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Starch granule sizes and degradation in sweet potatoes during storage
Postharvest Biology and Technology 150 (2019) 137–147
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Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio
Starch granule sizes and degradation in sweet potatoes during storage a,b
b,⁎
b,c
Suyan Niu , Xiu-Qing Li , Ruimin Tang , Guodong Zhang Loretta Mikitzelf, Muhammad Haroonb
b,d
e
T
a
, Xiubao Li , Bo Cui ,
a
Institute of Bioengineering, Zhengzhou Normal University, Zhengzhou, 450044, China Fredericton Research and Development Centre, Agriculture and Agri-Food Canada, Fredericton, New Brunswick, E3B 4Z7, Canada College of Life Sciences, Nanjing Agricultural University, Nanjing, 210095, China d College of Agronomy, Gansu Agricultural University, Lanzhou, 730070, China e Rizhao Academy of Agricultural Sciences, Rizhao, Shandong, 276500, China f Department of Agriculture, Aquaculture and Fisheries of New Brunswick, Wicklow, NB, E7L 3S4, Canada b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Sweet potato Post-harvest storage Dry matter Starch content Starch granule size
Starch granule sizes can greatly influence the quality of both table and processed products of sweet potato (Ipomoea batatas (L.) Lam), an important food, feed and industrial crop. Sweet potatoes require storage under suitable temperatures for year around supply, but there is very little research on starch granule size variation during storage. We characterized dry matter content (DMC), starch content, soluble carbohydrate (SoluCarb), and starch granule length before and after storage at 13 °C for 60 d in the cultivars, SP3388, SP3391, ‘Beauregard’, and ‘Covington’. Tuberous roots with higher DMC tended to have greater DMC, starch content, SoluCarb, and starch granule length after storage. Starch granule sizes ranged nearly continuously from small to large, without frequency peaks of small granules or large granules. Smaller starch granules degraded faster than larger ones during storage. The findings provided insights into starch degradation in plants, can help predict the processing quality of sweet potatoes during storage, and may also assist in their dry matter and starch-related breeding.
1. Introduction Sweet potato (Ipomoea batatas (L.) Lam), a species in the Convolvulaceae family, is an important food, feed (Zhang and Li, 2004) and industrial processing (Ganguli et al., 2003) crop. The tuberous roots of this crop can have very high dry matter contents, up to 42% (Li and Zhang, 2003), which is mainly starch (Woolfe, 1992). Food products from sweet potatoes include starch, noodles, fries, spirits and sugar syrup products (Kitahara et al., 2017; Liu, 2004), while industrial products include both biodegradable plastics (Ganguli et al., 2003; Liu et al., 2015) and bioethanol (Jusuf and Ginting, 2014; Kitahara et al., 2017; Liu, 2004). Both fresh/table and processed sweet potatoes, such as fries (Sato et al., 2018; Sylvia et al., 1997), are very popular in North America, China, and many other countries. Storage of sweet potatoes is critical for year-round supply for both table use and processing purposes, Sweet potato originated from a tropical region but is also adapted to relatively cool regions such as Canada and is a cash crop for farmers.
This adaptability provides valuable resilience of this crop to climate change. Both the aboveground parts and the tuberous root can be used as food and feed. The proven health properties (Oluyori et al., 2016), the crop adaptation, and the whole plant edibility make this crop important for both human health and world food security in the current era of rapid increase of world human population. Starch granule size influences the quality of processed products. Starch granule size distribution also influences bread characteristics (Sahlström et al., 1998). The feasibility of starch recovery from the wastewater during processing of fries and chips can be affected by the size of starch granules (Grommers and van der Krogt, 2009). Starch granule size affects starch hydrolysis in industrial processes. Smaller size of the starch granules results in a higher percentage of hydrolysis (Franco et al., 1992). Small granules are digested faster than large granules during enzymatic digestion in vitro (Kang et al., 1985). Wheat and tapioca starch are hydrolyzed faster than potato starch, likely due to structural or surface area differences (Sarian et al., 2012). Small and large starch granules from potatoes and sweet potatoes have different
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Corresponding author. E-mail addresses: [email protected], [email protected] (S. Niu), [email protected], [email protected] (X.-Q. Li), [email protected] (R. Tang), [email protected] (G. Zhang), [email protected] (X. Li), [email protected] (B. Cui), [email protected] (L. Mikitzel), [email protected] (M. Haroon). https://doi.org/10.1016/j.postharvbio.2019.01.004 Received 6 August 2018; Received in revised form 10 January 2019; Accepted 10 January 2019 Available online 16 January 2019 0925-5214/ © 2019 Published by Elsevier B.V.
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2. Materials and methods 2.1. Plant materials Four genotypes (lab codenames SP3388, SP3391, SP3392, SP3393) of sweet potato (Ipomoea batatas (L.) Lam) were used in this study (Fig. 1). SP3392 and SP3393, both with orange flesh, are known commercially as ‘Beauregard’ and ‘Covington’, respectively. The original slips of ‘Beauregard’ and ‘Covington’ were obtained from the Steele Plant Company, Gleason, Tennessee, USA. SP3388, (creamy white flesh), and SP3391 (purple flesh) were collected from the local market. The genotypes were propagated in the greenhouses of Agriculture and Agri-Food Canada (Fredericton, New Brunswick, Canada).
Fig. 1. Longitudinal sections of sweet potato tuberous roots, showing flesh color. Cultivars/clones from left to right: SP3388, SP3391, ‘Beauregard’, and ‘Covington’.
2.2. Plant growth conditions Slips of the cultivars were planted on the experimental farm of the Fredericton Research and Development Centre of Agriculture and AgriFood Canada (Fredericton, New Brunswick, Canada) on 29th June 2015, and harvested on 7th October. Fredericton has coordinates of 45.957319 °N and 66.647818 °W. The average daily maximum temperatures for July, August, and September 2015 were 25.7, 27.3, and 24.0 °C, respectively, and the average daily minimum temperatures for July, August, and September 2015 was 12.1, 14.4, and 9.6 °C, respectively according to Daily Data Report for 2015 by Environment Canada. (http://climate.weather.gc.ca/climate_data/). Unrooted cuttings from mother plants in a greenhouse were directly planted in the field. The space was 91 cm between rows and 30 cm between plants. Large weeds were manually removed in mid and late July. The plant plot was managed organically without using any plastic mulch, fertilizer, or chemical spray.
distribution of the acetyl group (Chen et al., 2005). Small granule fractions of potato and sweet potato starches can make better quality noodles than the large granule fractions (Chen et al., 2003). Li et al. (2018a) found that the emulsion droplet size increased with granule size when sweet potato starch granules were used as Pickering emulsion stabilizers. Knowledge of starch granule size variation can be useful for predicting and monitoring food quality and processing quality of tuberous roots. Storage of sweet potatoes can induce many changes including the carbohydrate composition, digestibility, amylase and trypsin inhibitor activity, and pasting properties (Morrison et al., 1993; Takahata et al., 1995; Zhang et al., 2002). Sweet potatoes, a tropical origin crop, cannot be stored at very cold temperature such as 4 °C because the tuberous roots get chilling injury (Li et al., 2018b). Therefore sweet potatoes are usually stored at ambient or relatively less cold temperatures such 13–15 °C (Picha, 1986; Ji et al., 2017; Li et al., 2018b). When stored at ambient temperatures, starch contents of tuberous roots decrease, and dry matter and sugar contents increase in some cultivars (Acedo et al., 1996). How the starch granule size changes and how dry matter, starch, and soluble carbohydrates co-evolve in sweet potato tuberous roots during cold storage are still largely unknown. In this present study, we characterized dry matter content, starch content, starch granule size, and soluble carbohydrate in comparison between the tuberous roots before storage and after storage in four sweet potato cultivars. The purpose is to increase knowledge about the variation of starch granule size, starch content, and soluble carbohydrate content of different sweet potato cultivars and about the potential interrelationship among these measured characteristics/properties during the cold storage treatment.
2.3. Sample preparation After harvest, the tuberous roots were cured for two weeks at 21 °C. At the end of curing (Day 0 for cold storage), the roots were moved to at 13 ± 1 °C at a relative humidity of approximately 60% in the dark for 60 days (Day 60). The tuberous roots did not show any sprouting at Day 60 of storage. Among the stored tuberous roots, three biological replicates for each cultivar-day combination were used for characterization and starch granule size analysis. The tuberous roots were washed thoroughly and surface-sterilized with 70% ethanol (v/v), then rinsed twice with distilled water. The tuberous roots were cut from the centre longitudinally (Fig. 1). One half of each tuberous root was used for determination of dry matter and starch contents, and the other half was used for the measurement of the starch granule size.
