Effect of Calcium Fertilization on Processing Properties and Storability of Frozen French Fries

Abstract

The effect of calcium fertilization on the processing properties and storability of frozen French fries was investigated using three different varieties of processing-type potatoes, namely Toyoshiro (TS), Kitahime (KH) and Snowden. We attempted to understand the possible mechanisms contributing to quality differences in the French fries, by comparing tuber, starch and cell wall material properties. Calcium fertilization significantly increased the calcium contents of cell wall materials of all varieties, while a significant increase in chelating-agent soluble pectin content was observed in TS. The oil content of French fries was not significantly affected by calcium fertilization. On the other hand, two-way ANOVA indicated a significant effect of calcium fertilization on the hardness of the center and bud-end of French fries after 24 weeks of frozen storage. Results of this study suggest that calcium fertilization may be an effective method to improve the processing properties and storability of French fries.

Introduction

French fries are a popular food world-wide because of their pleasant taste, which is created by the combination of a crispy crust, soft inside and typical fried potato flavor (Van Loon et al. 2005). Frozen French fries, a semi-processed food product that is a common form of French fries, are produced from raw potato generally by washing, peeling, cutting, blanching, drying, par-frying and freezing (Gould 1999). The frozen French fries are final-fried by consumers, e.g., households and restaurants, prior to serving. During frozen storage of French fries, however, the growth of ice crystals, known as an ice recrystallization, is promoted by prolonged periods of frozen storage (Sutton et al. 1996; Hagiwara et al. 2002; Pronk et al. 2005), and the presence of large ice crystals within the tuber tissue can result in physical damage, drip loss and a consequent reduction in product quality (Sun and Li 2003).

Texture is a major factor determining consumers' acceptance of French fries, and depends on both the characteristics of the raw material and the processing history (Du Pont et al. 1992). In addition, oil content is another important quality characteristic of French fries that contributes to consumer acceptability (Romani et al. 2008), notable because of the demand to reduce the oil content of fried foods in response to health concerns (Moreira and Barrufet 1998). According to various studies, the tuber, starch and pectin properties of potato contribute to the texture as well as the oil uptake of French fries (Kirkpatrick 1956; Johnston et al. 1970; Sayre et al. 1975; Aguilar et al. 1997; Khalil 1999; Tajner-Czopek 2003; Tajner-Czopek and Figiel 2003; Golubowska 2005). For example, Johnston et al. (1970) reported that the texture of French fries was positively correlated with starch content, dry matter content and specific gravity of raw potato tubers; and higher viscosity and swelling power of starches isolated from raw potatoes were associated with lower hardness of French fries. Moreover, blanching of potato strips in 0.7% CaCl₂ solution reduced oil penetration by up to 14% in French fries due to the formation of Ca-pectates (Khalil 1999).

The starch and pectin properties of potato can be modified by mineral fertilization. For instance, in vitro experiments demonstrated calcium (Ca) fertilization enhanced the Ca concentration of the potato tuber cell wall and increased its galacturonic acid content (McGuire and Kelman 1986). Ca fertilization of potato is also known to improve resistance against internal and external defects (Tawfik and Palta 1992; Karlsson et al. 2006; Ozgen et al. 2006), due to the formation of Ca²⁺ bridges between pectin molecules in the middle lamella, which provides cell wall rigidity (Palta 2010). In addition, a study assessing the effect of Ca fertilization on the pasting properties of potato starch revealed significant correlations between the Ca content of potato tuber and pasting properties, such as peak viscosity, breakdown and final viscosity, of isolated starches (Bogucka 2014). Furthermore, varietal and genetic differences affect the starch and pectin properties of potatoes, even if the tubers are grown under an identical fertilizer application regime (Clough 1994; Haase and Plate 1996; Marle and Recourt 1997; Park et al. 2005; Van Dijk et al. 2002; Karlsson et al. 2006).

