Potatoes: Factors affecting dry matter
Note Number: AG0323
Published: April 1995
Updated: August 2010
The selection of potatoes on the basis of dry matter (DM) content or solids is very important to most processors. Yields of French fries and potato crisps are directly related to dry matter levels.
Actual dry matter determinations can be time consuming but, since there is a very high correlation between dry matter and specific gravity, and since specific gravity (SG) readings are easily obtained, these are the usual yardstick by which quality is assessed.
Normally specific gravity readings vary from about 1.055 to 1.095. These readings correlate with 16.5% dry matter to 24% dry matter. Table 1 can be used as a ready reckoner.
Dry matter content directly influences yield of the processed product and it also affects the oil absorption rate in fried products (see Table 2). Costs of frying oil are major components in the processing costs and therefore the processor tries to obtain potatoes with optimum dry matter levels to maximise processed yield and to minimise oil costs.
Table 1. A ready reckoner for estimating dry matter (DM)
|Starch per cent||SG||DM |
Factors affecting dry matter levels
The most important thing, if growing for a processing contract, is to grow the right variety. Dry matter content varies considerably between varieties and is a strongly inherited characteristic. Irrespective of cultural conditions that can affect dry matter certain varieties are consistently high in dry matter, while others are consistently low.
Of the varieties grown in Victoria Atlantic is generally regarded as the best processing variety at present, specific gravity readings of greater than 1.090 are common. The variety Pontiac at the other extreme could be expected to have a specific gravity from 1.055 to 1.065 and it would never be considered as suitable for frying by a processor.
There are other factors that are also important in determining frying quality such as reducing sugar levels, which should be low. As a very general rule of thumb there is a correlation between sugar levels and dry matter, with varieties that have high dry matter generally being low in sugars and vice versa. Pontiac, which has low dry matter and high sugar levels, will not fry for this season.
Date of planting
Early planting helps increase dry matter content by lengthening the growth period; crops that mature early generally have higher dry matter than later plantings. The earlier the plants appear and can begin to produce solids from photosynthesis the better. However, if potatoes are planted into cold wet soils where subsequent emergence is slow then this advantage is lost. The actual date of emergence is therefore more critical than the date of planting in determining maturity and dry matter content.
The water-holding capacity, drainage, structure, fertility, and temperature of a soil can all affect dry matter. These can all affect dry matter separately, or they can work against each other with the result that they can cancel out the benefits of other factors.
For example, a sandy soil could be expected to drain better than a clay loam. In a wet season this could be an advantage and a higher dry matter may result. In a dry season yields may be lower although dry matter could still be high, but the net result would still be a low yield of dry matter per hectare.
If soil temperatures are high, dry matter can be lost through excessive respiration. In a hot season a moist soil will be several degrees cooler than a dry soil and dry matter will remain higher as a result of reduced respiration in the cooler soil.
Table 2. Effect of specific gravity on yield and oil content of potato chips
|Specific gravity||Yield of chips %||Oil content of chips %|
In general, loamy soils produce better dry matter levels than either lighter or heavier textured soils because their moisture and temperature relationships are closer to the optimum.
Soil pH is generally not regarded as having a direct effect on dry matter but can affect total dry matter per hectare by its effect on yield.
The main soil effect is that of soil moisture. Ideally there should be a uniform moisture supply right through the growth period. Excessive water, whether this be from rainfall or irrigation, will result in tubers with a low dry matter. Too much water at one time, with a long gap to the next watering, will result in misshapen, cracked tubers and possibly second growth characteristics. Excessive irrigation, or rainfall late in the season, will usually decrease dry matter. Conversely dry matter can be increased by withholding water late in the season.
As the general level of soil fertility increases the dry matter level decreases. Nitrogen levels have the most effect by promoting top growth, which if too lush can prolong the growing season with the result that the tubers may not be mature and therefore be of lower dry matter at harvest.
Results averaged over a 13-year period with three varieties indicate this decrease in specific gravity with increasing nitrogen levels quite clearly (Table 3).
Table 3. Effect of rate of application of nitrogen on specific gravity of potatoes
|Nitrogen applied kg/ha||Katahdin SG||Kennebec SG||Russet Burbank SG|
Phosphorus does not seem to have a marked effect on dry matter production; it may if anything increases dry matter.
Potassium, on the other hand, can have a significant effect. Muriate of potash (KCI) is the most commonly used form and at high rates this will decrease dry matter. This decrease is due to the chloride ion rather than the potassium itself. If potassium is applied as the sulphate dry matter levels can be increased considerably.
In Victoria a soil analysis of 150 ppm available potassium is considered sufficient for normal growth. Below this level potassium application has a marked effect on yield and dry matter. With soil availabilities of 200-300 ppm, applied potassium did not affect yield, but when it was applied as the muriate it still had a marked effect on dry matter (see Table 4).
Table 4. Reduction in dry matter with applications of muriate of potash
|Potassium kg/ha||DM reduction per cent|
|222 - 312||6.9|
The dry matter of the same variety may vary considerably from season to season in the same locality. These variations can be the result of differences in the time of planting, soil moisture, temperature, etc.
Temperature probably has one of the greatest effects. At high temperatures increased respiration rates mean that solids accumulated through photosynthesis are burnt up more quickly than they were formed, resulting in a decrease in dry matter. If night temperatures are also high this effect will be even greater.
The potato is classified as a "cool season" crop and so temperature probably has more effect on dry matter than any other single environmental factor. Continual cloud cover will decrease dry matter by reducing the photosynthetic rate, and also by the moisture effect if rain accompanies the cloud.
Increasingly efficient spraying programs have resulted in crops that have prolonged growth periods; previously these crops probably died back as a result of disease, particularly early and late blight.
If and when it becomes necessary to kill off a crop this can affect the dry matter levels. A rapid kill, whether this be mechanical or chemical, results in lower dry matter because there is no time for the transfer of foods from the tops to the tubers. This may also result in higher sugar levels along with the lowers starch levels, thus causing deterioration in fry quality as well.
It is therefore better to kill the tops slowly if possible, to maintain dry matter and processing quality.
Mature tubers are more desirable for processing and storage than immature ones. In order for the potatoes to achieve optimal maturity and reach good dry matter levels harvest should be left as long as possible, bearing in mind the comments made on time of planting and the growth period.
With late-planted crops it could be possible that low temperatures may affect fry quality; also increased soil moisture from autumn rains may lower dry matter or even cause harvesting problems. Digging should therefore not be delayed if there is a risk of this type of situation developing.
In a good storage environment dry matter levels will remain fairly constant. This is because as the potato breathes it loses weight equally from respiration and evaporation of moisture.
If badly damaged or immature tubers are stored respiration losses can outstrip evaporative losses, with a resultant decrease in dry matter. The same thing can occur if storage temperatures are too high, thereby stimulating respiration rates.
Humidity levels in storage can also affect dry matter levels.
Many factors influence dry matter levels. Although seasonal or climatic factors may often have an overriding effect, nonetheless there is a great deal the grower can do to maximise dry matter levels.
This Agriculture Note was developed in April 1995. It was reviewed by Neville Fernando, Farm Services and Tony Slater, Bio Sciences Research in August 2010.
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
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