Fig. 2. Microscopic images of sweet potato starch granules. 138
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software AxioVision Rel 4.7. The starch granules with the length no smaller than 4 μm were used to calculate the average length of starch granules (Fig. 2). 2.5. Dry matter content The fresh weights of chopped pieces, approximately 3 mm squares, in total about 6 g, from each tuberous root of each cultivar was recorded after sampling. These samples were then oven-dried at 65 °C for about 10 d until the weight stabilized and dry matter recorded. 2.6. Starch and soluble carbohydrate contents The samples were freeze-dried in a freeze dryer (Labconco Corporation, Kansas City, Mo, UNSA) under vacuum of 0.022 mBar at −55 °C for 30 h before being ground in a mortar without liquid nitrogen. About 100 mg of each powder sample was utilized for starch content determination using the protocol of Megazyme total starch kit (K-TSTA kit, Megazyme, Wicklow, Ireland). The total starch content was calculated by the formulized worksheet provided by Megazyme using the sample weight and measured optical density. The soluble carbohydrate content was determined according to the anthrone sulfuric acid colorimetric assay described by Li (2000) using 100 mg of each fresh sample. 2.7. Data and statistical analyses Clustered heatmap analysis of multiple traits for the four cultivars was conducted including dry matter content (DMC), starch content (Starch), starch granule length (Granule), soluble carbohydrate content (SoluCarb) before (CK) and after the cold storage treatment (T), and the percentage of change after storage of each trait was calculated (for example, DMCP = 100*(DMT-DMCK)/DMCK) using the heatmap.2 in the R package (https://www.r-project.org/). Pearson correlations and P values among different measured and derived traits were calculated using the multivariate correlation option in SAS Enterprise (Version 7.1). Principal Component Analysis (PCA) (SAS Institute Inc., Cary, NC, version 9.4) was conducted to examine the grouping of the cultivars using the following traits: DMC, Starch, Granule and SoluCarb traits, and the ratios such as Starch/DMC ratio, Granule/Starch ratio, SoluCarb/DMC ratio, SoluCarb/Starch ratio, and Granule/DMC ratio. Clustered heatmap analysis of eight cultivar-storage combinations was conducted using DMC, Starch, Granule and SoluCarb traits. The overall mean of each trait (DMC, starch content, soluble carbohydrate content, and starch granule length) of the four cultivars before storage was tested for significance with that of the samples after storage using the two-way ANOVA-Duncan multiple mean range test (SAS Enterprise Guide 7.1). These overall means were also subjected to paired t-test used the statistical package in Excel 2010. The statistical test between two starch proportion percentages was conducted using U test The significance of starch granule size difference between the roots before storage and the stored roots was determined using the t-test (two tails, unpaired) statistical package in Excel 2010 for each cultivar.
Fig. 3. Dry matter content, starch content, soluble carbohydrate content, and starch granule length of four sweet potato varieties before and after storage. D0: Roots before storage. D60: Roots after storage of 60 d at 13 °C. A: dry matter content. B: starch content. C: soluble carbohydrate content. D: Average of starch granule length (> = 4 μm) (the bars are small because they are standard errors from 517 to 1673 granules). Means (the two columns on the right): the mean value among the means of the cultivars. Bar: standard error of biological repeats (in A, B, C: n = 3).
2.4. Starch granule size (length) measurement The starch granule size of sweet potato tuberous roots was measured by a modified method similar to that described by Zhang et al. (2011). A section of cross cut from the middle region, after removing the skin, was used for squeezing juice using a garlic press. The sample amount (in volume) filled into the garlic press was about 2.2 cm x 1.5 cm. The juice was collected in an Eppendorf tube. Five μL of well mixed juice was added to 20 μL water in a new Eppendorf tube, and immediately mixed with the pipette tip. Ten μL of diluted juice was used for microscopic observation (50X) under a Carl Zesis light microscope equipped with a polarizer and AxioVision Rel 4.7 software. Three to six images were randomly taken. The lengths of starch granules (Fig. 2) were measured with a micrometer using the imaging processing
3. Results 3.1. Dry matter content (DMC) The DMC of tuberous roots was higher after storage than before storage in all four cultivars (Fig. 3A). Before storage, the DMC was 25.2%, 28.9%, 20.7% and 20.0% in SP3388, SP3391, SP3392 (‘Beauregard’) and SP3393 (‘Covington’), respectively; whereas, the DMC increased to 29.5%, 31.8%, 22.4% and 23.3% in the four cultivars, respectively, after storage at 13 °C for 60 d. Overall, the DMC in the tuberous roots of the cultivars increased from 23.5% before storage to 139
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Fig. 4. Comparison of length of starch granules in four sweet potato cultivars. A. D0 (before storage). B. D60 (after 60 d storage). Arrow: A peak in proportion at the interval 8–9 μm with midpoint at 8.5 μm. Cultivars/clones: SP3388, SP3391, ‘Beauregard’, and ‘Covington’.
3.4. Starch granule size
26.7% in the tuberous roots after storage at 13 °C (P < 0.05), likely due to water loss during storage. The percentage of water loss during storage was not significantly correlated with the Day 0 dry matter content but was significant with the Day60 dry matter content (P < 0.05). Moreover, among these four cultivars, the DMCs of ‘SP3388′ and ‘SP3391′ were greater than that of ‘Beauregard’ and ‘Covington’ both before and after storage.
The average length of starch granules was consistently greater in the tuberous roots after storage than before storage in all four cultivars (P < 0.01). The purple-fleshed sweet potato SP3391 had the greatest average length of starch granules among the cultivars both before and after storage. The two orange-fleshed cultivars ‘Beauregard’ and had similar granule lengths both before and after storage. The change of the average granule length in the creamy white flesh cultivar SP3388 was greater than in the other cultivars, with the average length increasing from 8.4 μm before storage to 9.5 μm after storage (Fig. 3D).
3.2. Starch content Overall, the starch content of the cultivars decreased from 32.3% before storage to 21.6% (P < 0.01) after 60 d of storage (Fig. 3B). Both before and after storage, the starch content was higher in the creamy white flesh cultivar SP3388 and the purple-fleshed cultivar SP3391 than the two orange-fleshed cultivars ‘Beauregard’ and ‘Covington’. For the t-test of starch content between the stages before and after storage within each cultivar, the storage-resulted starch content reduction was significant in the two orange cultivars but not in the creamy white (SP3388) and purple (SP3391) cultivars.
3.5. Starch granule size distribution curve The percentage of starch granules in the population was calculated for each micrometer interval of granule length (Fig. 4). The starch granule population was mainly predominated by small granules, and large granules had small percentages. Overall, the size distribution was quite continuous in all four cultivars, except for the frequency of granules at 8.5 μm (for the interval of 8–9 μm) which clearly peaked compared with the two adjacent intervals in the cultivars SP3388 and SP3391 (Fig. 4A and B). This proportion peak at 8.5 μm was not detected in the two orange flesh cultivars ‘Beauregard’ and ‘Covington’. Before storage, the percentage of each granule size group in sweet potatoes decreased with increase of granule size. The proportions of small granules that fell into the intervals from 4.5 to 7.5 μm, were higher in the orange-fleshed cultivars ‘Beauregard’ and ‘Covington’ than those in the other two cultivars (Fig. 4A). In SP3388 and SP3391, the two non-orange cultivars, the percentages of granules at the interval midpoint of 5.5 μm were much higher than those at the midpoint of 7.5 μm (Fig. 4A). For example, for SP3388, the percentage in the population was 15.2% for the interval midpoint 5.5 μm and 10.8% for the interval point 7.5 μm (Fig. 4A).
3.3. Soluble carbohydrate contents The soluble carbohydrate content was higher after 60 d of storage in all four cultivars (Fig. 3C). The overall mean in the tuberous roots after storage was 1.8-fold (p < 0.01) of that before storage. The soluble carbohydrate content was higher in SP3388 (7.6%) than the other three cultivars before storage, but was highest in SP3391 (12.5%) after storage. The difference in soluble carbohydrate content before and after storage was similar in the two orange-fleshed cultivars ‘Beauregard’ and ‘Covington’. The increase of soluble carbohydrate content after storage was highest (the after/before ratio was 2.1) in SP3391 and lowest (1.4 fold) in SP3388. 140
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Fig. 5. Distribution of starch granule length before and after storage in each sweet potato cultivar. A. SP3388. B: SP3391. C: ‘Beauregard’. D: ‘Covington’. D0: Day 0 (before storage). D60: Day 60 (after storage).
that of this region before storage. For example, after storage, for SP3388, the percentage in the population was 9.6% for the interval midpoint 5.5 μm and 11.1% for the interval point 7.5 μm (Fig. 4B). After storage, the distribution of large granules (8.5 to 18.5 μm) for the intervals was also clearly different between the two non-orange-fleshed cultivars and orange-fleshed cultivars (Fig. 4B). The percentages of granules in this interval were generally higher in the SP3388 and SP3391 than in the two orange-fleshed cultivars (‘Beauregard’ and
After storage, the distribution of small granules for the intervals from midpoint 4.5 to 7.5 μm was clearly different between the two nonorange-fleshed cultivars (SP3388 and SP3391) and orange-fleshed cultivars (‘Beauregard’ and ‘Covington’) (Fig. 4B) and between before (Fig. 4A) and after (Fig. 4B) storage. In the two non-orange-fleshed cultivars (SP3388 and SP3391), the percentages of granules at the interval midpoint of 5.5 μm were similar with those at the midpoint of 7.5 μm (Fig. 4B). This granule size distribution was very different from 141
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Fig. 6. Three groups of starch granules classified according to granule length. Y-axis: The percentage (A) and the average length (B) of the starch granules within each group. D0: Day 0 (before storage). D60: Day 60 (after storage). In A: * P < 0.05 and ** P < 0.01 significant levels in u-test for percentages between Day 0 and Day 60 within the same size group and within the same cultivar. In B: significant level of *P < 0.05 and **P < 0.01 in t-test analysis of average starch granule length in Day 60 vs. Day 0 comparison within each length group. Bar in B: standard error. Starch granule numbers in B: for range of 4–7 μm, n was from 205 to 847; for the range of 7–15 μm, n was from 260 to 793; for > 15 μm, n was from 5 to 122, depending on the cultivar.
‘Beauregard’ and ‘Covington’ (Figure S1). The results indicated larger changes in the two non-orange-fleshed cultivars than the two orangefleshed cultivars.