Thus, the objectives of the present study were to evaluate the effect of Ca fertilization on the tuber, starch and pectin properties of three different varieties of processing-type potatoes, and their influence on the processing properties and storability of frozen French fries.

Materials and Methods

Potato varieties    Three processing varieties of potatoes (Solanum tuberosum L.) were used in the present study, namely Toyoshiro (TS), Kitahime (KH) and Snowden (SD). TS is an early variety, while KH and SD are late varieties.

Potato production    Potatoes were cultivated from May to September 2014 under conditions commonly practiced by commercial farmers in Tokachi Subprefecture and based on the fertilizer application guide of Hokkaido Prefecture (Department of Agriculture of Hokkaido Prefecture 2010). The experimental field was divided into two sections, where conventional and calcium fertilizers were applied at 0 and 147 kg-Ca/ha, respectively, using calcium sulfate.

Raw materials    Approximately 50 kg of each harvested potato variety was stored, after harvesting on respective harvest dates, in a storage chamber at 4°C for 1 month, and then these potatoes were further stored at 14°C for 1 month for conditioning.

Determination of specific gravity and dry matter content of potato tuber    Specific gravity was determined by the method reported by Nzaramba et al. (2013). To measure dry matter content, 3 g of diced potato tuber was vacuum dried at 60°C and 30 cmHg to constant weight using a vacuum drying oven (DPF-41; Yamato Scientific Co., Ltd., Tokyo, Japan).

Preparation of starch and cell wall material (CWM) from potato tuber    Starch and CWM were prepared from potato tuber based on the method described by Noda et al. (2004) with slight modification. In addition, this procedure was designed to prevent β-elimination of pectin by maintaining a temperature lower than 80°C throughout preparation (Sila et al. 2009). One kilogram of potato tubers was washed using deionized water (DW), cut into 1 × 1 cm cubes, homogenized with 10 L of DW using a blender (JMM-1020-WY; YAMAZEN Co., Osaka, Japan) and filtrated through 710, 106 and 75 µm metal sieves. The residue was suspended in 3 L of DW, blended and sieved again, and this procedure was repeated twice more. The residue remaining on the sieves was dried in a hot air oven (WFO-700; TOKYO RIKAKIKI Co., Ltd., Tokyo, Japan) at 55°C for 16 h and the resultant material was used as CWM. The obtained starch slurry was filtrated through a glass filter (26G2; ASAHI GLASS Co., Ltd., Tokyo, Japan), and then the residual starch was suspended in 3 L of DW and stirred overnight. The starch slurry was again filtrated through a glass filter, and the washing procedure was repeated twice more. The obtained starch was dried in the hot air oven at 55°C for 4.5 h.

Analytical methods for measuring physical properties of starch    Mean particle size was determined using a compact laser diffraction particle size analyzer (LA-300; HORIBA Co., Ltd., Kyoto, Japan). The method reported by Noda et al. (2004) was employed to determine pasting properties, where a 4% starch slurry (on a dry weight basis) was subjected to analysis using a Rapid Visco Analyzer (RVA-4; Newport Scientific, Inc., Warriewood, Australia).

Analytical methods for chemical composition of CWM    Moisture content was determined using the standard methods of the Association of Official Analytical Chemists (AOAC) (AOAC International 2005). Ca content was determined by an inductively coupled plasma mass spectrometry (ICPS-8100; Shimadzu Co., Ltd., Kyoto, Japan) based on the method reported by George and Ridley (2004) with slight modification. CWM in a porcelain crucible was dry-ashed using a muffle oven (FM-35; Yamato Scientific Co., Ltd.) at 550°C for 8 h. The resultant ash was dissolved in 2N-HCl, and the solution obtained was subjected to mineral analysis by ICP. Chelating-agent soluble pectin (CSP) content was determined by extracting alcohol insoluble solid (AIS) from CWM based on the method described by Sabir et al. (1976) and Kato et al. (1997) with slight modification. AIS was prepared by washing CWM with 95% (v/v) ethanol, 99% (v/v) ethanol, acetone and diethyl ether. AIS (100 mg) was extracted with 50 mL of DW at room temperature overnight to remove the water-soluble pectin. The suspension was filtrated, and the residue was washed with DW. The residue was then re-suspended in 50 mL of 0.4% sodium hexametaphosphate and extracted for 1 h at 90°C. The suspension was filtrated, and the residue was washed with DW. The supernatant and washings were combined, and termed CSP. An aliquot of the CSP fraction was saponified with 0.2 M NaOH, and then subjected to the m-hydroxydiphenyl method described by Blumenkrantz and Asboe-Hansen (1973) to quantify galacturonic acid. Glucuronic acid exists only in trace amounts in the potato cell wall (Jarvis et al. 1981), and thus the glucuronic acid contribution was ignored.

