A non-destructive method for rapid evaluation of boiling quality of potato tubers

A non-destructive method for rapid evaluation of boiling quality of potatoes has been developed. The method is based on the degree of sloughing, after-cooking darkening and colour of tuber fractions (cores of 1.3 cm in diameter) cooked in test tubes at 118°C for 25 min. Using sixteen potato cultivars, with 20–30% dry matter content, boiling quality results obtained by the new method were compared to sloughing and after-cooking darkening values obtained by boiling quartered tubers in aluminum sauce pans on a standard electric stove. The correlation coefficient between sloughing data obtained by the two methods was 0.912 (P< -0.001). The new method has proven to be simple, reproducible and highly suitable for evaluating a large number of potato samples. This should make it attractive for routine use by potato breeders.     

Prevention of after-cooking darkening of irradiated potatoes

Summary Potatoes of the variety ‘Kufri Chandramukhi’ exposed to 10 krad of gamma rays for sprout inhibition, tended to develop after-cooking darkening, after storage at 15°C, for over three months. The darkening could be prevented or reduced (a) by pre-peeling of tubers prior to cooking: or (b) by reconditioning the potatoes by keeping them at 34–35°C for 4–6 days; or (c) by soaking and cooking the tubers in solutions of diaminoethane-tetra-acetic acid disodium salt (EDTA) or citric acid. Prepeeled tubers cooked in the presence of the peel tissues or added phenolics like caffeic and chlorogenic acids showed darkening. The results implicate that this could be due to the interaction of phenolics with Fe⁺⁺ present in the tissues. The storage of irradiated tubers at low temperature also showed enhanced disapperance of carotenoids.     

Determination of disodium acid pyrophosphate in French-fried potatoes

An analysis procedure was developed to determine residual disodium acid pyrophosphate (SAPP) in French-fry strips after treatment. The uptake of SAPP in relation to concentrations and temperatures of the treating solution, lengths of treating time and loss of P₂O₇ from the finish-fried French fries are also considered.     

Relation of potato size to after-cooking blackening tendency

An observation made in the laboratory led to a two-year study showing a relationship between after-cooking discoloration and potato size. This relationship, the larger the potato the greater the blackening tendency, was consistent for all potatoes examined in this sudy whether they blackened severely or slightly. These findings may help future researchers correct the conditions that cause blackening.     

The Combined Effect of CO2 and Ethylene Sprout Inhibitor on the Fry Colour of Stored Potatoes (Solanum tuberosum L.)

Recently, it has been shown that the darkening of potato processing colour attributable to a trace concentration of ethylene gas is more severe when CO₂ is also elevated. In view of the increasing use of ethylene gas for sprout suppression in potato storage facilities, it was considered important to determine whether this effect also occurs at the much higher ethylene concentration used in commercial practice. Sprouting and processing colour of the French fry cultivars Russet Burbank, Shepody and Innovator and the potato crisp cultivar Dakota Pearl were tested during the November to June storage season of two consecutive years. Treatments were 0 or 2 kPa CO₂ and 0 or 10 μL?L^?1 ethylene in a factorial design, plus a chlorpropham check. The 0 CO₂?+?0 ethylene treatment constituted an untreated control. The ethylene exposure was commenced abruptly to maximize its effect on colour. The main effect of ethylene resulted in darker processing colour in all cultivars, whereas darkening attributable to the main effect of CO₂ was observed only in Innovator and Dakota Pearl. The statistical interaction of the CO₂ and ethylene was not significant except in Dakota Pearl Hunter a (redness) scores, although a tendency to darker colour when both gases were present was seen in Russet Burbank and Innovator at all evaluation dates. The results indicate that both gases can affect processing colour when ethylene is used to control sprouting, although considerable variability in the response exists among cultivars. This variability in combination with management of storage conditions such as temperature and CO₂ can be utilized to minimize the impact of these gases on the processing colour of stored potatoes.     

Potato after-cooking darkening

After-cooking darkening (ACD) is one of the most widespread, undesirable characteristics of cultivated potato. With the current expansion of the potato-processing industry around the world, there is a renewed interest in the development of new ways to prevent ACD. After-cooking darkening is caused by the oxidation of the ferri-chlorogenic acid in the boiled or fried potatoes. The severity of the darkening is dependent on the ratio of chlorogenic acid to citric acid concentrations in the potato tubers. Higher ratio normally results in darker tubers. The concentration of the chlorogenic and citric acids is genetically controlled and influenced by environmental conditions. This paper outlines the history of ACD and current status of knowledge of the chemistry of the dark pigment formation and its genetic and environmental determinants. Also discussed are the methods of chemical prevention of ACD presently used by the potato-processing industry and potential strategies for reducing tuber after cooking darkening using molecular approaches.     

