Potato genotype differences in nutritionally distinct starch fractions after cooking, and cooking plus storing cool

Journal of Food Composition and Analysis 22 (2009) 539–545

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Original Article

Potato genotype differences in nutritionally distinct starch fractions after cooking, and cooking plus storing cool John Monro a,*, Suman Mishra a, Esther Blandford a, John Anderson b, Russel Genet c a

New Zealand Institute for Crop & Food Research Ltd, Private Bag 11 600, Palmerston North, New Zealand New Zealand Institute for Crop & Food Research Ltd, Cronin Rd, RD 1, Pukekohe, New Zealand c New Zealand Institute for Crop & Food Research Ltd, Private Bag 4704, Palmerston North, New Zealand b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 3 March 2008 Received in revised form 21 November 2008 Accepted 24 November 2008

Rapidly digestible (RDS), slowly digestible (SDS) and resistant starch (RS) were measured in 9 New Zealand supermarket potatoes and in 37 lines from a potato breeding program by in vitro digestion immediately after cooking, and after storing at 4 8C for 44 h post-cooking. The aim was to measure the range in the tendency to form SDS and RS in the potato gene pool in New Zealand. Immediately after cooking, the potatoes contained (mean and across-cultivar range, dry matter basis) 68% RDS (range 62– 73%), 3% SDS (range 0–8.5%), and 3.9% RS (range 3–6.4%). Cool storage after cooking altered the distribution and ranges to 44% RDS (range 33–53%), 23% SDS (range 15–34%) and 7% RS (range 4.7– 15.8%). There was no significant relationship between RS and SDS in the cooked-cooled potatoes. In the 37 potato lines, SDS ranged from 7 to 37% of total starch, RS from 12 to 27% of total starch after the postcooking cool treatment. The results suggest that the glycaemic impact of some potatoes may be substantially reduced by cool-storing after cooking, and that the differences between cultivars in the tendency to form cold-induced SDS and RS are sufficient for these traits to be used in conventional plant breeding. ß 2008 Elsevier Inc. All rights reserved.

Keywords: Potato Solanum tuberosum L. Potato cultivar Starch Digestion In vitro Glycaemia Glycaemic index Slowly digested starch Resistant starch Biodiversity and nutrition Food composition

1. Introduction Potatoes (Solanum tuberosum L.) are an important source of carbohydrate energy throughout the world. However, in the context of the global incidence of the metabolic syndrome, marked by glucose intolerance and Type 2 diabetes, and the myriad of medical conditions that are long-term complications of diabetes (Brownlee, 2001), potatoes have become less favored, because their available carbohydrate is typically of high glycaemic index. There is growing evidence that obesity (Livesey, 2005), Type 2 diabetes (Jenkins et al., 2002) and the atherosclerotic complications of diabetes are promoted by postprandial blood glucose surges, due to both glycation and the insulin response (Brownlee, 2001) and ensuing damage to the arterial endothelium. The need to find carbohydrate staples of reduced glycaemic impact is therefore becoming increasingly urgent. As well as containing highly glycaemic carbohydrate, potatoes provide little of the non-digestible polysaccharide (dietary fibre) which may play an important role in maintaining large bowel

* Corresponding author. Tel.: +64 6 355 6137; fax: +64 6 351 7050. E-mail address: [email protected] (J. Monro). 0889-1575/$ – see front matter ß 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jfca.2008.11.008

