Sergey Banadysev, Doctor of Agricultural Sciences,
LLC "Doka - Gene Technologies"
This season, there are signals from consumers about the bitter taste of potatoes without visible greening of the tubers. The reason for the bitterness in taste is the content of glycoalkaloids over 14 mg/100 g.
Glycoalkaloids (GCAs) are naturally occurring, bitter-tasting, heat-resistant toxicants in many plant species, including potatoes. They have fungicidal and pesticidal properties and are one of the natural defenses of plants.
It has now been proven that potato glycoalkaloids in therapeutic concentrations have many beneficial properties for human health: antitumor, antimalarial, anti-inflammatory, etc. Technologies for the commercial extraction of these substances during the industrial processing of potatoes are being developed, but this is a separate topic for publications, and the goal is summarized below. information - outline the options available to prevent excessive accumulation of glycoalkaloids in ware potatoes.
The main HCAs contained in potato tubers are α-solanine and α-chaconine (Fig. 1), which account for about 95% of the total content of glycoalkaloids in this plant species.
Solanine and chaconine are nitrogen-containing steroidal alkaloids that carry the same aglycone, solanidine, but differ in the side chain of the tri-saccharide. The trisaccharide in α-solanine is galactose, glucose and rhamnose, while in α-chaconine it is glucose and two residues.
rhamnose. An ordinary potato tuber contains on average 10-150 mg/kg of glycoalkaloids, while a green one contains 250-280 mg/kg, and a green peel contains 1500-2200 mg/kg. The content of glycoalkaloids in commercial potato tubers is relatively low, and
distribution within the tuber is not uniform. The highest levels are limited to the peel, while the lowest levels are found in the core area. HCA is always found in tubers, and at doses up to 100mg/kg they combine to contribute to the good taste of potatoes.
French fries and potato chips typically contain HCA levels of 0,04-0,8 and 2,3-18 mg/100 g of product, respectively. Peel products are relatively rich in glycoalkaloids (56,7-145 and 9,5-72 mg/100 g of product, respectively). The production of potato products includes washing, peeling, cutting, blanching, drying and frying. The largest amount of glycoalkaloids is removed during cleaning, blanching and frying, and ready-to-eat french fries contain only 3-8% glycoalkaloids compared to raw materials, with the main destruction of HCA occurring during frying. It has been proven that peeling usually removes most of the glycoalkaloids in edible tubers. Potatoes cooked with the skin on may become more bitter than those that have not been peeled due to the migration of glycoalkaloids into the flesh during the cooking process. Boiling reduces the level of HCA only by 20%, baking and microwave cooking do not reduce the content of glycoalkaloids, since the critical temperature for the decomposition of HCA is about 170°C.
Cases of HCA poisoning in potatoes in the entire history of observations are rare. However, possible symptoms such as nausea, vomiting, diarrhea, stomach and abdominal cramps, headache, fever, rapid and weak pulse, rapid breathing, and hallucinations should be mentioned. The toxic dose of HCA for humans is 1-5 mg/kg of body weight, and the lethal dose is 3-6 mg/kg of body weight when administered orally. Therefore, most developed potato growing countries have set limits for glycoalkaloids of 20 mg/100 g fresh weight and 100 mg/100 g dry weight as safe limits in edible tubers.
It is known that potato tubers with HCA 14 mg/100 g are already slightly bitter, while
burning in the throat and mouth is caused by concentrations greater than 22 mg/100 g. Therefore, the best guideline for consumers is: "If the potato tastes bitter, don't eat it."
At the stage of growing, storing and selling potatoes, it is important to prevent the accumulation of potentially dangerous concentrations of HCA in tubers.
The accumulation of HCA inevitably occurs in the tubers, but is repeatedly activated under the influence of sunlight. Lighting also leads to the formation of chlorophyll and the resulting greening of the skin of the tubers. These are independent processes with different consequences. Chlorophyll is absolutely harmless and tasteless. At the same time, greening signals a prolonged exposure to light and, consequently, the accumulation of glycoalkaloids that has occurred. Potatoes that have turned green are usually not sold or taken off the shelves as soon as the color change becomes noticeable. The high content of glycoalkaloids causes complaints from consumers and reduces the commercial value of the products sold. A difficult case noted in the current season, namely, the bitter taste of potatoes without signs of visible greening, deserves a separate explanation and analysis of possible causes.
