Low Temperature Sweetening Research Articles

Improvement of the color of potato chips from cold-stored tubers by pretreatment with ultrasonication.

Low-temperature sweetening is a crucial aspect in the potato chips industry because of the initiation of browning. The present study aimed to investigate the effect of ultrasonication on the concentrations of reducing sugars, sucrose, asparagine and chip color on tubers with different characteristics. Therefore, samples of the cultivars (cvs.) Nicola, Laura and Golden Wonder were used in this study. The tubers were initially stored at 4 °C and a relative humidity of 90% for 18 weeks. Ultrasonication was applied at 25 and 50 °C for 10, 20 and 30 min. The results showed that ultrasonication contributed to improve the color of the chips produced from the chip cultivar (cv. Golden Wonder), as well as the non-chip cultivars (cvs. Nicola and Laura). The concentration of reducing sugars and sucrose is the main factor for the color development of the chips. Increased temperature and extended ultrasonication duration correlate with more pronounced cell wall disruption. The present study revealed that ultrasonication could be considered as a pretreatment for production of chips from cold-stored tubers. However, it is necessary to validate the feasibility of this application in an upscaled process. © 2024 Society of Chemical Industry.

Potatoes (Solanum tuberosum L.) grown at “Patata dell'alto Viterbese” PGI have different quality characteristics and storage responses

Four pre-selected potato cultivars (Solanum tuberosum L.) from the “Patata dell’Alto Viterbese” PGI area underwent physico-chemical and enzymatic characterization to assess their suitability for specific processing line. Key chemical parameters (reducing sugar content (RSC), total phenol content (TPC), dry matter (DM), physical attributes (firmness), physico-chemical aspects (color, browning development), and enzymatic factors (polyphenol oxidase enzyme activity, PPO) were analyzed at harvest and monthly during a 6-month storage period over two production seasons (2020 and 2021). The results of one-way within-subject ANOVA and Principal Component Analysis (PCA) highlighted the superior attributes of the ‘Fontane’ cultivar for the processing industry, particularly for frying applications (e.g., chips, French fries, etc.). In comparison to other tested cultivars, ‘Fontane’ exhibited: i) the highest DM (> 200 g kg−1), ii) the lowest TPC and RSC, iii) the greatest firmness, iv) low susceptibility to browning, and v) high tolerance to Low-Temperature Sweetening (LTS). The ‘Jelly’ cultivar displayed favorable attributes for versatile use, suitable for both fresh market uses (roasting, homemade “gnocchi”, etc.) and industrial applications (fresh-cut or dried potato). It demonstrated high DM content (200 g kg−1) and firmness, the lowest PPO activity, good TPC, but higher RSC than ‘Fontane’. However, elevated RSC for ‘Jelly’ renders it less suitable for industrial frying. ‘Constance’ and ‘Gaudi’ exhibited positive traits for fresh market use such as boiling and steaming, with moderate to low DM content and firmness. Nonetheless, both cultivars displayed low tolerance to LTS, and ‘Gaudi’ exhibited the poorest processing properties, showing the highest susceptibility to browning.

Developmental and postharvest physiological phenotypes of engineered potatoes (Solanum tuberosum L.) grown in the Columbia Basin