Preparation of French fries    French fries were prepared based on the procedure described by Agblor and Scanlon (2000). Potato tubers were washed and peeled using a commercial peeler. Strips (1.0 cm × 1.0 cm × length of tuber) were cut along the apical to basal axis from the pith of the parenchyma region of the tubers using a commercial French fries cutter. The strips were rinsed with tap water immediately after cutting. Subsequently, the strips were blanched at 85 ± 3°C for 2 min using tap water, and dried at 100°C for 25 min by a forced-air drier (Minimini DX II; TAIKISANGYO, CO., Ltd., Okayama, Japan). Dried strips were parfried at 182 ± 5°C for 1 min using an electric fryer (TEFL-45N; TANICO Corp., Osaka, Japan) and then frozen by a blast freezer (FFB-092FMD6-N; Fukushima Industries Corp., Fukushima, Japan) at −40°C for 30 min. Frozen French fries were packed into polyethylene freezer bags and stored at −20°C in a deep freezer (SRR-K1583C2; Panasonic Corp., Tokyo, Japan) for 24 weeks. Parfried and frozen French fries were finally fried at 166 ± 5°C for 2.5 min and then cooled for 30 min at room temperature, without the French fries overlapping each other, prior to analysis.

Analytical methods of processing properties of French fries    Moisture content was determined by the vacuum drying method, where 2 g of French fries was vacuum dried at 60°C and 30 cmHg to constant weight. The Soxhlet extraction method was applied to measure oil content according to the method described in the AOAC standard method (AOAC International 2005). Hardness of French fries was evaluated using a texture analyzer (TA-HDi; Stable Micro Systems Ltd., Surrey, UK) at three measurement points, i.e., stem-end, center and bud-end of potato strips, as shown in Fig. 1. The rupture force as hardness was measured by penetrating with a 2 mm diameter flat-ended cylindrical probe (P/2; Stable Micro Systems Ltd.) into French fries at a 1-mm/sec compression rate.

Fig. 1.

Photograph of potato tuber and French fries.

Statistical analysis    Statistical analyses were conducted using SPSS for Windows (ver. 17.0). t-Test and two-way analysis of variance (ANOVA) were carried out to assess the effect of Ca fertilization, variety and frozen storage on the physicochemical properties of raw tuber, starch and CWM, as well as processing properties and storability of French fries.

Results and Discussion

Effect of Ca fertilization on tuber, starch and CWM properties    Table 1 indicates specific gravities and dry matter (DM) contents of different potato tubers with conventional or Ca fertilizer application. Specific gravities and DM contents of potato tubers did not change significantly with Ca application (p < 0.05); similar observations were reported by Silva et al. (1991), Locascio et al. (1992) and Clough (1994).

Table 1. Tuber properties of three potato varieties.

