less internal damage to the tubers, the higher the marketability of the crop. Zonal build-up of layers of phellem cells can be the result of increased phellogen activity as a result of, for example, high soil temperature or strong adhesion of adjacent phellem cells so that they do not flake off during tuber development. This may also be due to increased suberization or higher levels of pectin and hemicellulose. As the tuber expands during development, the thick skin cracks, resulting in a reticulated or reddish-brown color.
Algorithms and the result of the formation of potato peel in different situations differ significantly. The formation of native and wounded potato periderm has been studied for many decades and the main attention has been paid to the nature of suberization of the phellem cell wall, i.e. process that gives the periderm its primary protective properties. In the last decade, the genetic aspects of the skin formation processes have been actively studied, the genes-sources of a certain skin color, and many patterns have been identified. Progress has been made in changing the skin color of known potato varieties by introducing the right genes. However, there is still no understanding of the exact biological mechanisms and possibilities for controlling the activation of phellogen cells for more active tuber skin formation during growth or mechanical damage and inactivation of these same cells during tuber maturation and final skin setting. An immature periderm has an actively dividing phellogen layer, and a mature periderm (typical of potatoes in storage) also has a phellogen layer, but it is inactive and does not form new cork cells.
The condition of the potato peel can be assessed both visually and by methods of precise instrumental control. Most production labs now use quality charts to help staff visually assess tuber quality against predetermined categories. (An example of such a diagram is in photo 4).
Quality charts are widely used because they are cheap to make (and often supplied by the customer) and can be used to train quality control personnel relatively quickly and easily. However, the ratings that a person gives based on their visual impressions are subjective and subject to error. Therefore, in recent years, optical scanners have been actively introduced into the field of evaluating the appearance of tubers, the condition of the peel. Optical sorting is highly productive, up to 100 tons per hour and ensures constant (24/7) product quality according to specified non-standard rejection criteria. This area of technology is rapidly progressing. If 5 years ago its capabilities were limited to inspection of washed potatoes by 3-4 parameters, by now optical sorting equipment for 7-8 parameters of unwashed potatoes is being mass-produced (photo 5). There are already advances in optical scanning of subcutaneous, internal defects in potatoes.
In order to examine the condition of the peel, serial gloss meters can also be used (photo 6). Shiny skin reflects more light, so the difference between varieties or batches of potatoes with different skin quality is measured digitally. There were attempts to manufacture special devices for potatoes, but this did not lead to mass production.
The most important agrotechnical factors that affect and can improve the condition of the potato skin include variety, soil texture, planting depth, nutrition, soil temperature, lack of water, waterlogging, the length of the growing season and the regimen of the treatment period after loading into storage.
The condition of the skin is significantly different in different varieties. The differences between varieties are well known in the packaging industry and retail chains, but the skin quality characteristics of the varieties are not uniform enough. Breeding firms use different terminology to describe cultivar skins. Previously, they mainly indicated color, depth of eyes and smoothness - reticulation of the peel. Recently, the term “skin finish” has become increasingly common, but the criteria for referring to the levels of this indicator “poor - average - good - excellent” have not been published. As a result, the actual state of the peel of any variety in specific soil-climatic and technological growing conditions is revealed only in practice. The duration of preservation of the smoothness of the peel determines the suitability and the possibility of using the variety for washing during the entire period of storage. Even for industrial varieties, a rough, rough peel is unacceptable, as the cost of washing and waste when cleaning tubers increase.
The type of soil affects the purity of the skin, but the effect of soil texture has not been scientifically characterized in detail. Tubers grown in sand have more layers of phellem cells than tubers grown in humus. It is known in the packaging industry that the skin washes best on tubers grown in silty or clay soils compared to more abrasive sandy soils. Tubers grown in peat soils may also have smooth skins, but the appearance of these tubers may be inferior in coloration. That is, on tubers grown in more abrasive soils, the cork layer is thicker, but the texture, smoothness, and shine look better on clay soils. Deep planting results in a thinner skin compared to shallow planting.
Under conditions of high soil temperature (28-33°C), tubers have a relatively thick skin and are more prone to browning and netting. In one experiment, the thickness of the periderm when grown at a temperature of 10,20,30оC was 120, 164, 182 µm, respectively. Waterlogging is thought to increase the netting and dullness of the peel, but there is little or no published evidence to support this. There are reports that skin luster is inversely related to the length of time from drying to harvest (i.e., shorter harvest intervals result in shinier potatoes).
Proper balanced nutrition reduces the incidence of skin diseases and improves the appearance of the peel, also affects the thickness of the peel, but not in all cases. It has been found that the combined application of N, P and K or the application of organic fertilizers increases the thickness of the phellem and the total thickness of the phellogen and phelloderm compared to the use of nitrogen alone. There are many publications on the effect of both macro- and micronutrients on skin quality, but most of the specific patterns identified are associated with only a few nutrients.
