Scientific Distinction

The Idaho® potato harvest is a race to beat the hard freeze, which comes in mid- to late October. From the time the potato vines die or are killed, the decision to dig is put off as long as possible. This period just before the potatoes are dug is the first phase of maturing that is essential to the distinctive appearance, size, and quality of the Idaho® potato.

The outer cells phase in the “aging” process takes place in the potato cellar. A “sweat” period, which allows the field heat to leave the tubers, puts the potatoes in dormancy and prevents sprouting from occurring during hard freeze, which comes in early months of storage. For example, with Russet Burbanks, the outer skin continues to take on the characteristics of coarse, reddish-brown homespun cloth. Idaho® potatoes are not ready for distribution until this phase of the maturing process has been completed.

Exclusion of light, 95 percent humidity and ventilation systems, thermostatically controlled to keep the air circulated at approximately 42–45°F, are conditions necessary to maintain quality, taste, and texture.

This quality control helps Idaho growers and shippers meet standards for the Grown in Idaho® seal and guarantee a year-round supply of Idaho® potatoes.

The Idaho® Potato Commission wishes to thank Dr. Gale Kleinkopf, Ph.D., professor of plant physiology, University of Idaho, Research and Extension Center, for his expertise and assistance with the preceding information.

Specific Gravity

Specific gravity in the potato industry is a measure of maturity and quality involving a number of factors.

Simply stated, specific gravity is a measurement of the solids or starch content relative to the amount of water contained in a potato. Low moisture means high solids content, the distinguishing characteristic that makes an Idaho® Russet Burbank potato light, fluffy, and mealy when baked, mashed, or fried.

Development of solids or starch begins in the fields as the tubers form. Temperature, irrigation, and controlled plant nutrition play an important role in producing the world’s highest-quality potatoes.

Warm Idaho days cause the plant’s leaves to make sugars from sunlight and carbon dioxide. Cool Idaho nights aid in transferring the sugars through the plant stems into the tubers, where they are converted into starch. This process, called translocation, is also affected by a closely monitored system that regulates plant nutrition and moisture. Technicians frequently test the soil and plant tissue to determine the plants’ needs. Excessive nutrition and irrigation will promote vine growth, which prevents the tubers from maturing and developing a high solid content.

Proper storage is one of the most important factors contributing to the high quality and specific gravity of Idaho® potatoes.

Potatoes are living organisms, since biological reactions continue to take place within the potato after it has been harvested. In order to keep these metabolic changes from occurring too rapidly, cooler temperatures are needed to decrease metabolism and prevent reduction of the solids content. Shed temperatures are controlled, ranging from approximately 41°F needed for seed potatoes to 42–45°F for potatoes that remain in storage.

Since 60 percent of the potatoes grown in Idaho are used for processing, specific gravity plays an extremely important role in the consistent production of high-quality Idaho® potato products. As perceived by both consumers and processors, the ideal French fry is light in color, crisp on the outside, fluffy on the inside, with minimum oiliness. Potatoes with high specific gravity are needed to produce such a product. When tested, if the specific gravity is less than 1.070, the potatoes will not meet the standards maintained in the industry.

A simple method that can be used to separate high- and low-specific-gravity potatoes is to prepare an 1 1 percent brine solution of one cup of salt per 9½ cups of water. The resulting solution will have a specific gravity of close to 1.080, the figure used to measure the high quality of solids content in a Russet Burbank potato. Potatoes that sink in the solution have a high specific gravity and a light, mealy texture when cooked. Low-specific-gravity potatoes will float, have lower starch content, and may have a waxy, soggy texture.

Sugar/Starch Transformation

Since potatoes are constantly undergoing biological changes, they are greatly affected by temperature, humidity, ventilation, and exposure to light. These are conditions that must be carefully monitored not only by Idaho shippers and processors, but by retail and foodservice operators. The Russet Burbank, as grown in Idaho, consistently averages 21 percent solids. Most of this is starch, which is particularly sensitive to temperature changes.

If potatoes are stored at temperatures below 40°F, part of the starch turns to sugar, bringing about an undesirable sweetness and discoloration when cooked. This darkening is not to be confused with the normal oxidation that occurs when a cut surface is exposed to air.

Under controlled storage conditions, the starch and sugars in potatoes are in a balance. This balance is altered when sugars slowly begin to accumulate at 45°F. Reconditioning, or reserving the sugar accumulation, may be possible, depending on how the tuber will be prepared for service, and how long it has been refrigerated.

If potatoes have been stored under refrigeration for several weeks, they should be placed in a dark, well-ventilated room at 60° to 70°F for one to two weeks. The higher temperature increases the tuber’s respiration, causing it to “burn up” the accumulated sugars. This reconditioning method is best for potatoes that are to be baked and boiled.

