USING PENETROMETER DATA IN SUGAR BEET VARIETY SELECTION
This post summaries the current information around how using data from a penetrometer could inform decisions around variety selection in sugar beet, including the particular cautions that need to be observed.
The data is informative, can be cheap to obtain, but does not provide a complete picture.
The exact terminology used in the food sciences can vary a little from that used in material sciences. This summary draws from the food sciences. In the food sciences, both forces and pressures can be reported. This summary will mainly focus on pressures. There is also a tendency to use the collective terms “mechanical properties” and “textural properties” interchangeably. “Textural properties” come from the food sciences and the connection to how food feels in the mouth. “Mechanical properties” is used here, owing to our reluctance to eat sugar beet.
From a laboratory penetrometer numerous mechanical property metrics can be measured. Using a small diameter plunger (2mm):
- Penetration resistance: resistance pressure at the point the plunger penetrates the periderm. The peaks in Figure 1- the dark blue curve has the lowest Penetration resistance.
- Tissue firmness: the average resistance pressure from approximately 0.5mm after penetration through to 5mm after the initial plunger contact with the beet surface. The average of the long plateau after the peaks in Figure 1 – the red curve has the lowest Tissue firmness.
- Apparent modulus of elasticity: The slope of the pressure-distance curve between initial contact and the point of penetration. The red curve has the lowest Apparent modulus of elasticity in Figure 1, even though its Penetration resistance is similar to the brown and light blue curves..
Early literature on the mechanical properties of sugar beet often used large diameter plungers, like those traditionally used in softer fruits (ca. 8 or 11mm). After Kleuker and Hoffmann (2019), it became standard to use a 2mm diameter plunger in the above tests.
The laboratory penetrometer can also be used to test compressive strength of beets. Given their variability in size, and the very high loads need to cause damage to a sugar beet root, compression tests are usually carried out on cores taken from across the beet material.
From a handheld penetrometer, it is only possible to measure the single metric of maximum pressure over the distance the penetrometer is inserted. This metric is commonly termed Firmness in the food sciences, but is here referred to as Handheld pressure to avoid confusion with the laboratory metrics. This is usually equivalent to the Pentration resistance, although it has been shown that Handheld penetration has stronger correlations with Tissue firmness.
CONNECTION TO STORAGE LOSS
The connection from textural properties to the rate of loss of processible sugar during storage is through damage during harvest and handling. The links are quite clear, and the statistical associations strong.
PRACTICALITIES OF TESTING
Sample method: The method for the application of the laboratory penetrometer is outlined in Kleuker and Hoffmann (2019), and the method for the application of the handheld penetrometer is outline in English et al (2022).
The laboratory equipment is more accurate/ controlled and gives better data. The handheld equipment is reliable and much lower cost.
Sample size: There is a relatively high level of variation between individual sugar beets. In a block design experiment, with four replicates, at least 6, preferably 15 observations from individual beets are needed. With the laboratory equipment, this would still require around 4 trials (so 16 plots per treatment) to find difference of around 0.10MPa as significant. From 18 varieties tested in Sweden in 2020, average Handheld pressure resistance ranged from ca. 5.85 to 6.75MPa. With the handheld penetrometer, 6 trials (or 6 reps x 4 trials = 24 plots) would be needed.
Time: The laboratory penetrometer has a sampling rate of around 105 samples per hour, with additional time required for harvest, transport, cleaning, disposal, and other handling of the material. The handheld penetrometer has a sampling rate of around 150-180 samples per hour, with only additional time for travel required.
Timing: The laboratory penetrometer is applied post harvest. The handheld penetrometer can be applied both pre- and post-harvest. The relative ranking of mechanical properties has been shown to be stable from at least August to November within Northern Europe. If applied in-situ, the handheld penetrometer can be used at least from 2 months prior to a normal harvest time, with the size of the beets and access to a sampling point being the limiting factor. Given this, it is lower risk to apply the handheld penetrometer in-situ in the weeks prior to harvest.
THE REST OF THE PICTURE
Penetrometers are a psuedo-static test. The impacts sugar beets experience that cause damage are dynamic. Some initial work has been done to look at the connection between static properties and damage during dynamic impacts. In general, the associations are strong. From the laboratory penetrometer, Tissue firmness and Apparent modulus of elasticity seem more strongly associated with damage from dynamic impacts. Handheld pressure is also well associated with dynamic damage.
The correlation between mechanical properties and loss during storage is not 1. Other processes are at play. Respiration is the major source of sugar loss, and while it is effected by rates of damage and the endogenous factors suggest respiration and mechanical strength should be correlated (high cellular sugar concentration needs strong cells and means there is more sugar to use in respiration), there are varietal differences that are independent of mechanical properties. Early testing suggests that mechanical properties are stronger indicators of likely storage losses, but that respiration does seem to explain an independent/ additional fraction of total storage losses.
There are other factors we know impact storage losses, such as high disease loads in a field, high rates of liming, or high water availability during the warmer months of the season. Most of these factors, however, should be relatively stable at a field level, so are not of concern in the measurement of mechanical factors.
G. Kleuker and C. M. Hoffmann (2019) Method development for the determination of textural properties of sugar beet roots, Zuckerindustrie/ Sugar industry, Vol. 144 Issue 7 Pages 392-400. DOI: 10.36961/si23306