Nitrogen. The timing and amount of nitrogen fertilization have a large impact on bruising susceptibility due to the relatively large effect on maturity. A lack of nitrogen can lead to early crop aging and increased susceptibility to brussing if the tubers are under dying stems for a long period before harvest. Excess nitrogen (especially late in the season) delays the ripening of the crop, which leads to a decrease in specific gravity, increased susceptibility to peeling and damage from bruises, poor skin setting. American potato growers believe that the total nitrogen application rate for irrigated potatoes should not exceed 350 kg d wt/ha, while in mid-August the nitrate content in the petioles should not exceed 15 parts per million. Excessive application of nitrogen has a negative effect on skin formation if desiccation is carried out in the early phases of plant development. Too much nitrogen often leads to defoliation. Nitrogen application should be adjusted according to the expected length of the season. Special care must be taken when using nitrogen on varieties that are notorious for poor skin set.
Phosphorus. Unlike nitrogen, phosphorus generally promotes tuber maturation, firm skin formation, and even netting. Phosphorus is absorbed by the root tips during active growth, so phosphorus fertilizers must be applied before planting.
potassium for potatoes should always be applied in the optimal amount and ratio to other nutrients. With a lack of potassium, the tubers are prone to darkening of the pulp after peeling. Excessive application of potassium reduces the specific gravity and overall development.
Calcium reduces susceptibility to bruising due to its effect on cell wall strength. Susceptibility to bruising is generally lowest when the calcium concentration in tubers exceeds 200-250 micrograms per kilogram dry weight. The most effective absorption of calcium occurs when applied to the soil before planting.
Sulfur reduces the level of common and powdery scab. The best effect is achieved when sulfur is applied to the soil in a readily available form at planting, however, foliar application of sulfur can also reduce infestation.
Bor helps stabilize calcium in cell walls and also affects calcium absorption, so calcium stores are important to ensure a balanced diet and maximize the benefits of calcium intake.
Zinc commonly used to suppress powdery scab. Only its introduction into the soil provides sufficient efficiency.
There is abundant evidence of improvement in skin condition with the skilled use of fertilizers during the growing season (photo 7). However, the effect is achieved mainly by reducing the development of diseases. There is no evidence of a direct effect of foliar dressings on the thickness, smoothness, and luster of the peel. Experiments with complex nutrition, for example, could not solve the problem of fragile skins in some varieties in England.
Photo 7. The effectiveness of improving the condition of the peel with the help of macro- and microfertilizers
Other crop management practices that improve potato skins include:
• Selection of fields with optimal fertility, agrochemical parameters and soil granulometric composition. Exclusion of fields where adverse factors are present, such as disease, poor drainage or low water retention capacity;
• Full use of agro-climatic resources for the full ripening of the peel. Use of quality seeds with less disease;
• The use of fungicides, microbiological preparations, biologically active substances in the preparation of seed material, during planting and during the growing season to reduce the spread of diseases;
• Irrigation to prevent or minimize diseases such as common scab;
• Timely desiccation and harvesting in good weather conditions to avoid physical damage and disease infestation;
• Avoid liming just before planting potatoes, as this encourages scab.
The system of chemical protection of the peel of tubers from diseases cannot be described in detail in the format of a section of this article. This is a separate big topic, the use of protective equipment is mandatory in large-scale potato growing. But it must be emphasized that many skin diseases are quite successfully controlled (rhizoctoniosis, common and silver scab) and many active substances are effective, the choice is extensive, and for a number of problems the possibilities of chemical remedies are insufficient (anthracnose, powdery scab, bacterial rot) and effective molecules of a single .
Additional possibilities for the control of peel diseases are provided by the use of a relatively new type of protective agents - microbiological preparations and growth regulators. For example, in the United States, the herbicide 50-D has been widely used for more than 2,4 years to improve and stabilize the color of traditional local red-skinned potato varieties. The effect of a more saturated color lasts for several months, and a noticeable reduction in the spread of scab is also achieved (photo 8). This intended use is included in the official regulation of herbicide 2,4-D:RED POTATOES (Grown for fresh market):Properly timed application of this product generally enhances red color, aids in storage retention of red color, improves skin appearance, increases tuber set, and improves tuber size uniformity (fewer jumbos). Crop response may vary depending on variety, stress factors, and local conditions. Consult with Agricultural Extension Service and other qualified crop advisors for local recommendations. Varieties with naturally dark red color generally benefit less from treatment. Apply 1.6 fluid ounces of this product per acre in 5 to 25 gallons of water using ground or aerial equipment. The specific spray volume selected should be sufficient for good coverage of plants. Make first application when potatoes are in the pre-bud stage (about 7 to 10 inches high) and make a second application about 10 to 14 days later. Do not exceed two applications per crop. Do not harvest within 45 days of application. Uneven application, or mixture with other pesticides and additives, may increase the risk of crop injury.