health (Miller Jones, 2004) and reducing the risk of colorectal cancer, a common form of cancer second only to breast cancer in prevalence in New Zealand (Keating et al., 2003). The rate of glucose release from starch during digestion is a major factor determining both the glycaemic potency of starchy foods (Ells et al., 2005), and the colonic loading of resistant starch (RS) due to intrinsic unavailability and/or ‘‘potential availability’’ that is not realized in the time available during gut transit in humans. Recent studies have shown that cool storage of potatoes after cooking results in a decrease in starch digestibility, marked by an increase in both slowly digested starch (SDS) and in digestionresistant starch (Leeman et al., 2005). RS is in its own right valuable as a form of prebiotic carbohydrate fibre that may protect against cancer of the colon (Bird and Topping, 2001). Therefore, with appropriate processing it may be possible to make potato products that are simultaneously of reduced glycaemic potency, and of enhanced prebiotic potency. The glycaemic potency of starchy foods has been shown in a number of studies to be predictable from the digestibility of the starch measured in vitro (Brighenti et al., 1995; Goni et al., 1997; Englyst et al., 1999; Rosin et al., 2002). We have, therefore, measured the in vitro digestion of starch in a sample of widely available main crop New Zealand potatoes, and in a selection of 37

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lines from the New Zealand Institute for Crop & Food Research potato breeding program. The aim was to broadly determine the degree of variation that exists across a set of commercially available cultivars, and potential cultivars from a breeding program, in terms of their content of nutritionally distinct starch fractions, rapidly digested starch (RDS), SDS and RS, and the tendency of the fractions to change as a result of cold treatment after cooking. The results are reported in this paper. 2. Materials and methods 2.1. Samples 2.1.1. Supermarket Potatoes were purchased in local supermarkets in Palmerston North New Zealand. In New Zealand two major supermarket groups are supplied nationally by potato growers, so potatoes were purchased from both in one location, Palmerston North. Varieties purchased were those available for a single sampling (to standardize time) and included Draga, Nadine and Frisia (waxy), Desiree, Karaka and Moonlight (general purpose), and Agria, Fronkia, White Delight (floury) as summarized in Table 1. 2.1.2. Lines from breeding program Thirty-seven potato lines from the Crop & Food Research potato breeding program were supplied from two locations in New Zealand. Sample consisted of five tubers of each line supplied in paper bags and stored at ambient temperature. The potatoes were analyzed within a week of reception. 2.2. Sample treatment (cooking and post-cooking cool treatment) The potatoes were examined, and three sound, average-sized tubers from each cultivar were selected and thoroughly scrubbed. Tubers from all lines were weighed, boiled whole and unpeeled for 30 min, at which point resistance to penetration of the raw potato had disappeared, and then reweighed. The three potatoes within each cultivar were then coarsely chopped, mixed and combined before passing once through a Kenwood chef model A720 electric mincer with a 9-mm aperture plate. The minced potato was again mixed, before being placed into 250 mL plastic pots with lids. The samples were then analyzed in duplicate for in vitro digestibility within 2 h of cooking, and the pot containing the remaining potato was placed in the refrigerator (4 8C) for nearly 2 days (44 h; cool treatment) before reanalyzing, again in duplicate.

