Specific gravity (SG) is the most widely accepted measurement of potato quality because of its close relationship to solids or dry matter content and rapidity of sampling. Using SG to predict processing texture of potatoes is a common procedure in the potato industry. Potatoes with a high SG produce a higher yield and preferable texture of french fries with a lower fat content than lower-SG potatoes. Since more and more emphasis is placed on final product consistency, raw tubers supplied must meet stringent standards. Current standards for high-quality french fries prefer potato varieties to have SG between 1.080 and 1.095.
Many attempts have been made to correlate variability of SG with cultural practices and environmental conditions; variety selection has always been considered the dominant factor. Due to the variation in SG between tubers within a variety and/or lot, sampling becomes important so truckloads or storage lots can be accurately measured. A study was initiated to investigate how variety selection and growing environments affect distribution and variation of individual tuber SG. The study used the weight in air to weight in water method to determine individual SG of two new fry processing varieties (Payette Russet and Easton) plus Russet Burbank grown on research plots in five states (Idaho, Minnesota, Oregon, Washington and Wisconsin) in 2013 and 2014. There were 360 individual US No.1 tubers measured at harvest for each variety per year and location, for a total of 10,800 tubers assessed for the study.
There is a small year effect on individual tuber SG distributions of Payette Russet and RB, but a big year effect on Easton tubers (Figure 1). For Payette and Burbank in both years, there are relatively narrow and symmetric bell-shaped distributions, indicating consistent and uniform SG between tubers within each variety. The average SG of Payette Russet falls within the desired standards (1.080 to 1.095) in both years, but the average Burbank SG is lower than the standards in 2014. In comparison, for Easton, the distribution in 2013 is wider than those of Payette Russet and Russet Burbank, and the distribution in 2014 is greatly right-skewed, which suggests that there are some outliers that are much higher than the standards. The averages of Easton SG in both years are lower than the standards. With regards to environmental effects, the results demonstrate low or high SG outliers in some growing locations that cause left-skewed or right-skewed distributions.
This study indicates that some varieties are more consistent in the distribution of individual tuber SG than others as well as from one year to the next. The consistency of Russet Burbank may be due to optimal fertilization and irrigation regimes standard to Burbank, whereas cultural practices for the other varieties have yet to be optimized. The results demonstrate the inherent variability within a variety, but also the impact that growing conditions, season and cultural practices have on SG.
For more information regarding this study, please contact Yi Wang at yiw@uidaho.edu.