‘Covington’) (Fig. 4B). The overall distribution curve of the starch granule size was compared directly before and after storage for each cultivar (Fig. 5). In the cultivar SP3388, the percentages of the granules with the size from 4.5 to 6.5 μm became much lower after storage, the percentage of granules with 9.5 to 18.5 μm was clearly increased after storage (Fig. 5A). In the cultivar SP3391, the percentages of the granule size interval from 4.5 to 5.5 μm became much lower after storage, the granule size interval region of 6.5 to 12.5 μm was clearly increased after storage (Fig. 5B). In the two orange-flesh cultivars, the percentages of the granule size of 4.5 to 7.5 μm became much lower after storage, and the region of 10.5 to 18.5 μm in ‘Beauregard’ (Fig. 5C) and 8.5 to 14.5 μm in ‘Covington’ (Fig. 5D) were clearly increased after storage. The common pattern was that the small granules decreased in all four cultivars and the relatively large granules increased in proportion even though the exact intervals showed differences between the cultivars. The Pearson correlation coefficient R between the distribution curves (Fig. 5) was calculated using each cultivar’s starch granule proportion changes (for each 1 μm size interval) after storage (Table S2). Significant positive correlations were found in the comparisons between SP3388 and SP3391 (R = 0.420), between the SP3388 and SP3392 (R = 0.706), between SP3388 and ‘Covington’ (R = 0.595), and between ‘Beauregard’ and ‘Covington’ (R = 0.691). SP3391 was not correlated with the two orange-fleshed cultivars. The mean value of absolute changes of starch granule number percentages within each interval after storage was calculated for 19 intervals (4.5 to 22.5 μm) for each cultivar (Figure S1). The order of change from the largest to the smallest was as follows: SP3388, SP3391,
3.6. Percentage change of small, medium and large starch granules after storage of tuberous roots The starch granules were classified into small (4–7 μm), medium (7–15 μm), and large granules (> 15 μm) (Fig. 6A). Most granules were in the 7–15 μm range, and very few granules were larger than 15 μm, in terms of percentage. Overall, the small granule portion decreased after storage, the medium portion increased, and the large granule portion increased or decreased depending on cultivar. The percentage of the small granule portion was 41.5%, 39.7%, 52.2%, and 54.8% before storage and 29.1%, 31.8%, 46.1%, and 50.6% after storage for the four cultivars SP3388, SP3391, SP3392 and SP3393, respectively. The percentage of small granules in the population between Day 0 and Day 60 within the same cultivar was highly significant in SP3388 and SP3391, significant in ‘Beauregard’, and nonsignificant in ‘Covington’. The results indicated that the percentage of small granules in the granule population decreased more obviously in the non-orange-fleshed cultivars than the two orange-fleshed cultivars. The percentage of the medium granule portion was 54.7%, 50.3%, 47.2%, and 44.4% before storage and 62.3%, 58.7%, 50.0%, and 47.4% after storage for the four cultivars SP3388, SP3391, ‘Beauregard’ and ‘Covington’, respectively (Fig. 6A). The percentage of medium granules in the population between Day 0 and Day 60 within the same cultivar was highly significant in SP3388 and SP3391, and non-significant in 142
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Fig. 7. The average length of the first 20% and 50% of the smallest and largest starch granules in four sweet potato cultivars. A and B: The first 20% (A) and 50% (B) of the smallest granules (counted from the 4 μm granules as the smallest to the larger ones until it reached the 20% of the population). C and D: The first 20% (C) and 50% (D) of the largest granules (counted from the largest in the population). D0: Day 0 (before storage). D60: Day 60 (after storage). * P < 0.05 and ** P < 0.01 significant levels in t-test analysis of average starch granule length in Day 60 vs. Day 0 comparison within each cultivar. Bar in B: standard error. Starch granule numbers (n) in A and C was 103–335, and in B and D was 259–837, depending on the cultivar.
‘Beauregard’ and ‘Covington’ (Fig. 6A). The results showed that the percentage of medium granules in the population increased more obviously in the non-orange-fleshed cultivars than the two orange-fleshed cultivars. The percentage of the large granule portion was 3.8%, 10.1%, 0.7%, and 0.9% before storage and 8.6%, 9.5%, 3.9%, and 2.0% after storage for the four cultivars SP3388, SP3391, ‘Beauregard’ and ‘Covington’, respectively (Fig. 6A). The percentage of large granules in the population between Day 0 and Day 60 within the same cultivar was highly significant in SP3388 and ‘Beauregard’, significant in ‘Covington’, and non-significant in SP3391 (Fig. 6A). The results demonstrated that the percentage of large granules in the population significantly increased in all the cultivars except the cultivar SP3391, the purple-fleshed cultivar.
3.8. Average length of the 20% and 50% smallest and largest starch granules The starch granules were ranked from smallest to largest, and the average length of the first 20% and first 50% of the smallest and largest granules was calculated. The first 20% of the smallest granules had an average length ranging from 4.5 to 4.6 μm before storage and 4.5 to 5.0 μm after storage (Fig. 7A). The average length of 50% of the smallest granules ranged from 5.3 to 5.8 μm before storage and 5.4 to 6.5 μm after storage (Fig. 7B). The average length for both the 20% and 50% smallest starch granules increased highly significantly after storage in SP3388 and SP3391, but there was no significant change in the two orange-fleshed cultivars ‘Beauregard’ and ‘Covington’. The top 20% of the largest granules had an average length ranging from 11.4 to 16.7 μm before storage and 12.1 to 15.9 μm after storage (Fig. 7C). The top 50% of the largest granules had an average length ranging from 9.3 to 12.8 μm before storage and 9.7 to 12.6 μm after storage (Fig. 7D). Starch granule average length was significantly increased in top 20% and 50% of the largest granules after storage in the SP3388, ‘Beauregard’, and ‘Covington’ (P < 0.01), but not changed in SP3391.
3.7. Mean length changes of small, medium and large starch granules after storage of tuberous roots The mean length of small granules (4 to 7 μm) between Day 0 and Day 60 increased significantly in SP3391, but decreased in ‘Beauregard’ and had no significant change in SP3388 and ‘Covington’ (Fig. 6B). The average length of medium granules (interval from 7 to 15 μm) between Day 0 and Day 60 was increased in SP3388, ‘Beauregard’, and ‘Covington’ but not in SP3391 (Fig. 6B). The average length of large granules was highly significantly decreased in SP3388 and SP3391, significantly increased in ‘Beauregard’, and had no significant change in ‘Covington’ after storage (Fig. 6B). The results indicated that the average length of large granules (> 15 μm) in the population varied differently among the cultivars - decreased in the two non-orange-fleshed cultivars but increased in the two orange-fleshed cultivars.
3.9. Correlation among traits The Pearson correlation coefficient (R) and statistical significance levels (P values) among all the measurements and derived traits are presented in Table S2. The following traits were analyzed: dry matter content, starch content, starch granule length, soluble carbohydrate content, and percentage of change after storage (CP) of sweet potatoes before storage (CK) and after storage (T). The DMCK value showed a 143
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Fig. 8. Heatmap clustering analysis of sweet potato cultivars using multiple traits of tuberous roots before and after storage. DM: dry matter content. Starch: starch content. Granule: starch granule length; SoluCarb: soluble carbohydrate content. CK: before storage. T: after storage. CP: percentage of change after storage (for example, DMCP = 100*(DMT-DMCK)/DMCK).
GranuleCK, GranuleT, GranuleCP, SoluCarbCK, SoluCarbT and DMCP were grouped into the same cluster. StarchCP and SoluCarb were not closely grouped together with any trait (Fig. 8).
positive significant correlation with DMT, StarchCK, SoluCarbT, GranuleCK and GranuleT values. Starch values (StarchCK and StarchT) were positively and significantly correlated with each other. StarchCK was positively correlated with DMCK, and StarchT was correlated with DMT. Also, StarchCP was positively correlated with StarchCK, StarchT, DMT, and GranuleT. GranuleCK value was positively correlated with DMCK, DMT, SoluCarbT, StarchCK, StarchT, and GranuleT values. After storage, the GranuleT value was positively correlated with DMCK, DMT, StarchCK, StarchT, StarchCP, and GranuleCK. SoluCarbCK was not positively correlated with any trait, but SoluCarbT was positively correlated with DMCK and GranuleCK.
3.11. Heatmaps clustering analysis of eight cultivar-storage combinations The clusters of eight cultivar-storage combinations (four cultivars, before and after storage) were analyzed using their shared traits, including dry matter content, starch content, granule size, and soluble carbohydrate contents (Fig. 9). Granule size and SoluCarb content are shown in the same cluster while the other traits are in the other cluster. Among the eight cultivar-storage combinations, orange cultivars (‘Beauregard’ and ‘Covington’) formed a cluster, and the creamy white (SP3388) and purple (SP3391) formed the other. Within each cluster, there were two subclusters: the before storage cluster and the afterstorage cluster (Fig. 9).
3.10. Heatmaps of multiple traits clustering four cultivars The heatmap and clusters analysis was based on 12 traits–DMCK, DMT, StarchCK, StarchT, GranuleCK, GranuleT, SoluCarbCK, SoluCarbT, DMCP, StarchCP, SoluCarbCP, and GranuleCP (Fig. 8). The cultivars were clustered into the non-orange-fleshed cultivars and the orange-fleshed cultivars. Each trait, before and after storage, was grouped in the same cluster. DMC and Starch contents in both before and after storage samples were grouped into a large cluster. The
3.12. Principal component analysis The principal component analysis identified the first four principal components (PC) as significant with eigenvalues greater than 0.1 and 144
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Fig. 9. Heatmap clustering of eight cultivar-storage combinations from two stages using multiple traits. SP3388CK, SP3391CK, BeauregardCK and CovingtonCK were from sweet potatoes before storage. SP3388 T, SP3391 T, BeauregardT, and CovingtonT were from sweet potatoes after storage of 60 d at 13 °C. Starch: Starch content. DMC: dry matter content. Granule: Starch granule length. SoluCarb: Soluble carbohydrate contents.
accounted for a total of 99.7% of the total variation (Table 1) based on nine traits (Table 2). The first PC (PC1) explained 54.3%, the second PC (PC2) described 39.3% of the total variation, and the third PC (PC3) represented 5.1% variation (Table 1). The first PC was dominated by the positive values for the SoluCarb/Starch ratio, the Granule/Starch ratio, and the SoluCarb/DMC ratio, and the first two highest negative values for the Starch/DMC ratio and Starch (Table 2). The second PC was dominated by the positive values for DMC and Granule, and the highest negative value for the Granule/DMC ratio (Table 2). The third PC was dominated by the positive values for SoluCarb, the Granule/ DMC ratio and the SoluCarb/DMC ratio, and the highest negative value for the Granule/Starch ratio (Table 2). PC1 and PC2 separated the eight cultivar-storage combinations into four groups: BeauregardCK and CovingtonCK, SP3388CK and SP3391CK, SP3388 T and SP3391 T, and BeauregardT and CovingtonT (Fig. 10A). The separation among the groups was mainly according to DMC, Granule, the starch/DMC ratio, and the SoluCarb/Starch ratio (Table 2). PC1 and PC3 together separated the eight combinations into a before storage group (SP3388CK, SP3391CK, BeauregardCK, and CovingtonCK) and an after storage group (SP3388 T, SP3391 T, BeauregardT, and CovingtonT) (Fig. 10B). Interestingly, PC1, PC2 and PC3 grouped BeauregardCK, CovingtonCK, BeauregardT, and CovingtonT together, which are all of the orange-fleshed cultivar combinations, suggesting that the two cultivars are very similar (Fig. 10C).