with the measurement of glucose release at 0, 20, 60 and 90 min after the addition of the amylases. The glucose released by 20 min was defined as RDS, the material digested between 20 and 90 min defined as slowly digested starch. RDS, SDS and RS therefore refer to relative digestibilities. The undigested matter was collected by centrifuging, washed and freeze-dried before a sub-sample was analyzed for resistant starch (RS), which, in this study, is defined as starch remaining undigested. The digestion protocol is summarized in Fig. 1. Glucose released during digestion was measured by the dinitrosalicylate method, as insufficient of other sugars are present in potato for a glucose-specific enzymatic method to be required. The results were expressed as glucose equivalents. Enzymes used in the in vitro digestion were pepsin from porcine stomach mucosa (Sigma, P7000; 800–2500 U/mL), porcine pancreatin (Sigma P7545; 8  USP specifications), amyloglucosidase from A. niger (Megazyme, E-AMGDF; 3260 U/mL), and invertase (Sigma I 9253, Grade V, 30 U/mg). Heat stable amylase used in RS analysis was Megazyme thermostable a-amylase (ex B. licheniformis, E-BLAAM, 120,000 U/40 mL). 2.4. Analysis of starch digest Aliquots of 1 mL of digesta were removed during digestion, added to 4 mL ethanol with mixing, and after 30 min, centrifuged. A 0.1 mL aliquot of the ethanolic solution was removed to tubes containing 0.5 mL invertase/amyloglucosidase in acetate buffer pH 5.2 (50 mg invertase + 0.1 mL amyloglucosidase per 10 mL acetate buffer) for a secondary digestion for at least 10 min at 37 8C. The secondarily digested sample, and glucose standards, were then analyzed directly by the dinitrosalicylate reaction for reducing sugars by adding 0.25 mL glucose (0.5 mg/mL), 0.25 mL 4 M NaOH and 1 mL DNS solution (10 g 3,5-dinitrosalicylic acid + 16 g NaOH + 300 g Na–K tartarate in 1 L of water), heating for 15 min in a boiling water bath, diluting to 10 mL with water and reading at 530 nm against a reagent blank. Glucose standards of 0.5 mL containing 1 or 2 mg/mL glucose were used. 2.5. Precision of method The precision of the in vitro digestion method was demonstrated using two digestion runs of a single cultivar made on separate days with duplicate sampling at each 20, 60 and 120 min time-point within each run. 2.6. Data analysis

2.3. In vitro digestion The in vitro digestions were carried out on 2.5 g samples of the potato, and involved a ‘‘gastric’’ pretreatment followed by adjustment to pH 7, addition of pancreatin and amyloglucosidase,

All data were analyzed in a Microsoft excel spreadsheet. The precision of the measurements was determined for the duplicate means and an average precision value calculated for each starch fractions.

Table 1 Potato cultivars used for digestibility analysis. Cultivar

Type

Origin

Dry matter (%)

Draga Frisia Nadine Desiree Karaka Moonlight Agria White Delight Fronkia

Waxy Waxy Waxy General purpose General purpose General purpose floury Floury Floury

2017 X mpi 19268 (Netherlands) ZPC69c160 x SVP 66-42 (Netherlands) Solanum vernei polycross (Scotland) Urgenta X Depesche (Netherlands) 002/9 X V394 (C&FR)a 1463-1 V394 (C&FR) Quatra X Semlo (Germany) 002/9 X Maris Piper (C&FR) Edzina X ZPC63-103 (Netherlands) Av SD

17.9 22.7 14.4 20.5 21.1 20.9 20.0 21.0 20.1 0.2

a

C&FR, New Zealand Institute for Crop & Food Research Ltd.

J. Monro et al. / Journal of Food Composition and Analysis 22 (2009) 539–545

Fig. 1. Protocol for in vitro digestive analysis of starch fractions in potato.

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Table 2 Precision of in vitro method in two digestion runs with a single cultivar (Nadine), and with each time-point sampled in duplicate within each run. Time

Run 1

Run 2

Between runs

Value

Mean

Range

Value

Mean

0

26.2 24.8

25.5

1.4

30.3 29.3

29.8

26

239.2 221.8

230.5

17.4

251.4 205.8

228.6

60

269.0 246.8

257.9

22.2

260.2 247.6

90

269.5 267.3

268.4

2.2

285.0 267.2

Range

Mean

1

Range

21.8

4.3

45.6

182.0

1.9

253.9

12.6

198.6

4

276.1

17.8

207.9

7.7

Fig. 2. Time course of digestion of cooked ‘‘Frisia’’ potato with and without cool treatment, with the RDS, SDS and RS fractions shown for the sample analyzed after a post-cooking cool treatment.

