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Idaho potatoes and “specific gravity”


Can you explain the term “specific gravity” when it comes to Idaho potatoes? People have told me I need to look for a potato with high solids or specific gravity to make decent fresh fries.


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 plan an important role in producing the world’s highest quality potatoes.

Warm Idaho day’s will cause the plant leaves to make sugars from sunlight and carbon dioxide.  Cool Idaho nights aid in transferring the sugar down through the plant stems into the tubers, where they are converted into starch. This process, called translocation, aid in transferring the sugar down through the plant stems into the tubers, where they are converted into starch. This process 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 plant’s needs. Excessive nutrition and irrigation will promote vine growth, which prevents the tubers from maturing and developing a high solids 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 to rapidly, cooler temperatures are needed to decrease the metabolism and prevent reduction of the solids content. Shed temperatures are controlled, from approximately 41*F. needed for seed potatoes to 48*F. need for potatoes that remain in storage.

Since 60 percent of the potatoes grown in Idaho are used for processed products, specific gravity plays an extremely important role in the consistent production of high quality Idaho® potato products. The ideal French fry, as perceived by both consumers and processors, fries light in color, is crisp on the outside and fluffy or mealy on the inside and has a minimum or 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 11 percent brine solution of one cup salt per 9 ½ 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 will have a high specific gravity and a light, mealy texture when cooked. Low specific gravity potatoes will float, have a lower starch content, and may have a waxy, soggy texture.

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.