Since potato greening is the main cause of deterioration in the quality of potatoes in the process of marketing and a significant commercial problem, all the features of this phenomenon have been studied quite thoroughly. At the same time, a lot of expert information was also obtained on the accumulation of HCA in tubers. Like underground stems, potato tubers are non-photosynthetic plant organs that lack the mechanism of photosynthesis. However, after exposure to light, starch-containing amyloplasts are converted to chloroplasts in the peripheral cell layers of the tuber, which causes the accumulation of the green photosynthetic pigment chlorophyll. Tuber greening can be influenced by genetic, cultural, physiological and environmental factors, including planting depth, physiological age of tubers, temperature, atmospheric oxygen levels and lighting conditions. The main factors influencing the level of greening and accumulation of glycoalkaloids are the intensity and spectral composition of light, temperature, genetic characteristics of varieties.
The synthesis of chlorophyll and HCA in the tuber occurs under the influence of visible light wavelengths from 400 to 700 nm (Fig. 2). According to the researchers, chlorophyll synthesis shows a maximum at 475 and 675 nm (blue and red regions, respectively), while the maximum synthesis of α-solanine and α-chaconine occurs at 430 nm and 650 nm. Chlorophyll synthesis is minimal at 525-575 nm, while HCA accumulates minimally at 510-560 nm (green area). These differences confirm the assumption of different pathways for the biosynthesis of chlorophyll and HCA. The chlorophyll concentration in potato tubers exposed to blue light (0,10 W/m2) was three times higher after 16 days of storage compared to potatoes exposed to blue light.
exposed to red light (0,38 W/m2). Fluorescent lamps (7,5 W/m2) emit 1,9 times more blue light (400-500 nm) than LED lamps (7,7 W/m2), while LED lamps emit 2,5 times more red light light (620-680 nm) than fluorescent tubes. Therefore, replacing fluorescent lamps with LED lamps in grocery stores can reduce the intake of the most harmful blue wavelengths.
Potato tubers stored in the dark do not contain chlorophyll. After entering the light, literally within a few hours, specific genes are activated to produce a chain of chlorophyll and HCA synthesis products. Molecular analysis technologies make it possible to identify the structure of genes, and it turned out that the mechanisms of genetic control of these processes have varietal specificity. The influence of monochromatic LED lamps with different and narrow spectral composition has been studied. Light regulation of potato tubers landscaping was carried out under constant illumination provided by light emitting diodes (LEDs). Light wavelengths B (blue, 470 nm), R (red, 660 nm) and FR (far red, 730 nm) and WL (white, 400-680 nm) were used for 10 days. Blue and red wavelengths were effective in inducing and accumulating chlorophyll, carotenoids, and the two main potato glycoalkaloids, α-solanine and α-chaconine, while none of them accumulated in darkness or under far red light. Key genes for chlorophyll biosynthesis (HEMA1, which encodes the rate-limiting enzyme for glutamyl-tRNA reductase, GSA, CHLH, and GUN4) and six genes (HMG1, SQS, CAS1, SSR2, SGT1, and SGT2) required for the synthesis of glycoalkaloids were also induced in white , blue and red light, but not in the dark or with far red light (Fig.3,4,5). These data indicate the role of both cryptochromic and phytochromic photoreceptors in the accumulation of chlorophyll and glycoalkaloids. The contribution of phytochrome was further supported by the observation that far red light can inhibit white light-induced accumulation of chlorophyll and glycoalkaloids and associated gene expression.
Different varieties of potatoes produce chlorophyll and green color at different rates, which has been confirmed by many studies. For example, Norway has identified differences in apparent color changes between cultivars and developed separate subjective rating scales for different cultivars based on accurate measurements of chlorophyll and color. Visual color changes of four varieties of potatoes stored for 84 hours under LED illumination are shown in Fig. 6.
The red-skinned cultivar Asterix (Fig. 6a) showed a significant increase in hue angle, going from red to brownish, while the yellow cultivar Folva (Fig. 6b) changed from yellow-green to green-yellow. The yellow Celandie (Fig. 6c) showed the least change of all color parameters upon exposure to light, while the yellow variety Mandel (Fig. 6d) changed color significantly, from yellow to greyish. In digital form, the graph of the color change of different varieties of potatoes in the light looks like this (Fig. 7).
In this trial, all varieties except Mandel showed a significant increase in total glycoalkaloids after more than 36 hours of light exposure. But the dynamics of changes and the level of HCA content differ significantly in different varieties: Asterix - from 179 to 223 mg/kg, Nansen - from 93 to 160 mg/kg, Rutt - from 136 to 180 mg/kg, Celandin - from 149 to 182 mg /kg, Folva - from 199 to 290 mg/kg, Hassel - from 137 to 225 mg/kg, Mandel - no change (192-193) mg/kg.