Photo-induced changes in the concentrations of individual chlorogenic acid isomers in potato (Solanum tuberosum) tubers and their complexation with ferric ions

Summary Changes in the concentrations of 3-, 4- and 5-caffeoylquinic acids in potato tubers exposed to light have been determined by high-performance liquid chromatography. In the first 24 to 48 hours the observed increases in total chlorogenic acid content was due primarily to an increase in 5-caffeoylquinic acid content but thereafter the rate of accumulation of the other isomers increased gradually. After 24 hours exposure 4-caffeoylquinic acid accounted for 10% of the total chlorogenic acid content of the tubers compared with 33% after 168 hours. The significance of this change in isomeric ratio on the spectral characteristics of potential ferrichlorogenic acid complexes was investigated in vitro. It was concluded that potato quality, as reflected in the development of after-cooking blackening, was dependent on total chlorogenic acid content and was unaffected by the relative concentrations of the individual isomers.     

Effect of Growth Regulators and Ethanol on Termination of Dormancy in Potato Tubers

The main objective of this study was to find the best practice of inducing the sprouting of dormant potato tubers. We compared two protocols of breakage of dormancy, which are based on dipping excised potato eyes in an aqueous solution of gibberellic acid (GA₃) and kinetin (standard 1) or in the aqueous solution of GA₃, thiourea, and daminozide (standard 2), with a newly reported approach based on ethanol. We tested the effect of ethanol alone or in combination with GA₃ and/or kinetin on dormancy release and sprouting of the potato tubers. As a model, we used two potato genotypes (cultivars Pasat and Dorota), with long dormancy of 5 and 10 weeks respectively. We showed that the standard 2 was the most effective treatment both for dormancy breaking and in promoting sprout growth, especially for cv. Dorota, for which the treatment induced 82.3% of tuber eye-plugs to sprout 28 days after treatment and to produce 93.2% of emerged plants after subsequent 28 days of cultivation in the greenhouse. For this cultivar, similar efficacy was observed for the combination of 4% ethanol with GA₃ and kinetin. The same concentration of ethanol combined with GA₃ but without kinetin was the most efficient treatment for breaking dormancy of cultivar Pasat. However, the difference between the various treatment combinations was statistically insignificant. Ethanol alone or in combination with kinetin poorly induced breakage of dormancy, confirming the main role of GA₃ in artificial dormancy breaking. Thus our study showed that the standard 2 is the most effective approach for breakage of dormancy at least with long term-dormancy cultivars.     

Some effects of plant spacing on potato quality

Summary As plant population was increased so the dry matter content of the tubers increased, but delaying harvest had little such effect. Tubers of 2–3 cm (or 2.5–3.0 cm) had a lower dry matter content than tubers of 4.5–5.5 cm size at any one harvest. Increases in dry matter content were associated with increases in the disintegration of the tubers when cooked. The N content of the tubers was influenced by population, and although N content decreased and after-cooking blackening decreased with increasing plant population, after-cooking blackening was better related (inversely) to the K/N ratio of the tubers than to N content. Tubers from the later harvests had higher N, lower K and lower K/N ratio than those of a similar size at the earlier harvests. Published effects of fertilizer application on quality are related to similar effects obtained by varying the plant population.     

Enzymatic Peeling of Potato: A Novel Processing Technology

Development of an innovative biotechnological method for potato peeling, closer to the ‘ideal’ peeling conditions of the product, was the main objective of the present research. Studies on enzymatic hydrolysis of potato peel were conducted. The efficacy of enzymatic peel hydrolysis was expressed in terms of reducing sugar content of the enzyme solution in which potato peel was incubated. Enzyme screening revealed that an enzyme solution containing a cellulase-xylanase mixture and amylase in a ratio of 1:1 showed good peel hydrolyzing efficiency of peeled peels. To further maximise the peel hydrolysis, condition parameters were optimised using response surface methodology (RSM). At the optimised conditions of 60 °C, pH 6 and 4 h, the reducing sugar yield in the solution was maximum. Characterization of the potato peel using microscopic techniques (scanning electron microscopy (SEM), atomic force microscopy (AFM)) further illustrated degradation of cell wall and reduction in the surface roughness of the peel after enzymatic treatment, which could enhance peel loosening from the intact potato. To further ascertain the efficiency of the process, studies were conducted with the selected enzyme on intact potato tubers under optimised conditions. Easy removal of peel was observed in enzyme-treated potato tubers, which showed 0.52% peel loss by abrasive peeling. The present process employing enzymes could be applied for peeling of intact potato as an alternative to conventional peeling process.     

