- Aug 31, 2012
Plant Growth & Development - PlSc 401
1) Tuber solids make up about 80 percent of fresh tuber weight. Starch makes up about 70 percent of total tuber solids. Starch is heavier than water, and, therefore, is the primary determinant of tuber density, which is commonly referred to as tuber specific gravity.
2) Low specific gravity potatoes, typical of red varieties, for example, tend to be best for boiling and canning.
3) Russet type varieties generally make good all-purpose varieties, but with some exceptions, are best for baking and frying.
4) Long-white and round-white varieties are divided into those suitable for general home use where potatoes are often boiled, and those that have high specific gravity and are intended only for making potato chips or French fries.
5) The predominant sugars found in potato tubers are sucrose, glucose, and fructose. Sucrose is formed during photosynthesis and is the sugar used to transport energy to the tubers. Speaking simplistically enzymes in the tubers divide the sucrose molecule into its component sugars—fructose and glucose. Fructose is subsequently converted to glucose, and the glucose molecules are then chained together to form starch.
6) Potato tubers usually have high sugar content early in their development because the rate of transport from the leaves exceeds the rate of con-version to starch. As the tubers grow and mature, the sugar content decreases, reaching the lowest point when the vines are nearing complete senescence. For the tubers, this point is known as physiological maturity. Tubers left in the field after reaching physiological maturity generally begin to increase in sugars.
7) The other temperature response that is important to understand is cold induced conversion of starch back to reducing sugars. This can occur in the field or in storage. At temperatures from 50°to 55°F the balance between starch and sugars remains relatively static. As temperatures drop below this range, starch conversion to sugar becomes evident in most varieties. The lower the temperature, the faster the conversion and the higher the final concentration of sugars.
8) Potatoes that have accumulated excess sugars after exposure to cold temperatures in the field or storage experience a partial recovery toward lower sugar concentrations when exposed to temperatures above 55°F (60°to 65°F is considered optimal).
9) Potato varieties exhibit large differences in sugar content, especially after storage. For that reason, it is critical to match varieties with intended use. In general, potatoes bred for the chipping industry are lowest in sugars. Potatoes bred for French fry processing typically have intermediate sugar contents, while those bred for the fresh market usually have the highest. Potato breeders are currently making a concerted effort to develop varieties that can be stored at temperatures as low as 40º F and still maintain sufficiently low sugar levels to be used for chip or French fry processing. Several such varieties are now available.
10) In addition to cold induced stress, a few other conditions in storage can produce an increase in tuber sugars. The most important of these is insufficient air movement. The tuber requires oxygen for respiration and low-level physiological activity. If a pile of potatoes becomes oxygen starved because of infrequent operation of the storage air system or
because of excess dirt or other air blockage, the normal physiology of the tuber can be disrupted and sugar levels increase.
11) During tuber growth, the enzyme that converts sucrose to reducing sugars is inhibited. In storage, however, this enzyme becomes active and, if there is a sufficiently large pool of sucrose available, sucrose conversion results in a high level of reducing sugars. Any stress on tubers in storage, such as low temperatures or insufficient air supply, can cause an increase in the sucrose pool.