In New Zealand, the entire national variety of potatoes was evaluated by the intensity of greening. The results showed that the amount of chlorophyll in tubers after 120 hours of illumination in different varieties differs by an order of magnitude - from 0,5 to 5,0 mg (Fig. 8).
Important practical conclusions follow from this expert information. Under the influence of light, chlorophyll is produced in the potato, which gives the flesh a green color, and the skin a greenish or brownish tint. Different varieties of potatoes develop different forms of discoloration and at different rates. The spectral composition of light somewhat changes the dynamics of chlorophyll accumulation, but the option of using the far red spectrum, as well as darkness (which do not lead to chlorophyll accumulation), is not relevant for shops selling potatoes. There are varieties that accumulate 10 times less chlorophyll under the same lighting conditions. The dynamics of accumulation of glycoalkaloids differs from the dynamics of greening. The main difference is that the initial amount of HCA in tubers before entering the trade and the beginning of intensive lighting is not equal to zero, unlike chlorophyll, and can be quite significant. The low intensity of greening of many varieties predetermines a longer presence of potatoes on store shelves, which leads to a higher accumulation of HCA.
Since complaints about bitter taste do not occur every year, it is necessary to find out other reasons for the increase in the level of glycoalkaloids in tubers that are not due to lighting or varietal characteristics at the implementation stage. In practice, the functional relationship between greening and the accumulation of glycoalkaloids means the need to analyze the causes of greening. Production factors affecting greening and HCA accumulation:
- Growth conditions. Being underground stems, tubers can naturally turn green in the field with insufficient soil coverage, through cracks in the soil, or as a result of wind and/or irrigation soil erosion. With this in mind, potatoes should be planted deep enough while maintaining sufficient soil moisture to ensure rapid and uniform emergence. A proportional increase in the intensity of tuber greening occurs with an increase in the nitrogen norm in the soil from 0 to 300 kg/ha. At the same time, the researchers note that the double norm of nitrogen during cultivation increases the content of glycoalkaloids by 10% in some varieties. Any environmental factor that affects the growth and development of plants of the nightshade family is likely to affect the content of glycoalkaloids. Climate, altitude, soil type, soil moisture, fertilizer availability, air pollution, harvest time, pesticide treatments and exposure to sunlight all matter.
- Tuber maturity at harvest. The effect of maturity at harvest on greening frequency is controversial. Young potatoes with smooth and thin skins can turn green faster than more mature tubers. Early maturing varieties may show greater accumulation of glycoalkaloids than late maturing tubers, but there is evidence to the contrary in specific studies.
- Injury to tubers does not affect the accumulation of chlorophyll in any way, but provokes the accumulation of HCA (the level of HCA increases as much as it does as a result of exposure to light (Fig. 9).
- Storage conditions. Tubers stored at low temperatures are less susceptible to greening and HCA accumulation. Potato skin tissues at 1 and 5°C under fluorescent light showed no color change after 10 days of storage, while tissues stored at 10 and 15°C turned green from the fourth and second days, respectively. A storage temperature of 20°C under lighting has proven to be optimal for chlorophyll production, comparable to most retail stores. Glycoalkaloids accumulate twice as fast at 24°C than at 7°C in a dark room, and light accelerates this process even more.
- Packaging materials. The choice of packaging for retail stores is a critical factor in controlling greening and accumulation of HCA. Transparent or translucent packaging materials encourage greening and HCA synthesis, while dark (or green) packaging slows down degradation.
Based on the experimentally proven regularities, we can confidently conclude that the higher level of glycoalkaloids in potato tubers of the current season compared to the usual level is due to unfavorable conditions for crop formation. A long period of heat and drought in July - early September delayed the maturation of tubers and the absorption of nitrogen, the soil in the ridges in the fields without irrigation cracked. The beginning of harvesting took place against the background of excessively dry soil and a large number of hard lumps, which led to increased injury to tubers. Subsequently, the pace of harvesting slowed down due to excessive rainfall. Fields after desiccation, i.e. without shading the surface of the soil, they waited a long time for harvesting. These unfavorable conditions contributed both to the greening of the tubers and the formation of more than usual amounts of HCA in them.