3. Results The characteristics of the supermarket potatoes analyzed in terms of their texture and use – waxy, floury, and general purpose – are summarized in Table 1. All of the varieties analyzed except for Nadine had a very similar dry matter content. The Nadine had a lower dry matter than other varieties. The in vitro digestion method used in the present analysis showed good analytical precision, with a mean coefficient of variation of less than 5% (Table 2). The results from digestive analysis of the freshly cooked supermarket potatoes expressed on an as-eaten basis are shown in Table 3. They show that in the freshly cooked state, on an equal weight basis there are differences between the potatoes in their contents of the various starch fractions. The RDS mean was 135 mg/g with a range of 90–150 mg/g across cultivars. SDS and RS were minor components of the potatoes digested immediately after cooking, and in some samples, digestion of available starch was almost complete after 20 min, so that there was very little SDS (Fig. 2). Nadine had not only the lowest content of all three starch fractions, but also the lowest dry matter content, and the difference between Nadine and other cultivars was eliminated by taking dry matter into account. The distribution of starch between the RDS, SDS and RS fractions changed markedly as a result of 2 days cool storage after cooking. On a wet weight basis the mean RDS across all supermarket cultivars after cooking and cool treatment was 86 mg/g, a 36% reduction in RDS compared with the samples analyzed immediately post-cooking. Mean SDS increased eightfold from 5.5 to 47 mg/g, and mean RS about doubled in quantity from 7.5 to 14.5 mg/g as a result of cool storage. The reduction in RDS as

a result of cool storage after cooking could therefore be attributed more to a reduced digestion rate than to the formation of resistant starch per se. Thus the decrease in RDS was complementary to the increase in SDS, with a correlation of r = 0.96 (p < 0.001) between changes in the two. In contrast, the increases in RS were not significantly related to the decreases in RDS, and averaged 6.5 mg/g on a wet weight basis. On an equal dry weight basis, for the freshly cooked potatoes the mean RDS across all cultivars was 679 mg/g with a range of 624–730 mg/g, so all mean values for RDS fell within 8% of the overall RDS mean. After post-cooking cool storage there was a substantial reduction in RDS to an average of 439 mg/g, which is a mean 36% reduction in RDS compared with the freshly cooked potato, and a corresponding increase in both SDS and RS (Table 4). The RDS values were within 23% of the RDS mean for the cooked cool-stored potatoes. As a result of the cool treatment SDS increased on average from 29 to 233 mg/g, a sevenfold increase, and RS increased from 39 to 75 mg/g, almost doubling. The results in Table 4 show that even after taking dry matter into account there were appreciable differences between cultivars in the degree to which starch digestibility changed as a result of cool treatment. The relative changes in digestibility of available carbohydrate fractions, RDS and SDS, are summarized in the RDS:SDS ratios, as shown in Table 5. Changes in the individual starch fractions, RDS, SDS and RS, as a result of cool-storing the different cooked cultivars, are shown in the histograms of Figs. 3–5 where the results for the starch

Table 3 Starch fractions (mg starch/g potato) in cooked New Zealand potatoes: bulked sample of three tubers analyzed in duplicate – wet weight basis. Cultivar

Freshly cooked

Cooked and refrigerated

RDS

SDS

RS

RDS

SDS

RS

Draga Frisia Nadine Desiree Karaka Moonlight Agria Fronika White Delight

128 149 90 149 138 145 138 132 150

0.0 10.2 9.9 0.5 2.4 2.5 6.8 17.2 0.0

5.8 10.5 9.3 6.3 7.6 7.0 6.7 8.1 6.3

80 75 73 92 73 105 89 80 111

37 77 24 51 55 32 53 67 32

16.6 14.4 15.7 14.0 19.6 16.4 9.6 10.7 13.8

Mean Mean S.E.M. Range

135 4.4 90–150

47 4.3 20–77

14.5 2.5 9.6–19.6

5.5 4.8 0–17

7.5 0.64 5.8–10.5

86 2.8 73–111

Fig. 3. Rapidly digested starch (RDS) in supermarket potatoes analyzed immediately after cooking and after post-cooking cool storage, ranked by RDS content of the cooked-cooled samples. Error bars indicate differences between duplicate digestion runs.