The Innate® potato cultivars, Glaciate, Acclimate, and Hibernate are engineered for bruise and cold sweetening resistance, low acrylamide forming potential and increased Late Blight resistance. Phenotyping trials from 2015 to 2017 characterized the in-field performance, postharvest physiology, and process quality of these cultivars in the Columbia Basin of Washington. Plant growth, tuber yields and specific gravities of Acclimate and Hibernate were comparable to their respective parental cultivars, Ranger Russet and Atlantic. Glaciate exhibited lower yield and specific gravity compared with Russet Burbank. All Innate® cultivars had lower reducing sugars at harvest than their conventional counterparts resulting in superior process fry color. Asparagine (Asn) increased many-fold during development of Russet Burbank and Atlantic tubers but remained relatively low in Innate® tubers due to silenced Asn synthetase. Tuber respiration rates during sequential periods of low temperature sweetening (LTS, 4 °C), reconditioning (16 °C), and subsequent cold storage (4 °C) were largely comparable for Acclimate versus Ranger and for Hibernate versus Atlantic. Glaciate tubers, however, maintained significantly higher respiration rates than Russet Burbank tubers. Silenced invertase conferred resistance to reducing sugar buildup at 4 °C, resulting in superior process quality of the Innate® cultivars regardless of storage temperature. However, cold storage (4 °C) induced substantial increases in Suc concentration of Innate® tubers, which was then lowered 43–73 % by reconditioning at 16 °C. Subsequent storage at 4 °C had no further effect on Suc concentrations, demonstrating the efficacy of reconditioning as a management technique to reduce and minimize Suc levels during prolonged cold storage of Innate® tubers. While heat stress exacerbated the cold-induced buildup of Suc in Innate® tubers and reducing sugars in the parental cultivars, the increases in total sugar (Suc + Glc + Fru) concentrations remained equal, indicating similar elevated levels of starch catabolism during LTS of heat-stressed tubers. However, by virtue of silenced invertase, Innate® tubers were highly tolerant of heat stress for retention of process quality.

Respiratory and low-temperature sweetening responses of fresh-cut potato (Solanum tuberosum L.) tubers to low oxygen

Enzymatic browning (EB) has impeded the commercialization of fresh-cut (FC) potato. However, recently introduced Innate®-engineered cultivars with silenced polyphenol oxidase (PPO) have overcome this obstacle. As a supplement to refrigeration, low O2 atmosphere may extend the shelf-life of FC potato by reducing wound-induced respiration, associated dry matter loss, EB, and low-temperature sweetening (LTS). Determining the O2 concentration that minimizes these deteriorative physiological processes without invoking anaerobic metabolism is prerequisite to designing effective modified atmosphere packaging (MAP). Accordingly, FC tubers of cultivars Russet Burbank (RB), Ranger Russet (RR), and their Innate® counterparts, CultivATE®, GenerATE®, and GlaciATE®, were stored (4 °C) in ten O2 atmospheres ranging from 0 to 21 kPa to determine the lower oxygen limit (LOL) for aerobic respiration and effects on LTS and EB. FC tissue stored at 21 kPa O2 displayed a prominent cold-induced respiratory acclimation response (RAR), characterized by an initial decline in respiration rate followed by a steady increase through 48 h. The RAR decreased with O2 and was extinguished at ≤1.5 kPa. Lowering O2 from 21-7 kPa had little effect on tissue respiration; however, rates fell from 4.23 to 3.41 μg kg−1 s−1 as O2 decreased from 7 to 3.5 kPa, followed by a further 79% reduction to 0.70 μg kg−1 s−1 at 0 kPa O2. Tissue lactate profiles revealed the onset of anaerobic metabolism at ca 1.5 kPa O2 for all cultivars. Importantly, lactate and ethanol accumulation were negligible through 16 d at ≥2 kPa O2 (= LOL) but increased considerably at ≤1 kPa. Low O2 attenuated the cold-induced synthesis of sucrose and reducing sugars in FC tissue from RB, and sucrose from GlaciATE® tubers in which acid invertase is silenced. Enzymatic browning of FC RB and RR tubers was inhibited by anoxia, but extensive at ≥0.5 kPa O2. FC Innate® tubers exhibited only minor EB at all O2 concentrations. Collectively, these data inform the further development of MAP for FC potato.

Heat stress affects carbohydrate metabolism during cold-induced sweetening of potato (Solanum tuberosum L.).