Table 1 Eigenvalues from the principal component analysis. Eigenvalues of the Correlation Matrix
1 2 3 4
Eigenvalue
Difference
Proportion
Cumulative
4.88384083 3.53704607 0.45935459 0.09355436
1.34679476 3.07769148 0.36580023
0.5426 0.393 0.051 0.0104
0.5426 0.9357 0.9867 0.9971
Table 2 Eigenvectors for the traits and first four significant principal components. Eigenvectors Trait
Prin1
Prin2
Prin3
Prin4
DMC SoluCarb Starch Granule R(SoluCarb/DMC) R(SoluCarb/Starch) R(Starch/DMC) R(Granule/Starch) R(Granule/DMC)
0.020906 0.316127 −0.377079 0.006181 0.417965 0.447447 −0.446786 0.423203 −0.061457
0.528577 0.346583 0.289273 0.519207 0.106653 −0.043456 0.007169 −0.153276 −0.45872
−0.066103 0.400189 0.03761 0.200381 0.440907 −0.159099 0.145754 −0.278309 0.689566
0.274745 −0.312795 −0.13986 0.533781 −0.376307 −0.063842 −0.376401 0.193932 0.444096
Note: DMC: dry matter content. Starch: starch content. Granule: starch granule length. SoluCarb: soluble carbohydrate content. R: Ratio.
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results are also in agreement with the previous observation that starch content decreased, sugar content increased, and in some genotypes dry matter increased during storage at ambient temperatures (Acedo Jr et al., 1996). The following could be the causes why the higher dry matter roots had higher soluble carbohydrates after storage than the lower dry matter roots: 1) Higher dry matter roots had greater amounts of starch available for degradation even though the total percentage of degradation was not necessarily higher; 2) Starch degradation during storage further reduced the starch amounts available for degradation in low dry matter cultivars. Therefore, the difference between high dry matter and low dry matter cultivars became larger. These results may also suggest that sweet potato tuberous roots of high dry matter contents can be sweeter than low dry matter cultivars after long term storage. Further research is required to investigate whether there is a biochemical or genetic difference, such as the activity of invertases between high dry matter and low dry matter cultivars. Why did the starch granule size become greater after storage? Our hypothesis is that the increase is due to a more rapid reduction in size of smaller granules than large granules. Some small granules became so small that they could not be detected, which lead to a large average size of the granule population. It is known that smaller granules degrade faster under enzymatic treatment in vitro than large granules (Franco et al., 1992; Kang et al., 1985). This hypothesis gained support from the percentage reduction of smaller granules (4.5 to 7.5 μm) as shown by the starch granule size distribution curve (Fig. 5). It is unclear why the small granules degraded faster, but it could be due to the large relative surface area of the smaller granules compared with the large granules. Dry matter content is a trait that greatly influences processing quality. Sweet potatoes with higher dry matter contents have much less oil absorption during frying (Hagenimana et al., 1998). In potatoes, high dry matter content reduces the absorption of oil during frying for fries and chips and makes the fries have a better texture (Li et al., 2006). Sweet potato is a crop of increasing importance in Canada. Both sweet potato fries and fresh sweet potatoes are popular to consumers in Canada. Dry matter content and starch properties can greatly influence the quality of fries and the suitability for certain cooking methods. ‘Beauregard’ (SP3392) and ‘Covington’ (SP3393) have high yield but the dry matter content is not high (Fig. 3A). This is likely due to the small average size of starch granules (Fig. 3D). The change of starch granule size during postharvest storage should be monitored because starch granule sizes influence the quality of processed products (Chen et al., 2003; Sahlström et al., 1998). The present study demonstrated that cultivars have very different starch granule sizes. The variation of starch granules of the different cultivars during storage had certain features in common, such as, a reduction of the proportion of small granules and an increase in relatively larger granules (Fig. 5). There are certain differences in variation, such as the difference between SP3391 (Fig. 5B) and ‘Beauregard’ (Fig. 5C). Starch granule size and dry matter content are positively correlated before and after storage. Sweet potatoes have genetic variation between cultivars for cold sweetening when stored at low temperature (Picha, 1987). The present study characterized the postharvest variation patterns and detected the correlations among the characters including dry matter, soluble carbohydrate contents, and starch granule size of various cultivars, including the important cultivar Beauregard. This study also provides an effective method to characterize starch granule size distribution. The knowledge learned from the present study and the characterization of the cultivars may be useful for the parental clone selection in breeding, for the management of postharvest processing quality, and for investigating the mechanisms of starch granule degradation in vivo.
Fig. 10. Principal Component Analysis of cultivar-storage combinations. The eigenvalues from the principal components are shown in Table 1. The eigenvectors for the traits used in the analysis and first four significant principal components are presented in Table 2. Beau: ‘Beauregard’; Coving: ‘Covington’.
4. Discussion This study investigated effects of storage on the starch granule size of four sweet potato cultivars, Our finding of a positive correlation between the dry matter before storage (DMCK) with the after storage traits (DMT, SoluCarbT, and GranuleT) suggests that tuberous roots of higher dry matter sweet potato cultivars tended to also have greater after-storage dry matter content, soluble carbohydrate content and starch granule size. In addition, we also provide support to the previous observations that there is a correlation between dry matter and starch content before storage (Wang et al., 1989) and a correlation between dry matter and starch granule size in potato (Zhang et al., 2011). The
5. Conclusion In summary, 1) Tuberous roots with higher DMC tended to have also higher starch content, soluble carbohydrate content, and starch granule 146
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size after storage; 2) Smaller starch granules degraded faster than larger starch granules during storage; 3) ‘Beauregard’ and ‘Covington’ were more similar in starch granule distribution patterns than with the other two cultivars. The knowledge learned from this study can be useful for industry to predict the variation of both food quality and processing quality including DMC, soluble carbohydrate contents, starch granule size of cold-stored sweet potatoes and may assist breeders to determine parental clones for breeding to improve sweet potato yield, postharvest storability, processing quality, and starch production.
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Declarations of interest The authors declare no competing interest. Acknowledgments We thank the A-base funding of Agriculture and Agri-Food Canada (to XQL) for its support to this research. We also thank the China Scholarship Council for its support to RMT and GDZ, and the farm management team of the Fredericton Research and Development Centre of Agriculture and Agri-Food Canada for the support in growing the sweet potato plants. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.postharvbio.2019.01. 004. References Acedo Jr, A.L., Data, E.S., Quevedo, M.A., 1996. Genotypic variations in quality and shelflife of fresh roots of Philippine sweet potato grown in two planting seasons. J. Sci. Food Agric. 72, 209–212. https://doi.org/10.1002/(sici)1097-0010(199610) 72:2<209::aid-jsfa640>3.0.co;2-n. Chen, Z., Schols, H.A., Voragen, A.G.J., 2003. Starch granule size strongly determines starch noodle processing and noodle quality. J. Food Sci. 68, 1584–1589. https://doi. org/10.1111/j.1365-2621.2003.tb12295.x. Chen, Z., Huang, J., Suurs, P., Schols, H.A., Voragen, A.G.J., 2005. Granule size affects the acetyl substitution on amylopectin populations in potato and sweet potato starches. Carbohydr. Polym. 62, 333–337. https://doi.org/10.1016/j.carbpol.2005.07.035. Franco, C.M.L., do Rio Preto, S.J., Ciacco, C.F., 1992. Factors that affect the enzymatic degradation of natural starch granules ‐effect of the size of the granules. Starch 44, 422–426. https://doi.org/10.1002/star.19920441106. Ganguli, S., Dean, D., Benjamin, A., 2003. Preparation of biodegradable plastics based on nanoengeveered sweet potato starch/mPE blends. In: Cohen, L.J., Ong, C., Arendt, C. (Eds.), Advancing Materials in the Global Economy - Applications. Emerging Markets and Evolving Technologies, Long Beach, CA, pp. 2545–2558. Grommers, H.E., van der Krogt, D.A., 2009. In: BeMiller, J., Whistler, R. (Eds.), Chapter 11 - Potato Starch: Production, Modifications and Uses, starch (third edition). Academic Press, San Diego, pp. 511–539. https://doi.org/10.1016/B978-0-12746275-2.00011-2. Hagenimana, V., Karuri, E.G., Oyunga, M.A., 1998. Oil content in fried processed sweetpotato products. J. Food Process. Preserv. 22 (2), 123–137. https://doi.org/10. 1111/j.1745-4549.1998.tb00809.x. Ji, C.Y., Chung, W.H., Kim, H.S., Jung, W.Y., Kang, L., Jeong, J.C., Kwak, S.S., 2017. Transcriptome profiling of sweetpotato tuberous roots during low temperature storage. Plant Physiol. Biochem. 112, 97–108. https://doi.org/10.1016/j.plaphy.2016. 12.021. Jusuf, M., Ginting, E., 2014. The Prospects and Challenges of Sweet Potato as Bio-Ethanol Source in Indonesia. Elsevier Ltd, pp. 173–179. https://doi.org/10.1016/j.egypro.