Variety	Ca fertilizer	Specific gravity (g/cm3)	Dry matter (%)
TS	−	1.087 ± 0.002	25.80 ± 0.30
	+	1.087 ± 0.002	25.35 ± 0.39
KH	−	1.072 ± 0.001	23.95 ± 1.01
	+	1.072 ± 0.003	20.88 ± 1.63
SD	−	1.085 ± 0.004	25.38 ±0.98
	+	1.087 ± 0.003	24.31 ± 2.14

Abbreviations: TS, Toyoshiro; KH, Kitahime; SD, Snowden n=4, specific gravity; n=3, dry matter content

Table 2 summarizes mean particle sizes and pasting properties of starches isolated from three different varieties of processing type potato tubers. Ca fertilization significantly decreased the mean particle size of starches isolated from TS and KH (p < 0.05). The effect of mineral fertilization on starch granule morphology has not been reported sufficiently for discussion. For example, Bogucka (2014) reported that foliar application of fertilizers containing manganese or boron did not result in significant changes in potato starch morphology. The present study demonstrated that Ca fertilization significantly decreased the mean starch granule sizes of the processing-type potatoes TS and KH. Among pasting properties, starch isolated from TS and SD grown with Ca fertilization showed significantly lower peak viscosity than the non-Ca applied samples (p < 0.05). Similarly, breakdown of starch isolated from all potato varieties grown with Ca fertilization was significantly lower than the corresponding non-Ca treated counterparts (p < 0.05). On the other hand, Sulaiman (2005) reported that Ca fertilization had no effect on the pasting properties of starches isolated from potato cv. Saturna. Therefore, the effect of Ca fertilization on the pasting properties of potato starch may depend on varietal or genetic differences.

Table 2. Mean particle sizes and pasting properties of starch isolated from potato tubers.

Variety	Ca fertilizer	Mean particle size	Peak viscosity	Break down	Setback	Pasting temperature
		(µm)	(cP)			(°C)
TS	−	43.34 ± 1.28 *	2672.67 ± 12.58 *	1570.67 ± 18.72 *	146.67 ± 9.87	66.97 ± 0.08
	+	39.98 ± 0.82	2564.33 ± 11.15	1347.00 ± 7.55	145.33 ± 8.08	67.53 ± 0.46
KH	−	44.81 ± 0.34 *	4030.33 ± 6.66	2623.33 ± 33.86 *	143.00 ± 40.78	68.85 ± 0.52
	+	41.97 ± 0.71	4082.67 ± 27.47	2483.67 ± 50.77	119.67 ± 12.01	68.03 ± 0.53
SD	−	39.47 ± 0.01	3198.33 ± 10.60 *	2072.33 ± 13.58 *	145.00 ± 17.78	66.95 ± 0.05
	+	39.44 ± 0.05	3065.50 ± 3.54	1809.00 ± 72.27	140.00 ± 7.94	67.77 ± 0.10 *

Mean values in the same column with * are significantly higher than that of the corresponding same potato variety with different Ca fertilization (p < 0.05).

n=3

Table 3 presents Ca and CSP contents of CWM extracted from TS, KH and SD. Ca fertilization significantly increased Ca contents of CWM extracted from TS, KH and SD (p < 0.05). Ca fertilization also increased CSP contents of TS (p < 0.05) and SD. McGuire and Kelman (1986) reported that an increase in the Ca concentration of soil leads to a higher Ca content of CWM and a concomitant increase of galacturonic acid contents in vitro. Sriamornsak (2003) reported an increased probability for the formation of cross-linkages by a given amount of Ca²⁺ in the presence of a sufficient amount of low esterified pectin. Indeed, Ng and Waldron (1997) reported that the availability of Ca²⁺ might be a limiting factor in the crosslinking of pectic polysaccharides in potato tissue. In addition, CSP mainly contains ionically cross-linked pectin (Sila et al. 2009). Thus, the significant increase in CSP contents of CWM extracted from TS may result from the formation of Ca²⁺ bridges between pectin molecules. On the other hand, even though KH and SD showed a significant increase in Ca contents of CWM, significant changes in CSP concentrations were not observed. Quantification of CSP was conducted by sequential extraction using a chelating-agent, and the possibility exists that the chelating-agent was not sufficient to fully solubilize the Ca-pectate in potato tuber tissue (Zykwinska et al. 2005). In addition, the maturation of potato tuber has a significant effect on the degree of esterification of its periderm pectin (Sabba and Lulai 2002), i.e., the growth period may affect the degree of methyl/acetyl esterification of potato pectin, which consequently contributes to the formation of Ca-pectate.