Tolerance to heat stress for retention of low-temperature sweetening-resistant phenotype in potato is conferred by insensitivity of acid invertase activity to cold induction. Heat stress exacerbated cold sweetening (buildup of reducing sugars) of the LTS (low-temperature sweetening)-susceptible potato (Solanum tuberosum L.) cultivars, Ranger Russet and Russet Burbank, and completely abolished the resistance to cold sweetening in the LTS-resistant cultivars/clones, Sage Russet, GemStar Russet, POR06V12-3 and A02138-2. Payette Russet and EGA09702-2, however, demonstrated considerable tolerance to heat stress for retention of their LTS-resistant phenotype. Heat-primed Payette Russet and EGA09702-2 tubers accumulated fourfold more sucrose when subsequently stored at 4°C, while reducing sugar concentrations also increased marginally but remained low relative to the non-heat-tolerant LTS-resistant clones, resulting in light-colored fries. By contrast, sucrose concentrations in heat-primed tubers of the non-heat-tolerant clones remained unchanged during LTS, but reducing sugars increased fivefold, resulting in darkening of processed fries. Acid invertase activity increased in the LTS-susceptible and non-heat-tolerant LTS-resistant cultivars/clones during cold storage. However, Payette Russet tubers maintained very low invertase activity regardless of heat stress and cold storage treatments, as was the case for Innate® Russet Burbank (W8) tubers, where silenced invertase conferred robust tolerance to heat stress for retention of LTS-resistant phenotype. Importantly, heat-stressed tubers of Payette Russet, EGA09702-2 and Innate® Russet Burbank (W8) demonstrated similar low reducing sugar and high sucrose-accumulating phenotypes when stored at 4°C. Tolerance to heat stress for retention of LTS-resistant phenotype in Payette Russet and likely its maternal parent, EGA09702-2, is, therefore, conferred by the ability to maintain low invertase activity during cold storage of heat-stressed tubers.

In-season heat stress compromises postharvest quality and low-temperature sweetening resistance in potato (Solanum tuberosum L.)

The effects of soil temperature during tuber development on physiological processes affecting retention of postharvest quality in low-temperature sweetening (LTS) resistant and susceptible potato cultivars were investigated. 'Premier Russet' (LTS resistant), AO02183-2 (LTS resistant) and 'Ranger Russet' (LTS susceptible) tubers were grown at 16 (ambient), 23 and 29 °C during bulking (111-164 DAP) and maturation (151-180 DAP). Bulking at 29 °C virtually eliminated yield despite vigorous vine growth. Tuber specific gravity decreased as soil temperature increased during bulking, but was not affected by temperature during maturation. Bulking at 23 °C and maturation at 29 °C induced higher reducing sugar levels in the proximal (basal) ends of tubers, resulting in non-uniform fry color at harvest, and abolished the LTS-resistant phenotype of 'Premier Russet' tubers. AO02183-2 tubers were more tolerant of heat for retention of LTS resistance. Higher bulking and maturation temperatures also accelerated LTS and loss of process quality of 'Ranger Russet' tubers, consistent with increased invertase and lower invertase inhibitor activities. During LTS, tuber respiration fell rapidly to a minimum as temperature decreased from 9 to 4 °C, followed by an increase to a maximum as tubers acclimated to 4 °C; respiration then declined over the remaining storage period. The magnitude of this cold-induced acclimation response correlated directly with the extent of buildup in sugars over the 24-day LTS period and thus reflected the effects of in-season heat stress on propensity of tubers to sweeten and lose process quality at 4 °C. While morphologically indistinguishable from control tubers, tubers grown at elevated temperature had different basal metabolic (respiration) rates at harvest and during cold acclimation, reduced dormancy during storage, greater increases in sucrose and reducing sugars and associated loss of process quality during LTS, and reduced ability to improve process quality through reconditioning. Breeding for retention of postharvest quality and LTS resistance should consider strategies for incorporating more robust tolerance to in-season heat stress.

Tuber respiratory profiles during low temperature sweetening (LTS) and reconditioning of LTS-resistant and susceptible potato (Solanum tuberosum L.) cultivars

