- Effect of tuber growth
- Effect of availability of water
- Interaction between tuber growth and availability of water
- Effect of nutrition
- Effect of period between senescence / desiccation and harvest
- Changes after canopy destruction (PRG experiment)
Tuber dry matter concentration
- Tuber dry matter concentration is, effectively, the balance between starch and water is a tuber, as starch is the main constituent of dry matter.
- Cultivars differ in background dry matter concentration, see the AHDB Potato Variety Database (external website, opens in a new window).
- Patterns of changes in dry matter concentration will be the same for all cultivars, though the actual levels may differ.
- A low dry matter concentration reduces the risk of bruising but a relatively high dry matter concentration is required for processing for chips or crisps.
Effect of tuber growth
- Tuber dry matter concentration increases with increasing tuber size and weight.
- Tuber dry matter concentration increases with the duration of tuber growth.
- Tuber growth is the dominant influence on dry matter concentration, during the earlier part of the season.
- Tuber dry matter concentration may be low if temperatures are very low or high.
Effect of availability of water
- Tuber dry matter concentration may vary with availability of water: tubers may take up water under wet conditions and lose water under dry conditions.
- Dry conditions may result in high tuber dry matter concentration.
- Wet conditions (e.g. excessive irrigation) may result in low tuber dry matter concentration.
- Availability of water is the dominant influence on dry matter concentration, during the later part of the season.
Interaction between tuber growth and availability of water
- For any cultivar, tuber growth and availability of water may account for 80 – 85 % of the variation in tuber dry matter concentration.
- Tuber growth is the dominant affect in the earlier part of the season.
- Availability of water is the dominant influence during the later part of the season.
Effect of nutrition
- Nutrient deficiency may result in small tubers and thus low dry matter concentration.
- Excessive concentrations of nitrogen in soil (i.e. above the PRG optimum) or late application of nitrogen (i.e. after the onset of tuber bulking) may result in low dry matter concentrations.
- Excessive concentrations of potassium in soil (i.e. above the PRG optimum) may result in low dry matter concentrations.
- Excessive concentrations of both nitrogen and potassium in soil may result in lower dry matter concentrations than would high concentrations of either element alone.
Effect of period between senescence / desiccation and harvest
- Tuber dry matter concentration may decline between desiccation and harvest if:
- the crop is immature
- the canopy is removed or killed rapidly, before roots senesce
- soil is moist
- there is a long interval between desiccation and harvest
- Under these conditions roots may continue to take up water, which cannot be transpired by the canopy and is thus taken up by tubers (see also Desiccation and canopy maturity).
- Tuber dry matter concentration may continue to increase until natural crop senescence, if soil is dry.
Changes after canopy destruction (PRG experiment)
- After early mechanical or chemical haulm destruction, little change occurred in tuber size distribution, mean tuber size or mean tuber fresh weight.
- Similar results occurred after late haulm destruction.
- After haulm destruction, dry matter concentrations in tubers of the same size differed between treatments.
- Highest dry matters occurred in tubers from untreated plots and dry matters declined in the following order:
- late chemical treatment
- late mechanical destruction
- early chemical treatment
- early mechanical destruction
- Treatments did not influence the concentration of sucrose in tubers but early mechanical destruction of the canopy resulted in a higher concentration of reducing sugars, compared with other treatments.
- Published information indicates that:
- stopping irrigation before maturity can result in a higher dry matter concentration, compared with continuing irrigation until maturity;
- tubers can lose or gain water after canopy senescence or destruction.
More information: influences on tuber dry matter concentration are summed up in this review presentation.
Tuber composition and processing quality
- For production of chips and crisps a light fry colour is generally required.
- A dark colour results from an interaction between sugars and amino acids at high temperature in the Maillard Reaction.
- A dark fry colour results from a high concentration of sugars in tubers, particularly reducing sugars.
- Cultivars differ in sugar concentration.
- The concentration of sugars in tubers declines with tuber growth.
- Stress such as drought, extreme temperature or early haulm destruction may increase sugar concentrations.
- Sugar concentrations may decline during curing after harvest.
- Sugar concentrations may increase during storage, particularly at low temperatures.
- Low temperature “sweetening” may be reversed by a period at a higher temperature, i.e. reconditioning. More …
- Senescent sweetening occurs after long term storage and is not reversible.
- Crops which are infested with nematodes and infected with Rhizoctonia may produce tubers with uneven size distribution and thus poor colour after frying. More …
More information on sugar accumulation and changes.
- Production of chips or crisps requires a light colour after frying.
- A dark colour develops as a result of a chemical reaction between sugars and amino acids.
- Amino acids differ in their influence on colour after frying. More …
- The concentration of amino acids may be influenced by availability of zinc: zinc deficiency may result in a high concentration of amino acids and thus a dark colour after frying. More …
- Ascorbic acid may react with amino acids to influence colour after frying.
- Ascorbic acid concentration varies between cultivars and declines during storage. More …
Suggested effects of growing conditions
- Initial stages of a research project indicated that processing quality after storage may be reduced by:
- short growing season
- early desiccation
- late irrigation
- fast “pull down” rate in stores
- Effects of agronomic treatments were less than those of storage temperature: the lower the temperature the greater the decline in quality.
Taste and texture
- Dry matter concentration has been shown to be important for after cooking quality of pre-pack as well as processed potatoes:
- starch content influences texture
- Stored potatoes may be more susceptible to disintegration during cooking:
- this may be related to cell membranes becoming weaker and to water being lost
- Sulphur deficient potatoes may be less tasty
- a derivative of the amino acid methionone influences aroma, which is important for taste perception
- Cooking method influences aroma and texture of potatoes:
- microwave cooked potatoes may be relatively bland.
Notes on tuber composition and quality
Tuber quality 2020 (Includes an introduction to / review of dry matter concentration and fry quality)
Storage 2015 (Includes influence of air quality and reconditioning on fry quality)
Tuber quality review 2012 (Includes sugars and fry colour)
Tuber fry quality 2011 (Influence of nematode infestation and Rhizoctonia infection)
Tuber dry matter concentration 2010 (A review)
Potato taste and texture 2007 (Influence of cultivar, storage and cooking method)
Glycaemic Index and potatoes 2005 (The GI value depends on the whole meal, not individual constituents)
Fry colour 2002 (Interaction between sugars and amino acids; end of season management)
Haulm destruction 1999 (PRG experiment; includes effects of haulm destruction on dry matter and sugar concentrations)
Tuber dry matter 1998 (PRG experiments on nitrogen application and on canopy destruction)
Tuber fry colour 1998 (Influence of temperature, sugars and amino acids)
Tuber dry matter 1994 (Starch accumulation; tuber dry matter concentration in tubers of different sizes)
Seminar 3 Notes 1992 (Chapter 2: Sugar accumulation)