Table 3. Ca concentration and CSP contents of CWM extracted from three potato varieties.

Variety	Ca fertilizer	Ca	CSP
		(mg/100g, db)
TS	−	28.80 ± 0.37	2821.79 ± 224.86
	+	32.04 ± 0.36 *	3254.51 ± 80.75 *
KH	−	32.76 ± 1.24	4563.93 ± 39.31
	+	36.29 ± 0.47 *	4445.79 ± 40.63
SD	−	28.69 ± 1.29	4512.78 ± 371.13
	+	34.95 ± 1.99 *	4695.89 ± 484.40

Mean values in the same column with * are significantly higher than that of the corresponding same potato variety with different Ca fertilization (p < 0.05).

n=3

Effect of Ca fertilization on oil uptake in frozen French fries    Oil contents of French fries before and after 24 weeks of frozen storage are shown in Fig. 2. Significant differences were not observed between the oil content of French fries prepared from potatoes with/without Ca application within the same variety (p < 0.05). In the case of French fries, oil penetration occurs mainly after the removal of potato strips from the frying medium, via pores in the cellular structure created by vapor pressure and heat degradation (Mellema 2003; Dana and Saguy 2006). Although Ca fertilization did not have a significant effect on the oil absorption of French fries in this study, Khalil (1999) reported that blanching of potato strips in CaCl₂ solution reduced the amount of oil penetration into French fries because of the formation of Ca-pectates, which increase middle lamella-cell wall rigidity and resistance to degradation by the frying process. The blanching of potato strips using CaCl₂ solution appeared to be more effective for the formation of Ca-pectates compared to Ca fertilization. Thus, even though significant increases in Ca contents of CWM by Ca fertilization were observed (Table 3), the increase in Ca contents may have been insufficient to reduce oil absorption

Fig. 2.

Oil content of French fries prepared from before and after frozen storage.

n=3

Effect of Ca fertilization on the texture of frozen French fries    The hardness of French fries at the stem-end, center and bud-end before and after frozen storage is summarized in Table 4. Before frozen storage, French fries prepared from TS grown with Ca fertilization showed significantly greater hardness at the stem-end than the corresponding non-Ca application (p < 0.05). Ca fertilization significantly increased hardness at the center and bud-end of French fries prepared from TS and KH, respectively, after 24 weeks of frozen storage (p < 0.05).

Table 4. Hardness of French fries at the stem-end, center and bud-end prepared from six different potato tubers before and after 24 weeks of frozen storage.

Variety	Ca fertilizer	Hardness (N)
		Stem-end		Center		Bud-end
		0 week	24 weeks	0 week	24 weeks	0 week	24 weeks
TS	−	1.53 ± 0.57	1.23 ± 0.42	1.49 ± 0.68 †	0.79 ± 0.28	2.02 ± 0.74 †	1.21 ± 0.50
	+	2.41 ± 0.69 55 * †	1.52 ± 0.55	1.55 ± 0.53	1.28 ± 0.49 *	1.77 ± 0.65	1.44 ± 0.47
KH	−	1.68 ± 0.37 †	0.89 ± 0.26	1.15 ± 0.31 †	0.54 ± 0.19	1.27 ± 0.76 †	0.58 ± 0.20
	+	1.49 ± 0.37 †	1.13 ± 0.50	1.27 ± 0.41 †	0.50 ± 0.11	0.91 ± 0.17	0.78 ± 0.25 *
SD	−	2.25 ± 0.92 †	1.43 ± 0.60	1.48 ± 0.64 †	0.91 ± 0.42	1.40 ± 0.43 †	1.05 ± 0.41
	+	2.16 ± 0.62 †	1.44 ± 0.62	1.40 ± 0.42 †	0.89 ± 0.32	1.49 ± 0.33	1.32 ± 0.43

Mean values in the same column with * are significantly higher than that of the corresponding same potato variety with different Ca fertilization (p < 0.05).