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Starch granule sizes and degradation in sweet potatoes during storage a,b
b,⁎
b,c
Suyan Niu , Xiu-Qing Li , Ruimin Tang , Guodong Zhang Loretta Mikitzelf, Muhammad Haroonb
b,d
e
T
a
, Xiubao Li , Bo Cui ,
a
Institute of Bioengineering, Zhengzhou Normal University, Zhengzhou, 450044, China Fredericton Research and Development Centre, Agriculture and Agri-Food Canada, Fredericton, New Brunswick, E3B 4Z7, Canada College of Life Sciences, Nanjing Agricultural University, Nanjing, 210095, China d College of Agronomy, Gansu Agricultural University, Lanzhou, 730070, China e Rizhao Academy of Agricultural Sciences, Rizhao, Shandong, 276500, China f Department of Agriculture, Aquaculture and Fisheries of New Brunswick, Wicklow, NB, E7L 3S4, Canada b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Sweet potato Post-harvest storage Dry matter Starch content Starch granule size
Starch granule sizes can greatly influence the quality of both table and processed products of sweet potato (Ipomoea batatas (L.) Lam), an important food, feed and industrial crop. Sweet potatoes require storage under suitable temperatures for year around supply, but there is very little research on starch granule size variation during storage. We characterized dry matter content (DMC), starch content, soluble carbohydrate (SoluCarb), and starch granule length before and after storage at 13 °C for 60 d in the cultivars, SP3388, SP3391, ‘Beauregard’, and ‘Covington’. Tuberous roots with higher DMC tended to have greater DMC, starch content, SoluCarb, and starch granule length after storage. Starch granule sizes ranged nearly continuously from small to large, without frequency peaks of small granules or large granules. Smaller starch granules degraded faster than larger ones during storage. The findings provided insights into starch degradation in plants, can help predict the processing quality of sweet potatoes during storage, and may also assist in their dry matter and starch-related breeding.
1. Introduction Sweet potato (Ipomoea batatas (L.) Lam), a species in the Convolvulaceae family, is an important food, feed (Zhang and Li, 2004) and industrial processing (Ganguli et al., 2003) crop. The tuberous roots of this crop can have very high dry matter contents, up to 42% (Li and Zhang, 2003), which is mainly starch (Woolfe, 1992). Food products from sweet potatoes include starch, noodles, fries, spirits and sugar syrup products (Kitahara et al., 2017; Liu, 2004), while industrial products include both biodegradable plastics (Ganguli et al., 2003; Liu et al., 2015) and bioethanol (Jusuf and Ginting, 2014; Kitahara et al., 2017; Liu, 2004). Both fresh/table and processed sweet potatoes, such as fries (Sato et al., 2018; Sylvia et al., 1997), are very popular in North America, China, and many other countries. Storage of sweet potatoes is critical for year-round supply for both table use and processing purposes, Sweet potato originated from a tropical region but is also adapted to relatively cool regions such as Canada and is a cash crop for farmers.
This adaptability provides valuable resilience of this crop to climate change. Both the aboveground parts and the tuberous root can be used as food and feed. The proven health properties (Oluyori et al., 2016), the crop adaptation, and the whole plant edibility make this crop important for both human health and world food security in the current era of rapid increase of world human population. Starch granule size influences the quality of processed products. Starch granule size distribution also influences bread characteristics (Sahlström et al., 1998). The feasibility of starch recovery from the wastewater during processing of fries and chips can be affected by the size of starch granules (Grommers and van der Krogt, 2009). Starch granule size affects starch hydrolysis in industrial processes. Smaller size of the starch granules results in a higher percentage of hydrolysis (Franco et al., 1992). Small granules are digested faster than large granules during enzymatic digestion in vitro (Kang et al., 1985). Wheat and tapioca starch are hydrolyzed faster than potato starch, likely due to structural or surface area differences (Sarian et al., 2012). Small and large starch granules from potatoes and sweet potatoes have different
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Corresponding author. E-mail addresses: [email protected], [email protected] (S. Niu), [email protected], [email protected] (X.-Q. Li), [email protected] (R. Tang), [email protected] (G. Zhang), [email protected] (X. Li), [email protected] (B. Cui), [email protected] (L. Mikitzel), [email protected] (M. Haroon). https://doi.org/10.1016/j.postharvbio.2019.01.004 Received 6 August 2018; Received in revised form 10 January 2019; Accepted 10 January 2019 Available online 16 January 2019 0925-5214/ © 2019 Published by Elsevier B.V.
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2. Materials and methods 2.1. Plant materials Four genotypes (lab codenames SP3388, SP3391, SP3392, SP3393) of sweet potato (Ipomoea batatas (L.) Lam) were used in this study (Fig. 1). SP3392 and SP3393, both with orange flesh, are known commercially as ‘Beauregard’ and ‘Covington’, respectively. The original slips of ‘Beauregard’ and ‘Covington’ were obtained from the Steele Plant Company, Gleason, Tennessee, USA. SP3388, (creamy white flesh), and SP3391 (purple flesh) were collected from the local market. The genotypes were propagated in the greenhouses of Agriculture and Agri-Food Canada (Fredericton, New Brunswick, Canada).
Fig. 1. Longitudinal sections of sweet potato tuberous roots, showing flesh color. Cultivars/clones from left to right: SP3388, SP3391, ‘Beauregard’, and ‘Covington’.
2.2. Plant growth conditions Slips of the cultivars were planted on the experimental farm of the Fredericton Research and Development Centre of Agriculture and AgriFood Canada (Fredericton, New Brunswick, Canada) on 29th June 2015, and harvested on 7th October. Fredericton has coordinates of 45.957319 °N and 66.647818 °W. The average daily maximum temperatures for July, August, and September 2015 were 25.7, 27.3, and 24.0 °C, respectively, and the average daily minimum temperatures for July, August, and September 2015 was 12.1, 14.4, and 9.6 °C, respectively according to Daily Data Report for 2015 by Environment Canada. (http://climate.weather.gc.ca/climate_data/). Unrooted cuttings from mother plants in a greenhouse were directly planted in the field. The space was 91 cm between rows and 30 cm between plants. Large weeds were manually removed in mid and late July. The plant plot was managed organically without using any plastic mulch, fertilizer, or chemical spray.
distribution of the acetyl group (Chen et al., 2005). Small granule fractions of potato and sweet potato starches can make better quality noodles than the large granule fractions (Chen et al., 2003). Li et al. (2018a) found that the emulsion droplet size increased with granule size when sweet potato starch granules were used as Pickering emulsion stabilizers. Knowledge of starch granule size variation can be useful for predicting and monitoring food quality and processing quality of tuberous roots. Storage of sweet potatoes can induce many changes including the carbohydrate composition, digestibility, amylase and trypsin inhibitor activity, and pasting properties (Morrison et al., 1993; Takahata et al., 1995; Zhang et al., 2002). Sweet potatoes, a tropical origin crop, cannot be stored at very cold temperature such as 4 °C because the tuberous roots get chilling injury (Li et al., 2018b). Therefore sweet potatoes are usually stored at ambient or relatively less cold temperatures such 13–15 °C (Picha, 1986; Ji et al., 2017; Li et al., 2018b). When stored at ambient temperatures, starch contents of tuberous roots decrease, and dry matter and sugar contents increase in some cultivars (Acedo et al., 1996). How the starch granule size changes and how dry matter, starch, and soluble carbohydrates co-evolve in sweet potato tuberous roots during cold storage are still largely unknown. In this present study, we characterized dry matter content, starch content, starch granule size, and soluble carbohydrate in comparison between the tuberous roots before storage and after storage in four sweet potato cultivars. The purpose is to increase knowledge about the variation of starch granule size, starch content, and soluble carbohydrate content of different sweet potato cultivars and about the potential interrelationship among these measured characteristics/properties during the cold storage treatment.
2.3. Sample preparation After harvest, the tuberous roots were cured for two weeks at 21 °C. At the end of curing (Day 0 for cold storage), the roots were moved to at 13 ± 1 °C at a relative humidity of approximately 60% in the dark for 60 days (Day 60). The tuberous roots did not show any sprouting at Day 60 of storage. Among the stored tuberous roots, three biological replicates for each cultivar-day combination were used for characterization and starch granule size analysis. The tuberous roots were washed thoroughly and surface-sterilized with 70% ethanol (v/v), then rinsed twice with distilled water. The tuberous roots were cut from the centre longitudinally (Fig. 1). One half of each tuberous root was used for determination of dry matter and starch contents, and the other half was used for the measurement of the starch granule size.