Potato cultivars Premier Russet, GemStar Russet, Defender and Russet Burbank differ substantially in resistance to low temperature sweetening (LTS) and associated metabolism. ‘Gemstar Russet’ and ‘Premier Russet’ have moderate and high resistances, respectively, while ‘Defender’ loses processing quality progressively during storage at 9°C, and similar to ‘Russet Burbank’, has virtually no resistance to LTS at 4°C. The different mechanisms of LTS resistance or susceptibility in these cultivars were indicated by changes in sucrose (Suc), fructose (Fru) and glucose (Glc) concentrations in relation to tuber respiratory profiles during wound healing (9°C), LTS (4°C) and reconditioning (16°C). At 4°C, ‘Premier Russet’ tubers maintained low levels of Suc and reducing sugars (RS, Glc+Fru), while ‘GemStar Russet’ tubers accumulated Suc with little inversion to RS. ‘Defender’ and ‘Russet Burbank’ tubers accumulated RS during LTS but only moderate levels of Suc. Changes in RS content reflected the combined activities of acid invertase and its endogenous inhibitor. In response to an immediate drop from 9°C to 4°C, tuber respiration decreased to a minimum and then increased to a new maximum over the next approximately 5 days, before decreasing to a constant basal rate at 4°C. Relative changes in respiration from the minimum to maximum rate during cold acclimation (respiratory acclimation response, RAR) were 80% for ‘GemStar Russet’ and ‘Defender’, 51% for ‘Russet Burbank’ and 26% for ‘Premier Russet’. The RARs correlated with total sugar (Suc+Glc+Fru) accumulation during LTS and likely reflected the metabolic energy required to catabolize starch to Suc, Glc and Fru. The relative ratio of Fru/Glc was also demonstrative of LTS-resistance, discriminating genotypes that accumulated Suc versus RS under LTS conditions. Changes in carbohydrates, invertase, respiration rates and RARs in response to temperature over the wound healing, LTS and reconditioning phases of storage characterized the LTS phenotypes unique to each cultivar, and revealed different mechanisms of resistance to LTS.

Translucent tissue defect in potato (Solanum tuberosum L.) tubers is associated with oxidative stress accompanying an accelerated aging phenotype

Translucent tissue defect (TTD) is an undesirable postharvest disorder of potato tubers characterized by the development of random pockets of semi-transparent tissue containing high concentrations of reducing sugars. Translucent areas turn dark during frying due to the Maillard reaction. The newly released cultivar, Premier Russet, is highly resistant to low temperature sweetening, but susceptible to TTD. Symptoms appeared as early as 170 days after harvest and worsened with time in storage (4-9 °C, 95 % RH). In addition to higher concentrations of glucose, fructose and sucrose, TTD resulted in lower dry matter, higher specific activities of starch phosphorylase and glc-6-phosphate dehydrogenase, higher protease activity, loss of protein, and increased concentrations of free amino acids (esp. asparagine and glutamine). The mechanism of TTD is unknown; however, the disorder has similarities with the irreversible senescent sweetening that occurs in tubers during long-term storage, where much of the decline in quality is a consequence of progressive increases in oxidative stress with advancing age. The respiration rate of non-TTD 'Premier Russet' tubers was inherently higher (ca. 40 %) than that of 'Russet Burbank' tubers (a non-TTD cultivar). Moreover, translucent tissue from 'Premier Russet' tubers had a 1.9-fold higher respiration rate than the average of non-translucent tissue and tissue from non-TTD tubers. Peroxidation of membrane lipids during TTD development resulted in increased levels of malondialdehyde and likely contributed to a measurable increase in membrane permeability. Superoxide dismutase and catalase activities and the ratio of oxidized to total glutathione were substantially higher in translucent tissue. TTD tubers also contained twofold less ascorbate than non-TTD tubers. TTD appears to be a consequence of oxidative stress associated with accelerated aging of 'Premier Russet' tubers.

Alleviation of low temperature sweetening in potato by expressing Arabidopsis pyruvate decarboxylase gene and stress-inducible rd29A : A preliminary study.

The acceptability of potatoes for processing chips and French fries is largely dependent on the color of the finished product. Most potato cultivars and varieties stored at temperatures below 9-10°C are subjected to low temperature sweetening (LTS) which result in the production of bitter-tasting, dark colored chips and French fries which are unacceptable to consumers. However, storing tubers at low temperatures (i.e., <10°C) has many advantages such as lowered weight loss during storage, natural control of sprouting, and reduction/elimination of chemical sprout inhibitors. Our earlier research results on LTS suggested a role for pyruvate decarboxylase (PDC) in LTS-tolerance. In the present study, the role of PDC was examined whereby the potato variety Snowden was transformed with Arabidopsis cold-inducible pyruvate decarboxylase gene 1 (AtPDC1) under the control of promoter rd29A. Two transgenic plants were selected and storage studies were conducted on tubers harvested from one of the transgenic lines grown under green house conditions. Transgenic tubers showed higher Agtron chip color score indicating lighter chip and lower reducing sugar and sucrose concentrations compared to the untransformed tubers during the storage periods studied at 12°C and 5°C. These results suggest that overexpression of pyruvate decarboxylase gene resulted in low temperature sweetening tolerance in the transgenic Snowden.