Mean values in the same row with † are significantly higher than that of the corresponding French fries under frozen storage for 24 weeks among stem-end, center or bud-end (p < 0.05).

n=15

French fries texture is characterized by both a crispy outer crust and a soft mealy interior (Agblor and Scanlon 2000), and the interior texture is similar to that of cooked potatoes (Van Loon et al. 2007). The crust thickness was about 1 mm for all French fries samples (data not shown), and a similar observation was reported by Lima and Singh (2001). Since peak rupture force was detected at a position deeper than 1 mm from the French fries surface, the force required to rupture the interior was considered as hardness in this study. Thus, it is assumed that the texture of French fries is affected by a combination of tuber properties, such as specific gravity and DM content (Barrios et al. 1963; Jaswal 1970), starch properties as swelling behavior and granule size (Whittenberger 1951; Barrios et al. 1963; Johnston et al. 1970), and pectin properties of quality and quantity (Khalil 1999; Tajner-Czopek 2003; Ross et al. 2011).

Significantly higher Ca and/or CSP contents of CWM extracted from TS and KH may contribute to higher hardness of those frozen French fries, as a result of the formation of Ca-pectate in the middle lamella. It is known that Ca-pectate contributes to firm cohesion among contiguous non-esterified galacturonic acids and consequently provides rigid middle lamella-cell wall linkages (Grant et al. 1973) as well as firmer cell-cell adhesion (Knox 1992; Parker et al. 2001). Ormerod et al. (2002) reported that weakening of potato tissue on cooking is primarily controlled by thermal degradation of the middle lamella, and Tajner-Czopek (2003) revealed that soaking of potato strips in 0.4% CaCl₂ solution results in texture increase in French fries. Therefore, the formation of Ca-pectate induced by Ca fertilization in the middle lamella may have increased the hardness of French fries owing to improved resistance against degradation of the tuber tissue structure during the frying process and frozen storage.

On the other hand, tuber properties, such as specific gravity and DM content, and starch properties, such as granule morphology and pasting properties, seemed to have relatively little influence on the texture of French fries. For TS and KH, there were significant differences in hardness between French fries grown with and without Ca fertilization, although there were no significant differences in specific gravities and DM contents. This limited contribution of tuber density in cooked potato texture is in agreement with Van Dijk et al. (2002) and Hejlová and Blahovec (2008). In addition, Johnston et al. (1970) reported that the higher viscosity and swelling power of starches isolated from raw potatoes were associated with the lower hardness of French fries. This is in accordance with the observation in French fries prepared from TS in the present study.

Furthermore, a significant decrease in the hardness of frozen French fries at the stem-end, center and bud-end after frozen storage was observed in all varieties (p < 0.05). This is may be due to the formation of ice crystals and recrystallization, which causes rupture of the cell structure in potato tuber tissue (Alvarez and Canet 2000; Ullah et al. 2014). The rigid and firm potato tissue structure, resulting from the formation of Ca²⁺ bridges between pectin molecules in the middle lamella and cell wall, therefore, may provide greater resistance against structural damage by the frying process as well as expansion by the growth of ice crystals during prolonged frozen storage. In addition, since French fries were stored at −20°C for 24 weeks in the present study, lower temperature storage, such as −50°C, may suppress ice crystal growth (Hagiwara et al. 2005).

Meanwhile, in SD as a late variety, a significant effect of Ca fertilization on French fries texture was not observed, even though the Ca contents of CWM was significantly increased by Ca application. Karlsson et al. (2006) reported that the incidence of blackspot bruising of potato tuber is dramatically reduced when the tuber Ca concentration approaches approximately 250 ppm. Therefore, at Ca lower than 250 ppm or variety-specific concentration, sufficient modification of the tuber tissue structure to contribute to textural improvement of French fries might not be achieved.