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Table 4 Starch fractions (mg starch/g potato) in cooked New Zealand potatoes: bulked sample of three tubers analyzed in duplicate – dry weight basis. Cultivar

Freshly cooked

Cooked and refrigerated

RDS

SDS

RS

RDS

SDS

RS

Draga Frisia Nadine Desiree Karaka Moonlight Agria Fronika White Delight

714 656 624 729 653 695 690 657 716

0.0 44.9 68.6 2.6 11.2 12.1 34.1 85.4 0.0

32.5 46.2 64.5 30.6 36.0 33.7 33.6 40.1 30.1

445.8 328.7 508.9 447.2 346.0 502.7 445.0 397.3 527.8

209.5 340.8 169.2 247.1 260.8 152.2 264.8 331.7 151.9

92.6 63.2 109.3 68.6 92.8 78.6 47.8 53.1 66.0

Mean Mean S.E.M. Range % Total starch

679 22.4 106 (624–730) 91

29 25.8 85 (0–85) 4

39 3.5 34 (30–64) 5

439 12.5 199 (328–527) 59

233 27.8 188 (152–340) 31

75 17.1 60 (47–107) 10

Table 5 Effect of cool treatment after cooking on the RDS:SDS ratio in potatoes. Cultivar

Freshly cooked (RDS:SDS)

Cooked and refrigerated (RDS:SDS)

Draga Frisia Nadine Desiree Karaka Moonlight Agria Fronika White Delight Mean

NAa 15:1 9.1:1 276:1 58:1 57:1 20:1 7.7:1 NAa 23:1

2.1:1 1:1 3.6:1 1.8:1 1.3:1 3.3:1 1.7:1 1.2:1 3.5:1 1.8:1

a

No measurable SDS in the freshly cooked potato.

fractions are expressed on a percent total starch basis. Fig. 3 shows that in all cultivars, freshly cooked starch in the supermarket potatoes was almost completely digested within 20 min after the addition of the amylase, but that the cool treatment caused a substantial reduction in the amount of RDS, to about 40–70% of its value in freshly cooked sample. The proportion of starch that was RDS in the cool-stored potatoes differed from about 45% (Frisia) to 70% (White Delight). SDS increased dramatically as a proportion of total starch as a result of cool treatment of the cooked potato (Fig. 4). All cultivars showed the increase in SDS, and there were large differences between the cultivars, despite all cultivars being similarly digestible in the freshly cooked state. Fig. 5 shows that

Fig. 5. Resistant starch (RS) in supermarket potatoes analyzed immediately after cooking and after post-cooking cool storage, ranked by RS content of the cookedcooled samples. Error bars indicate differences between duplicate digestion runs.

the proportion of starch that was resistant to digestion under the conditions used did not increase evenly across cultivars as a result of cool treatment. The greatest increases occurred in Draga, Karaka, Moonlight and Nadine (Table 6). None of the changes in starch digestibility caused by cool treatment of the cooked potato bore any obvious relationship to the culinary categorization of the potatoes – waxy, general purpose or floury. The amylose:amylopectin ratio in the different cultivars did not show any significant relationship to the measured increases in SDS, RS or SDS + RS as a result of the cool treatment (results not shown).

Table 6 Changes in rapidly digested (RDS) and resistant (RS) starch as a result of cool storage of different cooked potato cultivars per gram dry weight calculated from values in Table 3.

Fig. 4. Slowly digested starch (SDS) in supermarket potatoes analyzed immediately after cooking and after post-cooking cool storage, ranked by SDS content of the cooked-cooled samples. Error bars indicate differences between duplicate digestion runs.

