Whoever can make two ears of corn or two blades of grass to grow where only one grew before, would deserve better of mankind and do more essential service to his country than the whole race of politicians combined.-- Jonathon Swift (1667-1745)

INTRODUCTION:

Selecting suitable varieties is an essential first step in any successful potato operation. Varieties must not only suit the intended market but also be well adapted to local growing conditions. 

Varieties differ in a number of characteristics including: yield; skin and flesh color; tuber shape and eye depth; time of maturity; disease and pest resistance; carbohydrate composition; usage potential; dormancy and storability; and any number of additional particulars. Processors typically specify varieties in grower contracts, but not always. Fresh market growers, on the other hand, are free to choose.

Potato varieties have traditionally been categorized according to shape and color as Round Whites, Russets, or Reds. Recently, "Specialty" and "Gourmet" have entered the potato vocabulary to describe almost any variety not fitting into the three major divisions. Broadly speaking, round-white varieties are used for chipping and, to a lesser extent, as table stock in central and eastern states. Reds and specialty gourmet varieties are used exclusively for the fresh market. Russets, which tend to produce oblong tubers, are used for both frozen processing and fresh market.

The carbohydrate (starch and sugar) composition of tubers plays an important role in determining variety usage. Processing varieties, for example, must have relatively high starch and low reducing (glucose/fructose) sugar levels. Starch content is directly related to specific gravity or dry matter in tubers. Typically, 60-80% of the dry matter is present as starch. Therefore, high-gravity or high-solids tubers contain high levels of starch.

Relationship Between Tuber Dry Matter and Optimum Use
Specific Gravity Dry Matter, %  Texture Typical Uses
Below 1.060 (very low) Below 16.2 Very soggy Pan frying, salads, canning
1.060-1.069 (low) 16.2-18.1 Soggy Pan frying, salads, boiling, canning
1.070-1.079 (medium) 18.2-20.2 Waxy Boiling, mashing; fair to good for chip processing and canning
1.080-1.089 (high) 20.3-22.3 Mealy, dry Baking, chip processing, frozen french fry processing; some cultivars tend to slough when boiled
Above 1.089 (very high) Above 22.3 Very mealy or dry Baking, frozen french fry processing, chip processing; tendency to produce brittle chips and to slough when boiled

Adapted from: Mosley, A.R. and R.W. Chase. 1993. Selecting Cultivars and Obtaining Healthy Seed Lots. In: Potato Health Management, APS Press, 1993. Pp. 19-27.

Starch has profound effects on product texture and oil consumption during processing. Frying drives much of the water out of chips, for example, and replaces it with oil. Therefore, low dry matter/high water tubers tend to absorb more oil, which is typically the most expensive component of fries, and become more soggy and/or oily.

In contrast to starch, which primarily affects product texture and oil content, reducing sugars (primarily glucose and fructose) play a critical role in the processed color of potato chips and french fries. Even very low levels of sugar cause chips and fries to turn dark brown during frying. For that reason, varieties intended for processing either as chips or fries, especially during the winter and early spring, must have low levels of reducing sugars both in the field and storage. Sugar levels typically make up less than 3% of total tuber dry matter.