Because the higher temperatures can cause moisture loss and rapid deterioration, it is important to monitor the process. Diabetic tape, purchased from a local drugstore, can also be used to determine the sugar level in potatoes. Simply run a piece of tape across the cut surface of a raw potato. If the glucose (sugar) level registers a dark color on the tape, this will mean that the potato may taste sweet or darken when cooked.

For potato processors, the starch/sugar content in the tuber is especially important because it directly affects the color and texture of products such as chips and fries.

Tubers that have been held under refrigeration for longer periods of time may not be able to be completely reconditioned. The residual sugars can lead to “streaks” appearing when the potatoes are fried. The high temperatures needed for deep-fat frying bring about an interaction between sugars and amino acids, known as the Maillard Reaction. This causes the surface of the fries to darken before completely cooking on the inside. The high sugar and low starch contents also result in excess oil absorption. Blanching cut potatoes in hot water (170°F) for several minutes will leach out sugars, cleansing the surface of the fries, to allow them to brown evenly.


A great deal of planning, time, money, and effort are required to produce a high-quality Idaho® potato crop. However, all of these investments by Idaho growers and shippers are of little importance when potatoes which reach the foodservice operator in prime condition and are then damaged by mishandling.

Despite their hardy appearance, potatoes can be bruised as easily as a banana or an apple.

Potatoes are living organisms made up of a network of cells that form skin (cork layer) and inner tissue (cortex). Bruising occurs when the tissue is crushed and cells rupture, releasing enzymes that produce a black discoloration. There are two types of bruising: internal bruising and shatter bruising.

INTERNAL BRUISING. Sometimes referred to as blackspot, internal bruising happens when potatoes are dropped more than six inches, or if something heavy is placed on top of them. The degree of bruise is directly related to the fall. It can appear beneath the surface of the skin, or penetrate deep into the tuber. The damage does not appear immediately, but becomes noticeable after one or two days in storage. Since the skin is not broken, the damage may not be found until the potato is cut or pared.

This type of internal bruising frequently takes place when potatoes are piled too high or dumped into a display bin, dropped into a shopping cart, or dragged along a storeroom floor.

Shatter Bruising

Shatter bruising occurs when the skin of the tuber has been broken. The potato then produces a substitute covering known as wound or scar tissue. This is usually a thick, unsightly layer that is hard to peel and results in excessive waste. Shatter bruising happens most often when potatoes have been refrigerated. The inner tissue becomes brittle and susceptible to impact damage.

One Positive Note

Nature provides its own protection. Dirt on the potatoes can act as a natural protection barrier against storage loss and abrasion.

To avoid bruising, potatoes should be handled as little as possible. Store them in a cool, dark, well-ventilated place at 45–48°F to keep the bruised area from spreading or rotting and possibly damaging surrounding spuds.

The Idaho® Potato Commission wishes to thank Robert Dwelle, Ph.D., associate professor of potato physiology, Idaho Research & Extension Center, for his expertise and assistance with the preceding information.


Controlled temperature, humidity, and light contribute to maintain the high quality of Idaho® potatoes. Variations in one of these important factors can cause significant changes in appearance and taste. Greening is the result of one such change.

The “greenish” hue sometimes seen on potato skin occurs when the tubers have been exposed to natural, artificial, or fluorescent lights in storerooms or in supermarket displays.

The color is actually chlorophyll developing in the skin. Insome varieties, it is green; in others, purple. Along with this change, an increased quantity of solanin is also formed. Solanin, a glycoalkaloid present in all potatoes, is actually part of the flavoring complex that gives the potato its taste.

More of this naturally occurring substance is found in some varieties than in others. In the Russet Burbank, the level is very low. However, in all varieties, green potato skin is an indication that excessive solanin is present. The brighter the color is, the higher the level or solanin and the more bitter the taste.

Solanin is generally concentrated close to the potato’s surface and is easily removed when peeled. Only if the potato has had prolonged exposure to light will the bitter taste and color penetrate into the tuber. The green portions can easily be discarded in preparation.

There is little concern about solanin being harmful. At levels that could cause an adverse reaction, the solanin level would have to be so high that the potato would be inedible. Furthermore, solanin, if accidentally eaten, does not accumulate in the body. Animal research shows that it is poorly absorbed and rapidly excreted.

Careful measures are taken by the potato industry to keep greening at a minimum. During storage, the tubers are held in darkened cellars and are carefully inspected before shipping. The poly film, burlap, and cardboard containers used are designed to filter or block out light. Even the dirt left on the potato can have a protective effect in blocking light.

Similarly, in your foodservice operation, fresh potatoes should be stored in a cool, dry, dark, well-ventilated place to maintain quality. When potatoes are on display in retail settings, they should be rotated regularly and covered whenever possible to reduce overexposure to light.