Using Ethylene Gas and Chlorpropham Potato Sprout Inhibitors Together

Marketplace preference for lower pesticide residues in foods has led to research to reduce the residue of chlorpropham (isopropyl N-3-chlorophenyl carbamate; CIPC), a postharvest-applied sprout inhibitor which is widely used around the globe to prevent sprouting of stored potatoes (Solanum tuberosum L.). Ethylene gas, an effective, safe and non-toxic sprout inhibitor used in several countries, sometimes has negative effects on the colour of processed potato products when used alone. Trials were conducted over 3 years using cv. Shepody (French fry) and cv. NorValley (potato chips/crisps) to determine whether a combination of these two sprout inhibitors, at reduced dosages, could inhibit sprouting while maintaining good processing colour. CIPC applied at 0, 0.1, 0.25 and 1.0 times the recommended dosage was combined with 4 μll⁻¹ of ethylene gas applied or no ethylene at all (0 ethylene), for 1 day in 4 days, for 1 day in 2 days or continuously, in a factorial design. Sprout inhibition in both cultivars was excellent at all levels of CIPC application except the 0 rate. In both cultivars, sprouting was inhibited by the continuous ethylene treatment. However, all levels of ethylene exposure except the 0 rate negatively affected processing colour in both cultivars. The darkening was dose dependent, whereby the colour was darkest in continuous ethylene and was less affected by the intermittent exposures. In continuous ethylene, the colour was progressively lighter during storage after initial darkening. Shepody tubers appeared to be more sensitive to ethylene than the NorValley tubers. In Shepody only, colour in the ethylene of 1 day in 4 days treatments was progressively darker with increasing time in storage.     

Soft-rot resistance combined with other tuber characters in long day-adaptedSolanum phureja

Summary Offspring were produced from a cross between two long-day-adaptedSolanum phureja clones which carried resistance to tuber soft rot (Erwinia carotovora subsp.atroseptica). In tests carried out on the produce of field-grown plants raised from tubers, over fifty per cent of the 173 offspring were found to be highly resistant. Assessments were also carried out of tuber yield, mean tuber weight, tuber number, shape, regularity, flesh colour, texture of the steamed flesh, fry colour, after-cooking blackening, sprout length after storage and overall dormancy. There were statistically significant differences between clones for all characters (P<0.001). Twelve of the clones were selected on the basis of high resistance, yield, tuber weight, regularity of shape and absence of after-cooking blackening. The value of resistant long-day-adapted diploid material for commercial breeding is discussed.     

Survival and development of Colorado potato beetles on potatoes treated with phosphite

Phosphite is a general term used to describe the salts of phosphorous acid H₃PO₃. It is effective in suppressing a number of plant diseases caused by oomycetes and has been shown to reduce populations of several insect species. We investigated the effects of phosphite on the Colorado potato beetles in the field and laboratory. Beetle numbers and defoliation on phosphite-treated plots were lower compared to the control plots during one out of two years of the study. No phosphite effects were detected in the field during the second year of the study. However, larval mortality was significantly higher the second year in the laboratory when larvae were fed on potato foliage excised from the potato plants treated with phosphite in the field. Laboratory tests with excised leaves dipped in a solution of phosphite revealed lower beetle survivorship and prolonged development on the treated foliage. Because of its dual properties as a fungicide and an insecticide, as well as its low toxicity to vertebrates, phosphite is a potentially good fit for integrated pest management programs.     

Ascorbic acid-ferrous iron (Fe++)complexes and after cooking darkening of potatoes

A complex of ascorbic acid plus ferrous iron (Fe^{+ +}) forms a purple pigment similar to the after cooking darkening pigment (ACD) in potatoes. The ascorbic acid-Fe^{+ +} complex is similar to the ACD pigment in the following ways:

1. Both require O₂ in an oxidation reaction to form the final pigment.
2. The color changes with pH are similar.
3. Strong chelating or complexing agents such as citric acid, EDTA and sodium acid pyrophosphate will cause loss of color.

The ascorbate-Fe^{+ +} complex is much less stable than the chlorogenic acid-Fe⁺⁺⁺ complex, which is the currently accepted cause of after cooking darkening.     

Relationship of harvest sucrose content to processing maturity and storage life of potatoes

Potatoes capable of reaching low sucrose levels prior to harvest demonstrate superior processing maturity for chipping directly from intermediate temperature storage (11.7C, 53F). Forty-nine of fifty-three potato clones with a harvest sucrose rating (SR= mg sucrose/g tuber) between 1.0 and 2.8 chipped long term from storage (7.0 to 11.0 months). Eleven of twelve clones with an SR greater than 2.8 accumulated reducing sugars at a rapid rate and showed short term storage potential (0.5 to 5.3 months). A correlation coefficient of -0.845 was obtained by comparing the harvest SR value to the log₁₀ of storage life with all clones sampled over two growing and storage seasons. A standard sucrose rating (SR) procedure is described to rapidly detect annual variation in processing potato maturity to aid in determining proper post-harvest storage and utilization practices.     