Fig. 2. Microscopic images of sweet potato starch granules. 138
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software AxioVision Rel 4.7. The starch granules with the length no smaller than 4 μm were used to calculate the average length of starch granules (Fig. 2). 2.5. Dry matter content The fresh weights of chopped pieces, approximately 3 mm squares, in total about 6 g, from each tuberous root of each cultivar was recorded after sampling. These samples were then oven-dried at 65 °C for about 10 d until the weight stabilized and dry matter recorded. 2.6. Starch and soluble carbohydrate contents The samples were freeze-dried in a freeze dryer (Labconco Corporation, Kansas City, Mo, UNSA) under vacuum of 0.022 mBar at −55 °C for 30 h before being ground in a mortar without liquid nitrogen. About 100 mg of each powder sample was utilized for starch content determination using the protocol of Megazyme total starch kit (K-TSTA kit, Megazyme, Wicklow, Ireland). The total starch content was calculated by the formulized worksheet provided by Megazyme using the sample weight and measured optical density. The soluble carbohydrate content was determined according to the anthrone sulfuric acid colorimetric assay described by Li (2000) using 100 mg of each fresh sample. 2.7. Data and statistical analyses Clustered heatmap analysis of multiple traits for the four cultivars was conducted including dry matter content (DMC), starch content (Starch), starch granule length (Granule), soluble carbohydrate content (SoluCarb) before (CK) and after the cold storage treatment (T), and the percentage of change after storage of each trait was calculated (for example, DMCP = 100*(DMT-DMCK)/DMCK) using the heatmap.2 in the R package (https://www.r-project.org/). Pearson correlations and P values among different measured and derived traits were calculated using the multivariate correlation option in SAS Enterprise (Version 7.1). Principal Component Analysis (PCA) (SAS Institute Inc., Cary, NC, version 9.4) was conducted to examine the grouping of the cultivars using the following traits: DMC, Starch, Granule and SoluCarb traits, and the ratios such as Starch/DMC ratio, Granule/Starch ratio, SoluCarb/DMC ratio, SoluCarb/Starch ratio, and Granule/DMC ratio. Clustered heatmap analysis of eight cultivar-storage combinations was conducted using DMC, Starch, Granule and SoluCarb traits. The overall mean of each trait (DMC, starch content, soluble carbohydrate content, and starch granule length) of the four cultivars before storage was tested for significance with that of the samples after storage using the two-way ANOVA-Duncan multiple mean range test (SAS Enterprise Guide 7.1). These overall means were also subjected to paired t-test used the statistical package in Excel 2010. The statistical test between two starch proportion percentages was conducted using U test The significance of starch granule size difference between the roots before storage and the stored roots was determined using the t-test (two tails, unpaired) statistical package in Excel 2010 for each cultivar.
Fig. 3. Dry matter content, starch content, soluble carbohydrate content, and starch granule length of four sweet potato varieties before and after storage. D0: Roots before storage. D60: Roots after storage of 60 d at 13 °C. A: dry matter content. B: starch content. C: soluble carbohydrate content. D: Average of starch granule length (> = 4 μm) (the bars are small because they are standard errors from 517 to 1673 granules). Means (the two columns on the right): the mean value among the means of the cultivars. Bar: standard error of biological repeats (in A, B, C: n = 3).
2.4. Starch granule size (length) measurement The starch granule size of sweet potato tuberous roots was measured by a modified method similar to that described by Zhang et al. (2011). A section of cross cut from the middle region, after removing the skin, was used for squeezing juice using a garlic press. The sample amount (in volume) filled into the garlic press was about 2.2 cm x 1.5 cm. The juice was collected in an Eppendorf tube. Five μL of well mixed juice was added to 20 μL water in a new Eppendorf tube, and immediately mixed with the pipette tip. Ten μL of diluted juice was used for microscopic observation (50X) under a Carl Zesis light microscope equipped with a polarizer and AxioVision Rel 4.7 software. Three to six images were randomly taken. The lengths of starch granules (Fig. 2) were measured with a micrometer using the imaging processing
3. Results 3.1. Dry matter content (DMC) The DMC of tuberous roots was higher after storage than before storage in all four cultivars (Fig. 3A). Before storage, the DMC was 25.2%, 28.9%, 20.7% and 20.0% in SP3388, SP3391, SP3392 (‘Beauregard’) and SP3393 (‘Covington’), respectively; whereas, the DMC increased to 29.5%, 31.8%, 22.4% and 23.3% in the four cultivars, respectively, after storage at 13 °C for 60 d. Overall, the DMC in the tuberous roots of the cultivars increased from 23.5% before storage to 139
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Fig. 4. Comparison of length of starch granules in four sweet potato cultivars. A. D0 (before storage). B. D60 (after 60 d storage). Arrow: A peak in proportion at the interval 8–9 μm with midpoint at 8.5 μm. Cultivars/clones: SP3388, SP3391, ‘Beauregard’, and ‘Covington’.
3.4. Starch granule size
26.7% in the tuberous roots after storage at 13 °C (P < 0.05), likely due to water loss during storage. The percentage of water loss during storage was not significantly correlated with the Day 0 dry matter content but was significant with the Day60 dry matter content (P < 0.05). Moreover, among these four cultivars, the DMCs of ‘SP3388′ and ‘SP3391′ were greater than that of ‘Beauregard’ and ‘Covington’ both before and after storage.
The average length of starch granules was consistently greater in the tuberous roots after storage than before storage in all four cultivars (P < 0.01). The purple-fleshed sweet potato SP3391 had the greatest average length of starch granules among the cultivars both before and after storage. The two orange-fleshed cultivars ‘Beauregard’ and had similar granule lengths both before and after storage. The change of the average granule length in the creamy white flesh cultivar SP3388 was greater than in the other cultivars, with the average length increasing from 8.4 μm before storage to 9.5 μm after storage (Fig. 3D).
3.2. Starch content Overall, the starch content of the cultivars decreased from 32.3% before storage to 21.6% (P < 0.01) after 60 d of storage (Fig. 3B). Both before and after storage, the starch content was higher in the creamy white flesh cultivar SP3388 and the purple-fleshed cultivar SP3391 than the two orange-fleshed cultivars ‘Beauregard’ and ‘Covington’. For the t-test of starch content between the stages before and after storage within each cultivar, the storage-resulted starch content reduction was significant in the two orange cultivars but not in the creamy white (SP3388) and purple (SP3391) cultivars.
3.5. Starch granule size distribution curve The percentage of starch granules in the population was calculated for each micrometer interval of granule length (Fig. 4). The starch granule population was mainly predominated by small granules, and large granules had small percentages. Overall, the size distribution was quite continuous in all four cultivars, except for the frequency of granules at 8.5 μm (for the interval of 8–9 μm) which clearly peaked compared with the two adjacent intervals in the cultivars SP3388 and SP3391 (Fig. 4A and B). This proportion peak at 8.5 μm was not detected in the two orange flesh cultivars ‘Beauregard’ and ‘Covington’. Before storage, the percentage of each granule size group in sweet potatoes decreased with increase of granule size. The proportions of small granules that fell into the intervals from 4.5 to 7.5 μm, were higher in the orange-fleshed cultivars ‘Beauregard’ and ‘Covington’ than those in the other two cultivars (Fig. 4A). In SP3388 and SP3391, the two non-orange cultivars, the percentages of granules at the interval midpoint of 5.5 μm were much higher than those at the midpoint of 7.5 μm (Fig. 4A). For example, for SP3388, the percentage in the population was 15.2% for the interval midpoint 5.5 μm and 10.8% for the interval point 7.5 μm (Fig. 4A).
3.3. Soluble carbohydrate contents The soluble carbohydrate content was higher after 60 d of storage in all four cultivars (Fig. 3C). The overall mean in the tuberous roots after storage was 1.8-fold (p < 0.01) of that before storage. The soluble carbohydrate content was higher in SP3388 (7.6%) than the other three cultivars before storage, but was highest in SP3391 (12.5%) after storage. The difference in soluble carbohydrate content before and after storage was similar in the two orange-fleshed cultivars ‘Beauregard’ and ‘Covington’. The increase of soluble carbohydrate content after storage was highest (the after/before ratio was 2.1) in SP3391 and lowest (1.4 fold) in SP3388. 140
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Fig. 5. Distribution of starch granule length before and after storage in each sweet potato cultivar. A. SP3388. B: SP3391. C: ‘Beauregard’. D: ‘Covington’. D0: Day 0 (before storage). D60: Day 60 (after storage).
that of this region before storage. For example, after storage, for SP3388, the percentage in the population was 9.6% for the interval midpoint 5.5 μm and 11.1% for the interval point 7.5 μm (Fig. 4B). After storage, the distribution of large granules (8.5 to 18.5 μm) for the intervals was also clearly different between the two non-orange-fleshed cultivars and orange-fleshed cultivars (Fig. 4B). The percentages of granules in this interval were generally higher in the SP3388 and SP3391 than in the two orange-fleshed cultivars (‘Beauregard’ and
After storage, the distribution of small granules for the intervals from midpoint 4.5 to 7.5 μm was clearly different between the two nonorange-fleshed cultivars (SP3388 and SP3391) and orange-fleshed cultivars (‘Beauregard’ and ‘Covington’) (Fig. 4B) and between before (Fig. 4A) and after (Fig. 4B) storage. In the two non-orange-fleshed cultivars (SP3388 and SP3391), the percentages of granules at the interval midpoint of 5.5 μm were similar with those at the midpoint of 7.5 μm (Fig. 4B). This granule size distribution was very different from 141
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Fig. 6. Three groups of starch granules classified according to granule length. Y-axis: The percentage (A) and the average length (B) of the starch granules within each group. D0: Day 0 (before storage). D60: Day 60 (after storage). In A: * P < 0.05 and ** P < 0.01 significant levels in u-test for percentages between Day 0 and Day 60 within the same size group and within the same cultivar. In B: significant level of *P < 0.05 and **P < 0.01 in t-test analysis of average starch granule length in Day 60 vs. Day 0 comparison within each length group. Bar in B: standard error. Starch granule numbers in B: for range of 4–7 μm, n was from 205 to 847; for the range of 7–15 μm, n was from 260 to 793; for > 15 μm, n was from 5 to 122, depending on the cultivar.
‘Beauregard’ and ‘Covington’ (Figure S1). The results indicated larger changes in the two non-orange-fleshed cultivars than the two orangefleshed cultivars.