Sweetening responses of potato tubers of different maturity to conventional and non-conventional storage temperature regimes

Low storage temperatures stimulate the accumulation of glucose and fructose (sweetening) in potato ( Solanum tuberosum L.) tubers, resulting in unacceptably dark processed products during frying. Information on how tubers respond to unorthodox storage temperature regimes is lacking, especially for the relatively new frozen-processing cultivars, ‘Umatilla Russet’ and ‘Ranger Russet’. Such knowledge could greatly facilitate the ability to manage immature, over-mature, or otherwise stressed potatoes. This study characterizes the sweetening responses of ‘Russet Burbank’, ‘Ranger Russet’ and ‘Umatilla Russet’ tubers of different maturity, over a range of conditioning and holding storage temperatures. Tubers from early- and late-planted crops were wound-healed (12 °C) for 17 d after harvest (DAH), conditioned for a month (17–48 DAH) at 4.5, 6.7, or 9 °C, and subsequently stored (48–230 DAH) at 4.5, 6.7 or 9 °C (nine temperature regimes) for an additional 182 d before reconditioning at 16 °C for 21 d. Reconditioning hastens catabolism of reducing sugars and restoration of processing quality. On average, tubers from the early planting had lower rates of respiration during wound-healing and emerged from dormancy sooner than those planted later, indicating relative immaturity of the latter. The early-planted crop of ‘Ranger Russet’ was more sensitive to low temperature sweetening than the late-planted crop, reflecting a tendency of tubers to become over-mature and reinforcing the benefit of a ‘green’ harvest for this cultivar. All cultivars sweetened rapidly when conditioned at 4.5 °C. Conditioning at 6.7 and 9 °C, however, decreased the extent of low temperature sweetening during subsequent storage at 4.5 °C through most of the storage season, expanding the options for managing potatoes with lower than normal temperature later in storage. The processing quality of ‘Ranger Russet’ was maintained for 230 d with higher conditioning/holding temperature regimes. Reducing sugar concentrations in cold-sweetened ‘Ranger Russet’ and ‘Russet Burbank’ tubers decreased more than in ‘Umatilla Russet’ tubers in response to reconditioning at 16 °C. Following an interval of wound-healing, the use of combinations of non-conventional conditioning and holding temperatures that do not stimulate excessive sweetening broadens the management options for storing potatoes for the frozen-processing industry.

Premier Russet: A Dual-Purpose, Potato Cultivar with Significant Resistance to Low Temperature Sweetening During Long-Term Storage

Premier Russet is a russet-skinned, high yielding potato cultivar suitable for processing or fresh-pack. Premier Russet is notable for its resistance to the accumulation of reducing sugars in its tubers. This characteristic allows tubers of Premier Russet to be stored at temperatures as low as 5.6°C for 250 days without the need for reconditioning prior to processing. The low temperature sweetening resistance of Premier Russet allows for its storage at colder temperatures which contributes to prolonged tuber dormancy and quality, and to a reduction in the incidence and severity of storage diseases associated with the higher storage temperatures typically used for industry-standard cultivars such as Russet Burbank and Ranger Russet. Premier Russet was released in 2006 by the USDA-ARS and the Agricultural Experiment Stations of Idaho, Oregon, and Washington, and is a product of the Northwest Potato Variety (Tri-State) Development Program. In full-season trials conducted over a 3-year period in the states of Idaho, Oregon, Washington, Colorado, California, New Mexico, and Texas, Premier Russet averaged 5% and 4% higher yields than Ranger Russet and Russet Burbank, respectively, and had a higher percentage of US No. 1 yield than either cultivar. Specific gravity of Premier Russet is consistently greater than that of Ranger Russet or Russet Burbank. Premier Russet is resistant to common and powdery scab, very resistant to the common strain of potato virus Y (PVYO), and moderately resistant to Verticillium wilt, early blight of the foliage and tuber, Pectobacterium (Erwinia) soft rot, and to galling of roots by powdery scab. Blackspot bruise reaction for Premier Russet is similar to that of Ranger Russet. The incidence of hollow heart in Premier Russet is higher than that of Ranger Russet or Russet Burbank. Cultural recommendations to manage hollow heart in Premier Russet are provided.