Effects of water stress on photosynthesis and chlorophyll fluorescence of five potato cultivars

Summary Reduction of leaf photosynthesis due to water stress has been analyzed into various components and genetic variation in these components has been evaluated. Five potato cultivars were grown on nutrient solution in a conditioned glasshouse. Water stress was imposed by adding polyethylene glycol to the nutrient solution. Photosynthesis, transpiration and chlorophyll fluorescence were measured on intact leaves during the stress period and after recovery from the stress. Water stress reduced photosynthesis, initially as a consequence of stomatal closure, but after 3 days increasingly by inhibiting directly the photosynthetic capacity (mesophyll limitation). Stomatal closure correlated with the reduction in photosynthesis, but it was not the sole cause of this reduction because the internal CO₂ concentration in the leaves was not affected by water stress, indicative of inhibitory factors other than stomatal ones. Chlorophyll fluorescence emission suggested that the Calvin cycle was inhibited, while quantum efficiency was not affected at 17°C. Increasing the temperature to 27°C reduced quantum efficiency but only in the stress environment. The recovery of young leaves after relief of the stress was associated with a lower stomatal conductance but a higher mesophyll conductance compared with the control, which caused a low internal CO₂ concentration and probably invoked photo-inhibition and leaf damage. Cultivar differences in photosynthetic rate were highly significant under both optimal and stress conditions, and corresponded with differences in mesophyll conductance.     

Influence of IAA,NAA and 2,4-D on ethylene production by potato discs (Solanum tuberosum L. cv. Red Pontiac)

Various auxins induced an increase of ethylene production by potato discs (Solarium tuberosum L. cv. Red Pontiac). 2,4-Dichlorophenoxyacetic acid (2,4-D) at 10 μM was the most effective in stimulating ethylene (C₂H₄) production. Napthaleneacetic acid (NAA) and indole-3-acetic acid (IAA) also stimulated ethylene production with optimal concentrations of 50 μM and 100 μM, respectively. The periderm of the tuber was found to produce the highest levels of C₂H₄ with sections taken toward the center of the tuber producing much lower levels. In this system, it was shown that pH 4.0 was the optimum pH tested and methionine was required.     

Color of single-screw extruded blends of soy-sweet potato flour – A response surface analysis

Blends of defatted soy flour and sweet potato flour were extruded in asingle-screw extruder run at varying pre-set rotational speeds. Diediameter of the extruder was also varied. A central composite, rotatablenearly orthogonal response surface design was used in studying theinteractive effects of extrusion variables on color of the extrudates.Color of raw and processed samples was determined on a DRLANGE TricolorLFM3 instrument. Measurements were made in duplicates to obtain the CIELABL* a* b* values. Results showed that whiteness (L*)decreased with increase in sweet potato in the blends during extrusion.Reduction in whiteness (darkening) evidenced in decreased L* valuesof samples was only affected by increase in sweet potatoes in the blends.Redness (*) significantly increased as sweet potatoes contentincreased, which confirmed the decrease in whiteness. Yellowness(*) also increased considerably, but as a result of the diediameter.     

Effects of soil characteristics on the relationship between potato cyst nematodes and yield. II. Acidity (soil pH)

Summary The sandy and sandy peat soils of the northeast of The Netherlands are mineralogically very poor and acid. Independently of potato cyst nematode infestations, there is a considerable negative effect of increasing soil pH on potato tuber yield. Potato cyst nematode infections aggravate the effect of soil pH in depressing yield. It was shown that both potato cyst nematode infection and increasing soil pH values reduce the buffering capacity of the root system. Testing for yield as well as for tolerance to potato cyst nematode infection should be made at a range of soil pH values, similar to those found in the target area. This refers to active breeding programmes as well as to the selection of cultivars for yield trials.     

A rapid method for the quantification of steroidal glycoalkaloids by reversed phase HPLC

Summary Methods were developed for clean-up of potato tuber extracts on solid phase extraction CN-cartridges and for analysis of steroidal glycoalkaloids by reversed phase HPLC. The alkaloids α-solanine, α-chaconine, α-solasonine and α-solamargine could be separated on C₈ or C₁₈ reversed phase columns using a mobile phase of acetonitrile/Tris-buffer (3∶2, v/v). The analysis time of tuber extracts containing these alkaloids was less then 4 min, if the pH of the mobile phase was adjusted to 7.4 and run at a flow rate of 0.5 ml/min.