‘Covington’) (Fig. 4B). The overall distribution curve of the starch granule size was compared directly before and after storage for each cultivar (Fig. 5). In the cultivar SP3388, the percentages of the granules with the size from 4.5 to 6.5 μm became much lower after storage, the percentage of granules with 9.5 to 18.5 μm was clearly increased after storage (Fig. 5A). In the cultivar SP3391, the percentages of the granule size interval from 4.5 to 5.5 μm became much lower after storage, the granule size interval region of 6.5 to 12.5 μm was clearly increased after storage (Fig. 5B). In the two orange-flesh cultivars, the percentages of the granule size of 4.5 to 7.5 μm became much lower after storage, and the region of 10.5 to 18.5 μm in ‘Beauregard’ (Fig. 5C) and 8.5 to 14.5 μm in ‘Covington’ (Fig. 5D) were clearly increased after storage. The common pattern was that the small granules decreased in all four cultivars and the relatively large granules increased in proportion even though the exact intervals showed differences between the cultivars. The Pearson correlation coefficient R between the distribution curves (Fig. 5) was calculated using each cultivar’s starch granule proportion changes (for each 1 μm size interval) after storage (Table S2). Significant positive correlations were found in the comparisons between SP3388 and SP3391 (R = 0.420), between the SP3388 and SP3392 (R = 0.706), between SP3388 and ‘Covington’ (R = 0.595), and between ‘Beauregard’ and ‘Covington’ (R = 0.691). SP3391 was not correlated with the two orange-fleshed cultivars. The mean value of absolute changes of starch granule number percentages within each interval after storage was calculated for 19 intervals (4.5 to 22.5 μm) for each cultivar (Figure S1). The order of change from the largest to the smallest was as follows: SP3388, SP3391,
3.6. Percentage change of small, medium and large starch granules after storage of tuberous roots The starch granules were classified into small (4–7 μm), medium (7–15 μm), and large granules (> 15 μm) (Fig. 6A). Most granules were in the 7–15 μm range, and very few granules were larger than 15 μm, in terms of percentage. Overall, the small granule portion decreased after storage, the medium portion increased, and the large granule portion increased or decreased depending on cultivar. The percentage of the small granule portion was 41.5%, 39.7%, 52.2%, and 54.8% before storage and 29.1%, 31.8%, 46.1%, and 50.6% after storage for the four cultivars SP3388, SP3391, SP3392 and SP3393, respectively. The percentage of small granules in the population between Day 0 and Day 60 within the same cultivar was highly significant in SP3388 and SP3391, significant in ‘Beauregard’, and nonsignificant in ‘Covington’. The results indicated that the percentage of small granules in the granule population decreased more obviously in the non-orange-fleshed cultivars than the two orange-fleshed cultivars. The percentage of the medium granule portion was 54.7%, 50.3%, 47.2%, and 44.4% before storage and 62.3%, 58.7%, 50.0%, and 47.4% after storage for the four cultivars SP3388, SP3391, ‘Beauregard’ and ‘Covington’, respectively (Fig. 6A). The percentage of medium granules in the population between Day 0 and Day 60 within the same cultivar was highly significant in SP3388 and SP3391, and non-significant in 142
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Fig. 7. The average length of the first 20% and 50% of the smallest and largest starch granules in four sweet potato cultivars. A and B: The first 20% (A) and 50% (B) of the smallest granules (counted from the 4 μm granules as the smallest to the larger ones until it reached the 20% of the population). C and D: The first 20% (C) and 50% (D) of the largest granules (counted from the largest in the population). D0: Day 0 (before storage). D60: Day 60 (after storage). * P < 0.05 and ** P < 0.01 significant levels in t-test analysis of average starch granule length in Day 60 vs. Day 0 comparison within each cultivar. Bar in B: standard error. Starch granule numbers (n) in A and C was 103–335, and in B and D was 259–837, depending on the cultivar.
‘Beauregard’ and ‘Covington’ (Fig. 6A). The results showed that the percentage of medium granules in the population increased more obviously in the non-orange-fleshed cultivars than the two orange-fleshed cultivars. The percentage of the large granule portion was 3.8%, 10.1%, 0.7%, and 0.9% before storage and 8.6%, 9.5%, 3.9%, and 2.0% after storage for the four cultivars SP3388, SP3391, ‘Beauregard’ and ‘Covington’, respectively (Fig. 6A). The percentage of large granules in the population between Day 0 and Day 60 within the same cultivar was highly significant in SP3388 and ‘Beauregard’, significant in ‘Covington’, and non-significant in SP3391 (Fig. 6A). The results demonstrated that the percentage of large granules in the population significantly increased in all the cultivars except the cultivar SP3391, the purple-fleshed cultivar.
3.8. Average length of the 20% and 50% smallest and largest starch granules The starch granules were ranked from smallest to largest, and the average length of the first 20% and first 50% of the smallest and largest granules was calculated. The first 20% of the smallest granules had an average length ranging from 4.5 to 4.6 μm before storage and 4.5 to 5.0 μm after storage (Fig. 7A). The average length of 50% of the smallest granules ranged from 5.3 to 5.8 μm before storage and 5.4 to 6.5 μm after storage (Fig. 7B). The average length for both the 20% and 50% smallest starch granules increased highly significantly after storage in SP3388 and SP3391, but there was no significant change in the two orange-fleshed cultivars ‘Beauregard’ and ‘Covington’. The top 20% of the largest granules had an average length ranging from 11.4 to 16.7 μm before storage and 12.1 to 15.9 μm after storage (Fig. 7C). The top 50% of the largest granules had an average length ranging from 9.3 to 12.8 μm before storage and 9.7 to 12.6 μm after storage (Fig. 7D). Starch granule average length was significantly increased in top 20% and 50% of the largest granules after storage in the SP3388, ‘Beauregard’, and ‘Covington’ (P < 0.01), but not changed in SP3391.
3.7. Mean length changes of small, medium and large starch granules after storage of tuberous roots The mean length of small granules (4 to 7 μm) between Day 0 and Day 60 increased significantly in SP3391, but decreased in ‘Beauregard’ and had no significant change in SP3388 and ‘Covington’ (Fig. 6B). The average length of medium granules (interval from 7 to 15 μm) between Day 0 and Day 60 was increased in SP3388, ‘Beauregard’, and ‘Covington’ but not in SP3391 (Fig. 6B). The average length of large granules was highly significantly decreased in SP3388 and SP3391, significantly increased in ‘Beauregard’, and had no significant change in ‘Covington’ after storage (Fig. 6B). The results indicated that the average length of large granules (> 15 μm) in the population varied differently among the cultivars - decreased in the two non-orange-fleshed cultivars but increased in the two orange-fleshed cultivars.
3.9. Correlation among traits The Pearson correlation coefficient (R) and statistical significance levels (P values) among all the measurements and derived traits are presented in Table S2. The following traits were analyzed: dry matter content, starch content, starch granule length, soluble carbohydrate content, and percentage of change after storage (CP) of sweet potatoes before storage (CK) and after storage (T). The DMCK value showed a 143
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Fig. 8. Heatmap clustering analysis of sweet potato cultivars using multiple traits of tuberous roots before and after storage. DM: dry matter content. Starch: starch content. Granule: starch granule length; SoluCarb: soluble carbohydrate content. CK: before storage. T: after storage. CP: percentage of change after storage (for example, DMCP = 100*(DMT-DMCK)/DMCK).
GranuleCK, GranuleT, GranuleCP, SoluCarbCK, SoluCarbT and DMCP were grouped into the same cluster. StarchCP and SoluCarb were not closely grouped together with any trait (Fig. 8).
positive significant correlation with DMT, StarchCK, SoluCarbT, GranuleCK and GranuleT values. Starch values (StarchCK and StarchT) were positively and significantly correlated with each other. StarchCK was positively correlated with DMCK, and StarchT was correlated with DMT. Also, StarchCP was positively correlated with StarchCK, StarchT, DMT, and GranuleT. GranuleCK value was positively correlated with DMCK, DMT, SoluCarbT, StarchCK, StarchT, and GranuleT values. After storage, the GranuleT value was positively correlated with DMCK, DMT, StarchCK, StarchT, StarchCP, and GranuleCK. SoluCarbCK was not positively correlated with any trait, but SoluCarbT was positively correlated with DMCK and GranuleCK.
3.11. Heatmaps clustering analysis of eight cultivar-storage combinations The clusters of eight cultivar-storage combinations (four cultivars, before and after storage) were analyzed using their shared traits, including dry matter content, starch content, granule size, and soluble carbohydrate contents (Fig. 9). Granule size and SoluCarb content are shown in the same cluster while the other traits are in the other cluster. Among the eight cultivar-storage combinations, orange cultivars (‘Beauregard’ and ‘Covington’) formed a cluster, and the creamy white (SP3388) and purple (SP3391) formed the other. Within each cluster, there were two subclusters: the before storage cluster and the afterstorage cluster (Fig. 9).
3.10. Heatmaps of multiple traits clustering four cultivars The heatmap and clusters analysis was based on 12 traits–DMCK, DMT, StarchCK, StarchT, GranuleCK, GranuleT, SoluCarbCK, SoluCarbT, DMCP, StarchCP, SoluCarbCP, and GranuleCP (Fig. 8). The cultivars were clustered into the non-orange-fleshed cultivars and the orange-fleshed cultivars. Each trait, before and after storage, was grouped in the same cluster. DMC and Starch contents in both before and after storage samples were grouped into a large cluster. The
3.12. Principal component analysis The principal component analysis identified the first four principal components (PC) as significant with eigenvalues greater than 0.1 and 144
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Fig. 9. Heatmap clustering of eight cultivar-storage combinations from two stages using multiple traits. SP3388CK, SP3391CK, BeauregardCK and CovingtonCK were from sweet potatoes before storage. SP3388 T, SP3391 T, BeauregardT, and CovingtonT were from sweet potatoes after storage of 60 d at 13 °C. Starch: Starch content. DMC: dry matter content. Granule: Starch granule length. SoluCarb: Soluble carbohydrate contents.
accounted for a total of 99.7% of the total variation (Table 1) based on nine traits (Table 2). The first PC (PC1) explained 54.3%, the second PC (PC2) described 39.3% of the total variation, and the third PC (PC3) represented 5.1% variation (Table 1). The first PC was dominated by the positive values for the SoluCarb/Starch ratio, the Granule/Starch ratio, and the SoluCarb/DMC ratio, and the first two highest negative values for the Starch/DMC ratio and Starch (Table 2). The second PC was dominated by the positive values for DMC and Granule, and the highest negative value for the Granule/DMC ratio (Table 2). The third PC was dominated by the positive values for SoluCarb, the Granule/ DMC ratio and the SoluCarb/DMC ratio, and the highest negative value for the Granule/Starch ratio (Table 2). PC1 and PC2 separated the eight cultivar-storage combinations into four groups: BeauregardCK and CovingtonCK, SP3388CK and SP3391CK, SP3388 T and SP3391 T, and BeauregardT and CovingtonT (Fig. 10A). The separation among the groups was mainly according to DMC, Granule, the starch/DMC ratio, and the SoluCarb/Starch ratio (Table 2). PC1 and PC3 together separated the eight combinations into a before storage group (SP3388CK, SP3391CK, BeauregardCK, and CovingtonCK) and an after storage group (SP3388 T, SP3391 T, BeauregardT, and CovingtonT) (Fig. 10B). Interestingly, PC1, PC2 and PC3 grouped BeauregardCK, CovingtonCK, BeauregardT, and CovingtonT together, which are all of the orange-fleshed cultivar combinations, suggesting that the two cultivars are very similar (Fig. 10C).