Cultivar

Decrease in RDS (mg)

Increase in SDS (mg)

Increase in RS (mg)

Draga Frisia Nadine Desiree Karaka Moonlight Agria Fronika White Delight

268.2 327.3 115.1 281.8 307 192.3 245 259.7 188.2

209.5 295.9 100.6 244.5 249.6 140.1 230.7 246.3 151.9

60.1 17 94.3 38 56.8 44.9 14.2 13 35.9

Mean

242.7

207.7

41.6

544

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Fig. 6. Slowly digested starch in 37 cooked and cool-treated potato lines from the Crop and Food Research potato breeding program.

Fig. 7. Resistant starch in 37 cooked and cool-treated potato lines from the Crop and Food Research potato breeding program. The same bar numbers in Fig. 6 and this figure refer to the same potato lines.

Results from the screening experiment in which 37 lines from the Crop and Food Research potato breeding program were tested for their generation of slowly digested and resistant starch as a result of the cooking-cooling process, are in Figs. 6 and 7, expressed on a percent total starch basis. There were large differences between cultivars. In the cultivars showing the effects to the largest extent about 30% of the total starch was SDS and 25% RS. The relationship between SDS and RS formation in the 37 lines was not strong (R2 = 0.34). Commercial sensitivity did not allow the cultivars involved to be identified. 4. Discussion The present study has confirmed earlier findings on the effects of cool treatment on digestibility of potato starches (Leeman et al., 2006 and citations therein), but has extended them by showing the degree of variation available, for possible exploitation in potato breeding and product development, in a large sample of potato lines. The scale of the present experiment did not allow the issue of within-line or between-tuber variation and possible causes of it to be addressed, but rather provided ‘‘snap-shot’’ of existing variation

which is widely relevant because the lines analyzed included European and American cultivars as well as many New Zealand lines derived from them. Furthermore, the precision of in vitro digestion as an analytical procedure, demonstrated here, allows the causes of variation observed to be attributed to the potatoes analyzed and not to the analytical method. The in vitro results reported here agree with the clinical literature; in the freshly cooked state most of the available starch in all of the freshly cooked potatoes was digested within 20 min after the addition of amylase, so was mostly RDS, which is consistent with the high and medium glycaemic indexes reported for freshly cooked potatoes (Soh and Brand-Miller, 1999; Henry et al., 2005). In contrast, in the cool-stored potatoes glucose release at 20 min was less and continued to be less at 90 min as it approached a plateau, which showed that the rate of digestion of starch, and to a lower degree its extent in the time available, was ˜ i, 2000; reduced by the cool treatment (Garcia-Alonso and Gon Leeman et al., 2005). If the observed reduction in rate of potato starch digestion as a result of cool treatment can be maintained in vivo the change in glycaemic potency could be substantial, and sufficient to move potato products from being classified as highly glycaemic, to moderately glycaemic, because glycaemic response reflects not the absolute intake of available carbohydrate, but the net balance between blood glucose loading and glucose disposal. In vitro reductions in digestibility are likely to under-predict in vivo reductions due to the non-linearity of the carbohydrate dose– blood glucose response relationship (Monro and Shaw, 2008), so the glycaemic benefits may be even greater than suggested by the present results. Further clinical trials are needed to establish the exact relationship between in vitro and in vivo responses for potatoes. The reduction in digestibility of potatoes during cool storage after cooking has been attributed to partial starch retrogradation (Karlsson et al., 2007). However, the decrease in RDS was not matched by the increase in indigestible RS, but was a result of a reduced rate of digestion evident as a large increase in SDS. It has been suggested that amylose chain alignment that normally leads to retrogradation occurs sufficiently in the disorganized amylopectin of gelatinized potato starch, to impede, but not prevent, digestion (Fredriksson et al., 2000). In this study differences in the amylose–amylopectin ratios between the supermarket cultivars were not detected (results not shown), possibly because the reason for differences in SDS formation between cultivars resides largely in the amylopectin fraction. For instance, differences in degrees of branching responsible for differences in degree of SDS formation may have been more subtle than could be measured by the standard iodine complexing methods, which are designed to distinguish between amylose and amylopectin as major starch classes (Peris-Tortajada, 2004). From a food labeling and nutritional perspective it is important that a substantial reduction in glycaemic potency could be achieved by increasing SDS, rather than by increasing RS, because it would mean that the energy availability from potato would be largely maintained, while both the glycaemic impact per given weight, and the glycaemic index would be reduced. If RS rather than SDS had been formed, glycaemic impact and energy availability would both have been reduced, but the glycaemic index would have remained high because it is, by definition, based on available carbohydrate, so any RS should be excluded from the calculation of GI (Monro, 2003). SDS, on the other hand is available, so any reduction in glycaemic effect that it causes will translate to a lower response per unit of available carbohydrate, that is, a lower GI. Based on the overall means in Table 3, in the freshly cooked potato 96% of the available (RDS + SDS) starch was RDS and as a