Two-way ANOVA for evaluation of the effect of variety and Ca fertilization on storability of frozen French fries.    Table 5 summarizes the result of two-way ANOVA to assess the effect of Ca fertilization and variety on the physicochemical properties of raw tuber, starch and CWM as well as processing properties of French fries. From the two-way ANOVA, both variety and fertilization significantly affected the Ca content of CWM (p < 0.05 and <0.001, respectively). Thus, it was confirmed that Ca fertilization significantly increased the Ca content of the middle lamella and/or cell wall. On the other hand, there was no significant effect of Ca fertilization on the CSP content (p = 0.181), although significant varietal differences were observed in the CSP content of CWM (p < 0.001). Thus, it was inferred that even if the Ca content is significantly increased by Ca fertilization, other factors, such as degree of methyl/acetyl esterification, may contribute to the formation of Ca²⁺ bridges between pectin molecules in the middle lamella. Furthermore, significant effects of Ca fertilization on hardness of French fries were observed at the center and bud-end after 24 weeks of frozen storage (p = 0.042 and 0.006, respectively), and significant interaction of Variety × Fertilization was shown in hardness at center after 24 weeks of frozen storage (p = 0.003). Hence, at the bud-end, a significant effect of Ca fertilization was observed on hardness. In the present study, hardness of French fries was evaluated at three measurement points, i.e., stem-end, center and bud-end of potato strips, due to differences in the distribution of DM and chemical components such as Ca (Park et al. 2005; LeRiche et al. 2009; Ross et al. 2010; Subramanian et al. 2011). Moreover, LeRiche et al. (2009) reported that the Ca content decreases from the stem-end to budend of potato tubers. Therefore, the effect of Ca fertilization on the hardness of French fries may be greater than that at regions of relatively lower Ca content in potato tuber, i.e., the bud-end.

Table 5. Two-way ANOVA to assess the effect of variety and Ca fertilization on the storability of frozen French fries.

		Variety	Fertilization	Variety × Fertilization
		p value
Tuber properties
Specific gravity		0.609	0.920	0.025
Dry matter		0.002	0.024	0.211
Starch properties
Mean particle size		<0.001	<0.001	0.004
Peak viscosity		<0.001	<0.001	<0.001
Break down		<0.001	<0.001	0.054
Set back		0.433	0.310	0.601
Pasting temperature		<0.001	0.291	0.004
CWM properties
Ca		0.002	<0.001	0.150
CSP		<0.001	0.254	0.172
French Fries properties
Crude fat	0	<0.001	0.491	0.042
	24th	<0.001	0.558	0.044
Hardness (Stem-end)	0	0.001	0.137	0.002
	24th	0.003	0.100	0.553
Hardness (Center)	0	0.063	0.733	0.731
	24th	<0.001	0.042	0.003
Hardness (Bud-end)	0	<0.001	0.136	0.272
	24th	<0.001	0.006	0.932

Values of p in bold are significant (p < 0.05)

Meanwhile, in this study, higher Ca or CSP contents of CWM did not consistently result in higher hardness of French fries. Potato tubers are reported to vary in chemical composition, and such variations were observed within tubers as well as plants grown under identical conditions (Kleinkopf et al. 1987; Dijk et al. 2002; Busse and Palta, 2006). Thus, variability among the sample tubers might be a reason for the unsystematic results of the present study, and careful investigation of individual tuber characteristics is necessary.

Conclusion

The present study demonstrated considerable improvement of the hardness of French fries prepared from three processing-types of potatoes, as a result of Ca fertilizer application. French fries prepared from TS and KH grown with Ca fertilization showed significantly greater hardness as compared with non-Ca treated counterparts even after frozen storage (p < 0.05). In addition, the significant effect of Ca fertilization in hardness of French fries was confirmed by two-way ANOVA. Therefore, results of this study suggest that Ca fertilization may be an effective method to improve the processing properties and storability of French fries made from processing-type potato varieties.

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