Table 1 Eigenvalues from the principal component analysis. Eigenvalues of the Correlation Matrix
1 2 3 4
Eigenvalue
Difference
Proportion
Cumulative
4.88384083 3.53704607 0.45935459 0.09355436
1.34679476 3.07769148 0.36580023
0.5426 0.393 0.051 0.0104
0.5426 0.9357 0.9867 0.9971
Table 2 Eigenvectors for the traits and first four significant principal components. Eigenvectors Trait
Prin1
Prin2
Prin3
Prin4
DMC SoluCarb Starch Granule R(SoluCarb/DMC) R(SoluCarb/Starch) R(Starch/DMC) R(Granule/Starch) R(Granule/DMC)
0.020906 0.316127 −0.377079 0.006181 0.417965 0.447447 −0.446786 0.423203 −0.061457
0.528577 0.346583 0.289273 0.519207 0.106653 −0.043456 0.007169 −0.153276 −0.45872
−0.066103 0.400189 0.03761 0.200381 0.440907 −0.159099 0.145754 −0.278309 0.689566
0.274745 −0.312795 −0.13986 0.533781 −0.376307 −0.063842 −0.376401 0.193932 0.444096
Note: DMC: dry matter content. Starch: starch content. Granule: starch granule length. SoluCarb: soluble carbohydrate content. R: Ratio.
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results are also in agreement with the previous observation that starch content decreased, sugar content increased, and in some genotypes dry matter increased during storage at ambient temperatures (Acedo Jr et al., 1996). The following could be the causes why the higher dry matter roots had higher soluble carbohydrates after storage than the lower dry matter roots: 1) Higher dry matter roots had greater amounts of starch available for degradation even though the total percentage of degradation was not necessarily higher; 2) Starch degradation during storage further reduced the starch amounts available for degradation in low dry matter cultivars. Therefore, the difference between high dry matter and low dry matter cultivars became larger. These results may also suggest that sweet potato tuberous roots of high dry matter contents can be sweeter than low dry matter cultivars after long term storage. Further research is required to investigate whether there is a biochemical or genetic difference, such as the activity of invertases between high dry matter and low dry matter cultivars. Why did the starch granule size become greater after storage? Our hypothesis is that the increase is due to a more rapid reduction in size of smaller granules than large granules. Some small granules became so small that they could not be detected, which lead to a large average size of the granule population. It is known that smaller granules degrade faster under enzymatic treatment in vitro than large granules (Franco et al., 1992; Kang et al., 1985). This hypothesis gained support from the percentage reduction of smaller granules (4.5 to 7.5 μm) as shown by the starch granule size distribution curve (Fig. 5). It is unclear why the small granules degraded faster, but it could be due to the large relative surface area of the smaller granules compared with the large granules. Dry matter content is a trait that greatly influences processing quality. Sweet potatoes with higher dry matter contents have much less oil absorption during frying (Hagenimana et al., 1998). In potatoes, high dry matter content reduces the absorption of oil during frying for fries and chips and makes the fries have a better texture (Li et al., 2006). Sweet potato is a crop of increasing importance in Canada. Both sweet potato fries and fresh sweet potatoes are popular to consumers in Canada. Dry matter content and starch properties can greatly influence the quality of fries and the suitability for certain cooking methods. ‘Beauregard’ (SP3392) and ‘Covington’ (SP3393) have high yield but the dry matter content is not high (Fig. 3A). This is likely due to the small average size of starch granules (Fig. 3D). The change of starch granule size during postharvest storage should be monitored because starch granule sizes influence the quality of processed products (Chen et al., 2003; Sahlström et al., 1998). The present study demonstrated that cultivars have very different starch granule sizes. The variation of starch granules of the different cultivars during storage had certain features in common, such as, a reduction of the proportion of small granules and an increase in relatively larger granules (Fig. 5). There are certain differences in variation, such as the difference between SP3391 (Fig. 5B) and ‘Beauregard’ (Fig. 5C). Starch granule size and dry matter content are positively correlated before and after storage. Sweet potatoes have genetic variation between cultivars for cold sweetening when stored at low temperature (Picha, 1987). The present study characterized the postharvest variation patterns and detected the correlations among the characters including dry matter, soluble carbohydrate contents, and starch granule size of various cultivars, including the important cultivar Beauregard. This study also provides an effective method to characterize starch granule size distribution. The knowledge learned from the present study and the characterization of the cultivars may be useful for the parental clone selection in breeding, for the management of postharvest processing quality, and for investigating the mechanisms of starch granule degradation in vivo.
Fig. 10. Principal Component Analysis of cultivar-storage combinations. The eigenvalues from the principal components are shown in Table 1. The eigenvectors for the traits used in the analysis and first four significant principal components are presented in Table 2. Beau: ‘Beauregard’; Coving: ‘Covington’.
4. Discussion This study investigated effects of storage on the starch granule size of four sweet potato cultivars, Our finding of a positive correlation between the dry matter before storage (DMCK) with the after storage traits (DMT, SoluCarbT, and GranuleT) suggests that tuberous roots of higher dry matter sweet potato cultivars tended to also have greater after-storage dry matter content, soluble carbohydrate content and starch granule size. In addition, we also provide support to the previous observations that there is a correlation between dry matter and starch content before storage (Wang et al., 1989) and a correlation between dry matter and starch granule size in potato (Zhang et al., 2011). The
5. Conclusion In summary, 1) Tuberous roots with higher DMC tended to have also higher starch content, soluble carbohydrate content, and starch granule 146
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size after storage; 2) Smaller starch granules degraded faster than larger starch granules during storage; 3) ‘Beauregard’ and ‘Covington’ were more similar in starch granule distribution patterns than with the other two cultivars. The knowledge learned from this study can be useful for industry to predict the variation of both food quality and processing quality including DMC, soluble carbohydrate contents, starch granule size of cold-stored sweet potatoes and may assist breeders to determine parental clones for breeding to improve sweet potato yield, postharvest storability, processing quality, and starch production.
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Declarations of interest The authors declare no competing interest. Acknowledgments We thank the A-base funding of Agriculture and Agri-Food Canada (to XQL) for its support to this research. We also thank the China Scholarship Council for its support to RMT and GDZ, and the farm management team of the Fredericton Research and Development Centre of Agriculture and Agri-Food Canada for the support in growing the sweet potato plants. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.postharvbio.2019.01. 004. References Acedo Jr, A.L., Data, E.S., Quevedo, M.A., 1996. Genotypic variations in quality and shelflife of fresh roots of Philippine sweet potato grown in two planting seasons. J. Sci. Food Agric. 72, 209–212. https://doi.org/10.1002/(sici)1097-0010(199610) 72:2<209::aid-jsfa640>3.0.co;2-n. Chen, Z., Schols, H.A., Voragen, A.G.J., 2003. Starch granule size strongly determines starch noodle processing and noodle quality. J. Food Sci. 68, 1584–1589. https://doi. org/10.1111/j.1365-2621.2003.tb12295.x. Chen, Z., Huang, J., Suurs, P., Schols, H.A., Voragen, A.G.J., 2005. Granule size affects the acetyl substitution on amylopectin populations in potato and sweet potato starches. Carbohydr. Polym. 62, 333–337. https://doi.org/10.1016/j.carbpol.2005.07.035. Franco, C.M.L., do Rio Preto, S.J., Ciacco, C.F., 1992. Factors that affect the enzymatic degradation of natural starch granules ‐effect of the size of the granules. Starch 44, 422–426. https://doi.org/10.1002/star.19920441106. Ganguli, S., Dean, D., Benjamin, A., 2003. Preparation of biodegradable plastics based on nanoengeveered sweet potato starch/mPE blends. In: Cohen, L.J., Ong, C., Arendt, C. (Eds.), Advancing Materials in the Global Economy - Applications. Emerging Markets and Evolving Technologies, Long Beach, CA, pp. 2545–2558. Grommers, H.E., van der Krogt, D.A., 2009. In: BeMiller, J., Whistler, R. (Eds.), Chapter 11 - Potato Starch: Production, Modifications and Uses, starch (third edition). Academic Press, San Diego, pp. 511–539. https://doi.org/10.1016/B978-0-12746275-2.00011-2. Hagenimana, V., Karuri, E.G., Oyunga, M.A., 1998. Oil content in fried processed sweetpotato products. J. Food Process. Preserv. 22 (2), 123–137. https://doi.org/10. 1111/j.1745-4549.1998.tb00809.x. Ji, C.Y., Chung, W.H., Kim, H.S., Jung, W.Y., Kang, L., Jeong, J.C., Kwak, S.S., 2017. Transcriptome profiling of sweetpotato tuberous roots during low temperature storage. Plant Physiol. Biochem. 112, 97–108. https://doi.org/10.1016/j.plaphy.2016. 12.021. Jusuf, M., Ginting, E., 2014. The Prospects and Challenges of Sweet Potato as Bio-Ethanol Source in Indonesia. Elsevier Ltd, pp. 173–179. https://doi.org/10.1016/j.egypro.
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