Roles of alcohol dehydrogenase, lactate dehydrogenase and pyruvate decarboxylase in low‐temperature sweetening in tolerant and susceptible varieties of potato (Solanum tuberosum)

Low‐temperature sweetening (LTS) results when tubers of potato (Solanum tuberosum) are stored at temperatures below 9–10°C with the accumulation of sucrose and reducing sugars glucose and fructose. Our earlier study showed that the LTS‐tolerant varieties have higher ethanol and lactate tissue levels compared with the LTS‐susceptible variety Monona (Blenkinsop et al. 2003), which led us to investigate the role of the anaerobic respiratory pathway in LTS tolerance. The anaerobic respiratory enzymes alcohol dehydrogenase (ADH), l‐lactate dehydrogenase (LDH) and pyruvate decarboxylase (PDC) were, therefore, investigated in LTS‐tolerant and ‐susceptible potato varieties. A positive correlation (P≤ 0.05) was observed between reducing sugar concentration and the KM of PDC, with the LTS‐tolerant ND 860‐2 possessing a lower KM and reducing sugar content than the LTS‐susceptible Monona variety. The moderately LTS‐tolerant variety, Snowden, exhibited intermediate behavior between the two aforementioned cultivars at 4°C. The isozyme profile of the tolerant varieties differed from the susceptible variety. Two groups of LDH isozyme families were observed in all varieties with the exception of ND 860‐2, where the second group appeared only during low‐temperature exposure. Moreover, the tolerant variety possessed one additional ADH isozyme. Gene expression levels of these enzymes were higher in ND 860‐2 as compared with Monona at 4°C. The above results suggest that the anaerobic respiratory enzymes contribute to LTS‐tolerance.

Changes in invertase, sucrose-6-phosphate synthase and UDP-glucose pyrophosphorylase activities, and their relations to reducing sugar content in Japanese processing potato varieties stored at low temperature

Invertase, sucrose-6-phosphate synthase (SPS) and UDP-glucose pyrophosphorylase (UGPase) activities of three Japanese processing varieties, differing in susceptibility to low temperature sweetening (susceptibile : Norin-1; middle : Toyoshiro; resistant : Hokkai-82), were measured at intervals during storage at 7°C in order to investigate the role of the three enzyme activities in reducing sugar accumulation during storage. The susceptible variety dramatically increased the reducing sugar content, and invertase, SPS and UGPase activities during storage, whereas the mid-susceptible and resistant varieties progressively increased them. In the three enzymes, both invertase and SPS activities correlated significantly (p<0.01) with the reducing sugar content, implying that invertase and SPS may play an important role in the regulation of the reducing sugar accumulation of processing potatoes stored at low temperature. This information may be useful for potato breeders to develop new resistant processing varieties.

Changes in sugar content and activity of vacuolar acid invertase during low-temperature storage of potato tubers from six Japanese cultivars.

The storage of potato tubers at low temperatures leads to the accumulation of sugars in a process called "low-temperature sweetening." To understand this phenomenon, we measured the sugar contents and the activity of acid invertase over several months in tubers of six Japanese cultivars stored at 4 degrees C or 20 degrees C. At 20 degrees C, few changes in sugar contents took place in any of the tubers. On the other hand, when stored at 4 degrees C, three types of changes were observed among the cultivars: (1) increased levels of reducing sugars during storage; (2) a pattern similar to that of type 1, but with 4- to 6-fold lower levels of reducing sugars throughout storage; and (3) increased sucrose, but not reducing sugars. The activity of vacuolar acid invertase increased in the type-1 cultivars, whereas, in the type-2 and type-3 cultivars, the activities were very low during storage at 4 degrees C. Reverse transcription-polymerase chain reaction analysis of acid invertase showed that the transcripts of the enzyme accumulated in the tubers stored at 4 degrees C in the type-1 cultivars but not in type-3. These results suggest that the activity of vacuolar acid invertase is related to the types of changes that occurred in sugar content during low-temperature storage among the potato cultivars.