As a rule, the appearance of the peel does not improve during storage, so the quality of the peel when it enters the store is of the greatest importance. In order for potatoes to provide the highest quality washed product on the market and maintain that quality throughout their shelf life, it is vital that field agronomy be efficient in achieving the best possible skin quality. With modern storage technologies, it is possible to maintain good skin quality for more than 35 weeks, but only if the quality is high at the time of harvest. Many aspects of the skin finish are already determined at harvest time and change little in storage. This applies to netting, growth cracks and some diseases such as common scab and rhizoctoniosis. At the same time, many peel parameters can deteriorate during storage: luster, lentil size, anthracnose, silvery and powdery scab.
To keep the skin in good condition during storage, it is recommended to refrigerate the crop as soon as possible after loading into storage (provided the skin is intact and firmly set and the variety is not susceptible to skin spotting). In addition, crops should be ventilated with dry air during early storage to remove surface moisture. Try to store potatoes below 4,0°C.
The surface of the tubers during storage often noticeably loses its luster. Special studies have shown that this deterioration is caused by the collapse of cells in the covering layer during the first two weeks of storage, if the cells lose moisture during the treatment period. A change in the structure of the periderm leads to a roughening of the skin surface, which worsens the shine, the peel becomes dull. The outer layers of the cork also peel off during storage, but are no longer replaced by anything, the peel from a smooth, shiny, bright one can become rough, dull and rough (photo 9). Therefore, maintaining a high relative humidity during the healing of damage and strengthening the periderm must be observed very strictly .
Optimal ventilation during the main storage period will generally have minimal effect on skin gloss reduction. But a number of varieties show the best cork condition at the highest humidity of 98% maintained in storage. Storage of tubers at high relative humidity reduces the loss of mass of tubers by 1-2%. At the same time, one must remember about the danger of moisture condensate in storage, the negative consequences of which for the quality and safety of the crop are many times higher than the possible savings in weight loss from shrinkage. In the modern phytopathological environment, maintaining a humidity of 90-95% (and this is the level of humidity that is formed due to the respiration of tubers in the intertuber space during periods without ventilation, i.e. this is a natural property of stored potatoes) is optimal. And for batches with the risk of spreading fungal and bacterial diseases, it is advisable to maintain a relative humidity level of 85-90%, which will prevent the physiological and bacteriological deterioration of the stored product. The luster of the skin of many red varieties deteriorates during long storage. Radical attempts are being made to maintain high quality with cling film coating. In one experiment, four different coating compositions were used. Alginate-based food coatings have significantly improved sensory evaluation, especially in terms of color, gloss, and overall acceptability of red-skinned potatoes. The results showed that the edible coating treatment significantly improved the color of the skins, especially the F1 and F2 formulations.
During pre-sale preparation, it is advisable to use technologies that allow maintaining and improving the appearance of tubers. Drum washers with rotating brushes (they are called polishers, photo 11) can increase the sheen of potato skins, i.e. some of the adverse effects of agricultural practices and storage can be largely eliminated by good washing. However, excessive polishing compromises the integrity of the tuber skin, which can lead to potato spoilage. It is always necessary to quickly evaluate the effect of washing on the skin of tubers when changing to a new batch or variety and adjust the washing procedure. At this stage, the level of microbiological contamination, including the water used, should also be monitored, and disinfectants and antimicrobials approved for the food industry should be applied. Until now, everyone is trying to protect and maintain the rules for processing washed potatoes with protective agents in the know-how mode.
Preservation of the quality of the potato peel at the stage of transportation and sale is ensured by the use of packaging with sufficient perforation for ventilation and the prevention of prolonged exposure to bright light, which inevitably leads to greening and accumulation of glycoalkaloids. The topic of potato peel greening during cultivation, storage and sale deserves separate consideration.
Thus, the peel performs important protective functions of tubers and predetermines the assessment of potato quality by consumers. As the volume of sales of washed and packaged products increases, the requirements for the appearance of tubers increase. Many regularities in the formation of a strong, smooth, shiny cork layer of the periderm have been identified, but there is no universal system algorithm for controlling this process. Effective opportunities for improving the condition of the potato peel are the selection of the best varieties and soil varieties, the full use of agro-climatic resources of the growing season, the prevention of diseases, stable water supply, balanced and complete fertilizer with macro- and microelements, the use of biologically active substances and growth regulators, timely desiccation, high-quality harvesting and qualified and accurate carrying out of the first stages of storage, prevention of mechanical damage, polishing of tubers with special equipment.
Photo 11. Polishing washer
Material author: Sergey Banadysev, Doctor of Agricultural Sciences, Doka-Gene Technologies