The green indicates an increase in glycoalkaloids. When the potato is green, chlorophyll and solanine levels dramatically increase. Chlorophyll's presence results in an appearance undesirable by consumers and solanine's increase may result in a health hazard. Greening is the result of exposure to light and this can occur in the field when potato tubers poke out of the ground. It can also occur in potato storage cellars, on the store shelf, and at home on the counter. Since this can be a major marketing and retailing problem, how can greening be prevented or inhibited. The bottom-line is AVOID LIGHT.
Many factors play a role in greening, "sunburn," of potato tubers in the field. Exposure to light in the field occurs when potato tubers protrude from the ground (Figure 3). Sometimes associated with light exposure in the field is heat necrosis, a hollowing of the center of the tuber caused by exposure to high temperatures (Figure 4).
Therefore, maintaining a sufficient soil cover above the potato seed piece and keeping a wide enough hill for new tubers to expand underground are absolutely essential.
- Choose potato varieties that set tubers deeper not shallow. But, if a shallow-setting variety such as Russet Norkotah is grown, make the hills high. Plant deep; six inches (15 cm) is a common recommendation.
- Although fertilization does not directly affect solanine content, excessive and late application of nitrogen can result in higher solanine content due to its effect on growth and maturity.
- Immature tubers contain higher levels of solanine than mature tubers. Therefore, don't harvest too early unless planting was early. Vine desiccate and allow tubers to mature before harvesting.
- The ideal hill structure is trapezoidal giving a wide and flat cropping row. This gives room for the new tubers to grow without sticking out.
- Avoid planting on ridges where rows can be exposed to dry soil conditions and wind which may erode or blow-off the row exposing the seed and making the hill to small to cover new tubers. If such an erosion occurs early enough in the season and herbicide application permit it, re-hill and build the row back up.
- Drought in itself does not have effect greening but will promote blow-off and will promote ground cracking. During the season, avoid excessive tillage. If the ground cracks during the end of the season, tubers near the surface may be exposed to light penetrating through the cracks. Avoid drying out of the soil especially after vine desiccation. It is often recommended in dry climates to irrigate just before desiccation. Not only will this avoid ground cracks but will make chemical vine desiccating more effective.
Basically, avoid exposing the pile to light. Use low-wattage incandescent light and don't leave it on longer than needed. Once potatoes turn green, it is irreversible. Sort out green potatoes before going to market. Not only will the buyer complain less but the lot grade will be higher.
Don't wash the tubers going into storage. Dirt remaining on potato tubers will offer some protection against exposure to light and greening. Several reports have shown that washed potatoes will green more readily than unwashed.
Note all commercial potato varieties grown in No. America a bred to have low levels of solanine. With thin-skinned varieties, whites and yellows, greening is easier to see, but russet and red varieties will also green. As in the field and storage, the key is AVOID LIGHT. Keep potatoes in the dark. Greening usually occurs at the retail level.
- Shut lights off at night over the potatoes, or cover them with burlap sacks or brown paper bags at night.
- Watch for the start of greening and cover displays or bag the potatoes.
- Locate potato displays in sections with low light intensity. Do not locate at front window or on sidewalks.
- Use a canopy or some decorative overhang to lower exposure to direct sunlight.
- Use incandescent light bulbs which release much less ultraviolet light than fluorescent ones. Do not use spotlights on potato display.
- Keep them as cool as possible without freezing (below 40F, 4C).
- Package potatoes in dark paper or dark plastic (vented) bags for this reason. Bags with green cellophane for viewing will inhibit greening and not promote solanine formation. Remember plastic bags must be vented else soft and wet rots will break down the tubers in the bag and you'll have "mush."
- Don't keep potatoes wet when displayed under light since wetness may magnify the light intensity on the skin.
- At home, store the potatoes for short periods in a dark cupboard preferably in a cool part of the house such as a basement.
- Wash potatoes only before cooking.
- Green areas, especially the peel, may be cut away and cook the rest for safe eating. But, if you have a tendency toward allergies, it's best to throw the whole potato tuber away to be safe. If the potato tastes bitter, throw it away.
- There has been a report that putting potatoes into a 3% dishwasher detergent solution for 30 minutes will protect them from light for 2 to 10 days, depending on the temperature, light intensity, etc.
- Waxes have not shown to be useful in retarding greening and they can promote tuber breakdown as unvented plastic bags can.
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