A proposed role for the anaerobic pathway during low‐temperature sweetening in tubers of Solanum tuberosum

Following exposure to low temperatures (i.e. &lt; 9–10°C), tubers of potato (Solanum tuberosum) undergo a phenomenon known as low‐temperature sweetening (LTS), which results in the conversion of starch to sugars. LTS in stored potato tubers was evaluated by monitoring glycolysis, the oxidative pentose phosphate pathway, anaerobic respiration and mitochondrial respiration, in order to obtain an overview of the regulation of sugar metabolism during LTS. Carbohydrate metabolism during 4°C storage was evaluated in four different storage seasons, comparing tubers of cold‐tolerant (low sugar‐accumulating) clones (ND 860‐2, V 0056‐1 and Wis 1355‐1) and cold‐sensitive (high sugar‐accumulating) cultivars (Novachip and Monona). Parameters measured included whole tuber respiration, concentrations of sucrose, reducing sugars, ATP, NADH, NADPH, lactate and ethanol, as well as tissue pH. In three of the four seasons of study, the cold‐tolerant tubers maintained lower levels of reducing sugars, and higher levels of lactate and ethanol, relative to cold‐sensitive tubers. A higher anaerobic flux in tubers of cold‐tolerant genotypes may be partially responsible for a lower accumulation of reducing sugars relative to cold‐sensitive tubers. This study represents the first known report on the role of the anaerobic pathway during LTS in potato tubers.

Sprouting rate, change of carbohydrate contents and related enzymes during cold treatment of lily bulblets regenerated in vitro

This study was performed to investigate the effect of low temperature storage on the dormancy of in vitro bulblets of Lilium Oriental hybrid ‘Casablanca’ and Lilium Asiatic hybrid ‘Mona’. All bulblets regenerated in vitro remained dormant, irrespective of cultivar, if they are not subjected to cold treatment. Physiological dormancy was released more rapidly and the highest percentage of sprouting occurred after a 9-week cold treatment at 4 °C, as compared with 25 and 10 °C. Exposure to low but non-freezing temperatures induced the net breakdown of starch and the accumulation of sucrose and glucose in lily bulblets regenerated in vitro (low temperature sweetening). Bulb sweetening was especially pronounced at 4 °C. Analysis of carbohydrate types revealed that sucrose was the dominant soluble carbohydrate in bulblets regenerated in vitro, with glucose and fructose present at significantly lower concentrations. Starch-degrading enzymes α- and β-amylase exhibited higher activities in both cultivars after cold treatment. Varying level of sucrose phosphate synthase, sucrose synthase and invertase activities were also observed after cold treatments.

Sugar Accumulation as a Mechanism for Freezing Tolerance in Cold-Stored 'Danshaku' Potato Tuber (Solanum tuberosum L.)

Tissue samples from cold-stored (0°C) 'Danshaku' potato tuber which underwent low temperature sweetening (LTS) were analyzed by differential scanning calorimetry (DSC) using a heating rate of 5°C /min. Results showed that LTS significantly lowered the onset temperature of melting. Tubers stored at 10°C did not sweeten and showed insignificant reduction of onset temperature. Total sugar levels were significantly related to Tp, To and Te while off was significantly related to moisture content of the sample. The primary endotherm (average To=-3. 86°C) representing extracellular melting of water was observed for tubers stored at 10°C and 0°C ; a secondary endotherm representing freezing of intracellular water was observed in 41.18% of tissue samples from tubers stored at 10°C (average To=-3.22°C). Only 11.11% of tissue samples from tubers stored at 0°C showed a secondary endotherm (average To=-12.20°C), implying that high sugar levels reduced lethal intracellular freezing in cellular compartments. Hence, sugar accumulation induced by cold conditions may be a mechanism to delay freezing and reduce its harmful effects on cell compartmentation and function.