The most effective ways to prevent unwanted accumulation of glycoalkaloids come down to a severe limitation of the exposure of tubers to light during cultivation, storage and sale, especially against the backdrop of high temperatures. Agricultural practices such as the correct planting depth, the formation of voluminous ridges, optimal fertilizer rates are used regularly in modern potato production technologies. Immature tubers contain higher levels of solanine than mature tubers. Therefore, it is very important not to harvest early, to dry the stems reliably, and to allow sufficient time (two to three weeks) for the tubers to mature. Guaranteed to prevent cracking of the ridges is possible only with the help of timely and sufficient periodic irrigation. It is possible to reduce the consequences of cracking in the pre-harvest period, after the introduction of desiccants, by rolling the ridges. To do this, special machines for rolling ridges are mass-produced, for example, GRIMME RR 600, there are options for combining with defoliators (Fig. 10). However, in the Russian Federation they are still used extremely rarely. At the same time, this agricultural method is simple, cheap, productive and effective. The level of HCA is strongly influenced by the combined effects of light quality, duration and intensity. Chlorophyll is green because it reflects green light while absorbing red-yellow and blue. The formation of chlorophyll is most intense under blue and orange-red illumination (Fig. 11). Under green lighting, potato greening practically does not occur, and under blue or ultraviolet light, it occurs to a weak degree. Fluorescent lights cause more greenery than incandescent lights. Sections, storage compartments for potatoes should be dimly lit and cool. Exposure of tubers in storage to sunlight should be avoided. Use low wattage incandescent bulbs and do not leave them on longer than necessary. The soil on the surface of the tubers provides some protection from light exposure and landscaping. Washed potatoes turn green faster. Once a potato turns green, it is irreversible and must be sorted before sale.
Modern Light Emitting Diode (LED) technology opens up new possibilities for preventing the formation of solanine in all post-harvest stages of potato production. Serially produced special lamps for the potato industry, operating in the spectrum of 520-540 nm (Fig. 12). Light, perceived as green by the human eye, effectively prevents the formation of chlorophyll and solanine and is thus a decisive factor in preserving the value of potatoes during storage and further processing. Such lamps are especially effective in areas of pre-sale preparation and pre-sale storage of packaged potatoes. And one more general rule: keep the storage temperature rationally low and keep the potatoes dry, as moisture increases the intensity of light on the skin.
The type and color of the packaging material affects the intensity of HCA accumulation. Marketing and advertising aside, it's best to pack your potatoes in dark paper or dark plastic bags to avoid exposure to light. There is even a recommendation that packaging materials for sensitive potato varieties should have a total light transmission of less than 0,02 W/m2. Such low levels of light penetration are only possible when packaged in two-layer black plastic with aluminium. Green cellophane viewing bags inhibit greening and do not promote solanine formation. It is clear that such recommendations fall into the category of good intentions when it comes to the retail sale of potatoes. Packaging colors in trade are selected only in the context of sales promotion.
Lighting conditions in retail stores are also difficult to standardize. There are hardly any commercial companies that design lighting based on the fact that the least HCA accumulation and greening is observed in the 525-575 nm spectrum. Even such a necessary and simple protection method as covering potatoes with light-insulating materials during off-hours is rarely practiced by shops.
The summary above lists all effective preventive methods to control the accumulation of glycoalkaloids in potato tubers. There have been many attempts to find more radical means of neutralization: treatment with oils, waxes, surfactants, chemicals, growth regulators and even ionizing radiation, which in many cases have shown high efficiency. However, these methods are not used in practice due to complexity, high cost and environmental problems.
Bright prospects are declared by adherents of new technologies for editing the genome and "turning off" the genes for the synthesis of chlorophyll and HCA. These works are being actively and thoroughly carried out in many countries, where this technology is not classified as a GMO variety (it is classified in the Russian Federation), there are many publications on this topic, but so far there is no need to talk about practical achievements. As with many previously proposed revolutionary breeding methods, the initial euphoria from the possibility of editing the genome is gradually replaced by an awareness of the extreme complexity of metabolic processes. It suffices to look at the diagram listing the already identified processes related to the synthesis of GCA and the potato genes involved in these processes (Fig. 13). Despite the apparent clarity of this diagram, the groups of enthusiastic researchers who have taken up this matter have not yet succeeded in managing such a complex process of interaction between numerous genes and the products synthesized by them. Blocking seemingly purely specific, single genes leads not only to the expected changes in specific levels of glycoalkaloids, but also significant changes in the formation of other biochemical products, for which the task of editing was not set.
However, even without waiting for future successes in genome editing, all commercial potato varieties currently grown have under normal conditions a low, absolutely safe content of glycoalkaloids, due to the consistent decrease in this indicator during many decades of classical breeding work. As for varieties with a relatively slow rate of accumulation of chlorophyll and greening of the peel, this is not a drawback and not a reason to refuse them. But when selling potatoes, it is necessary to officially inform trade organizations that the variety has a peculiarity in order to prevent excessively long exposure of tubers to the light and the resulting claims of buyers for an unexpectedly bitter taste in the absence of obvious greening.