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result of cool treatment the proportion of available carbohydrate that was RDS dropped to 64%. If a corresponding proportional decrease in GI occurred, potato would move from being high GI (>70) in the freshly cooked state (potato GI = 80; Foster-Powell et al., 2002) to low GI (GI < 55) in the cooked-cooled state. Assuming that RDS is a valid indicator of glycaemic impact, the cool treatment would lead to a reduced intake of glucose from 13.5 to 8.6 g per 100 g serving (equivalent to about one potato). As an intake of 10 g glycaemic glucose equivalents is considered the border between low and medium glycaemic loading per serving (Brand-Miller et al., 2003), cool treatment could lead to potato being classified as a low glycaemic impact food product, because, although predominantly carbohydrate, it has a high water content and therefore a low density of carbohydrate. Not only is the glycaemic potency of the potato as a result of post-cooking cool treatment likely to be reduced, but also the increase in resistant starch that occurred will have increased the dietary fibre content. Doubling of resistant starch (RS) as a result of cool treatment to 7.4% on a dry weight basis (Table 3) would result in dry potato being classified, in terms of food regulations, as high in dietary fibre (>6%), so that if used as an ingredient it would not reduce the possibility for a claim of ‘‘high in dietary fibre’’. On a fresh weight basis the formation of RS shown in Table 2 would lead to an increase in dietary fibre content by more than 100% for several cultivars, and possibly to health advantages in a potatobased diet due to the activity of RS as a prebiotic, the benefits of which have been demonstrated for cereal RS (Bird and Topping, 2001). The present study has illustrated the usefulness of in vitro glycaemic analysis as a screening tool. With reasonable economy, 2 treatments of 9 supermarket cultivars and 37 breeding lines have been compared. Potato lines showing most extreme responses to cool treatment after cooking may be taken forward into a variety of secondary food processes and food products, which may be subjected to further in vitro analysis prior to clinical confirmation of reduced glycaemic potency. The results have shown that even among potato cultivars presently available for public consumption in New Zealand, culinary practices may have a nutritionally significant impact on glycaemic potency. Furthermore, despite the fact that glycaemic potency has not until recently been a criterion for potato breeding or processing, very large differences exist between lines in the propensity to develop SDS. The amount of difference between lines suggests that the ability of potatoes to generate SDS from RDS during cool storage after cooking may be quickly exploited by conventional plant breeding and/or genetic modification. The results have provided a ‘‘snapshot’’ of existing population variability without regard to a large number of factors that might cause variation, such as growth conditions, storage, cooking time, retrogradation time and temperature, stability of the effect, intracultivar variation, and so on, which the present study was not designed to investigate, but which will be the subject of further research. And because most of the potato lines investigated here were derived from American and Europe sources the results will have wide relevance. 5. Conclusion Potato genotypes currently available in New Zealand are susceptible to a considerable reduction in glycaemic potency as a result of cool treatment after cooking. The observed range of differences in response to the cool treatment suggests that screening of germplasm and conventional plant breeding would be viable strategies for identifying or producing cultivars of potentially reduced glycaemic impact that may be useful in

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