472 Effects of Low-temperature Storage on Carbohydrate Metabolism in Potato Tubers

Following exposure to low temperatures (i.e., &lt;10 °C), potato tubers undergo low-temperature sweetening (LTS), the conversion of starch to sugars. This phenomenon is of great importance to potato chip processors because high levels of reducing sugars lead to undesirable nonenzymatic browning during potato chip frying operations. The purpose of this study was to elucidate the biochemical differences in carbohydrate metabolism between a tolerant (ND 860-2) and a sensitive (Novachip) cultivar during 4 °C storage. On chilling, there was an increase in the levels of sucrose, fructose, and glucose in both cultivars, with levels being at least 2-fold higher in the sensitive cultivar. Increased levels of ATP and NADH, along with a higher respiratory rate observed in the tolerant tubers, collectively indicate a higher metabolic rate in the LTS-tolerant cultivar. ATP- and pyrophosphate-dependent phosphofructokinase activity was similar in both cultivars. Higher levels of ethanol and lactate were also observed in ND 860-2, suggesting a greater flux of sugars via anaerobic respiration. No significant differences were observed in enzymatic activities in the oxidative pentose phosphate pathway (PPP) or in levels of NADPH, thereby suggesting that the PPP does not play a role in conferring LTS tolerance. Therefore, we propose that LTS-tolerant potatoes may maintain low tissue sugar concentrations via an overall increased metabolism, rather than differing in one specific metabolic step. This increased metabolic rate does not appear to be due to greater enzyme expression (i.e., coarse control) but, rather, to a greater overall flux of carbohydrates through glycolysis and respiration.

Kinetic model for carbon partitioning in Solanum tuberosum tubers stored at 2°C and the mechanism for low temperature stress-induced accumulation of reducing sugars

Exposure to low but nonfreezing temperatures induces the breakdown of starch and the accumulation of sucrose, glucose and fructose in potato tubers, a complex phenomenon known as low-temperature sweetening (LTS). A kinetic model for the degradation of starch to sucrose, fructose, glucose, hexose phosphates and carbon dioxide in 2°C-stored mature Solanum tuberosum cv. Norchip (LTS-sensitive) and Solanum tuberosum seedlling ND860-2 (LTS-tolerant) tubers is presented in this work. Analysis of sugar accumulation data in tubers grown in 1993 and 1994 showed no significant differences in the rates of conversion of starch to hexose phosphates and hexose phosphates to sucrose for both cultivars ( P > 0.05). The rate constant corresponding to invertase activity was 2.3 day −1 for Norchip tubers and 1.1 day −1 for ND860-2 tubers grown in 1993 ( P ≤ 0.05); however, no significant differences were observed in invertase activity for 1994-grown tubers ( P > 0.05). The accumulation of the reducing sugars fructose and glucose was found to be dependent on the relative difference in rate constants corresponding to invertase activity and glycolytic/respiratory capacity. This difference was 3–4 fold greater for Norchip in 1993, and 4–6 fold greater for Norchip in 1994, than for ND860-2 ( P ≤ 0.05). Results from the analysis also suggest that the amount of available starch for degradation was greater in Norchip tubers than ND860-2 tubers ( P ≤ 0.05). Our analysis suggests that tubers with decreased invertase activity coupled to increased glycolytic/respiratory capacity should be more tolerant to low-temperature stress.

The relationship of chip color with structural parameters of starch

Seven potato cultivars (Norchip, Monona, Onaway, Snowden, Atlantic, Saginaw Gold, Novachip) and the seedling ND860-2 differing in their susceptibility to low temperature sweetening were analyzed to determine if starch structural stability parameters could be used to consistently predict and/or monitor chip color (Agtron score) of potatoes during storage at 4 and 12 C. Fresh tissue gelatinization temperature (Tp) was significantly correlated (P≤0.05) with chip color for all cultivars except Onaway, Snowden and Atlantic for tubers harvested in 1993 when stored at 12 C. No correlations existed between Tp of isolated starch and chip color (P>0.05). In 1994 no correlations existed between Tp of fresh tissue, Tp of starch and chip color at either 4 or 12 C (P>0.05). Prediction of chip color based on starch stability parameters was not possible. Starch crystallinity, amylose content and starch granule size distribution could not be used to predict or monitor chip color in stored tubers.