Characterization of physicochemical and functional properties of selected Irish potato varieties developed through gamma irradiation

Abstract

Objective

This study investigated the physicochemical and functional properties of three improved Irish potato varieties (IP1, IP2, and IP3) irradiated at 15 Gy, 30 Gy, and 20 Gy, respectively, relative to the parent tubers (Asante and Sherekea).

Methods

A comparative analysis was adopted to study the tubers and their parents in a completely random design with three replicates, and data was analysed using R-software version 4.3.1.

Results

Key findings revealed a significant increase in tuber dimensions for IP1, with a thickness of 63.16 ± 9.25 mm, width (48.87 ± 7.60 mm), and length (82.5 ± 11.17 mm). Notably, IP1 with a shape index of 1.51 ± 0.27 (oval) was identified as optimal for French fries’ production, while IP2 and IP3 with shape indices of 1.13 ± 0.11 and 1.17 ± 0.10 (spherical) respectively are suited for crisps. Irradiation significantly increased specific gravity across all varieties, with IP1 (1.20 ± 0.00) classified as high and IP2 (1.07 ± 0.10) and IP3 (1.06 ± 0.10) as low. Crude protein decreased for IP2 (1.67 ± 0.06) and IP3 (1.53 ± 0.11) relative to Sherekea. The total ash was highest in Asante (4.15 g/100 g) and decreased post-irradiation in IP1. Irradiation also decreased the peak, breakdown, and final viscosities for IP1 relative to Asante, but varied effects were recorded for IP2 and IP3 relative to Sherekea, which was dose-dependent. Asante had the highest peak viscosity (308.99 ± 4.07), hence suitable for mashing. Overall, gamma irradiation enhanced tuber characteristics, influencing their suitability for processing.

Conclusions

Gamma irradiation can, therefore, be used to select the desired modification and make new commercial tuber varieties.

Keywords

Functional properties gamma irradiation Irish potatoes physicochemical properties proximate composition

Introduction

Irish potato (Solanum tuberosum L.) is a major food crop ranking third in production after rice and wheat worldwide.^1,2 Approximately 350 million metric tonnes are produced worldwide annually.^2–4 It is considered an excellent starch source compared to cereals and other tubers.⁵ The potato value chain in Kenya is a primary source of livelihood for approximately 3.5 million people directly and indirectly, with a substantive contribution to revenue.⁶ According to the Food and Agriculture Organization (FAO)⁷ and the Ministry of Agriculture (MOA),⁸ Irish potatoes can potentially help address food insecurity in Kenya and reduce overdependence on maize as the main staple due to its potential to yield more than the current production rates.

Nutritionally, Irish potatoes are rich sources of carbohydrates (17.5 g/100 g), mainly starch, and traces of vitamins such as vitamin C (3–40 mg/100 g).^9,10 However, the nutrient contents of Irish potatoes vary by climate conditions and across varieties.^11,12 Some of the widely used quality characteristics used to select Irish potatoes include dry matter content, sugar content, and the nutritional quality of the tubers.^13,14 Nutritional composition remains one of the criteria used to determine potatoes’ cooking and processing qualities. Irish potatoes have a vast and ready market since they are used in making various food products such as French fries (chips), bhajia, crisps, salads, soups, and other dehydrated, frozen, or canned foods.^3,15 Besides consumption, starch, the primary component in Irish potatoes accounting for 65.90−74.08 g dry weight.,^16–18 has been used in the pharmaceutical and food industries as a thickening, gelling, filling, binding and bulking agent.^19–21

Despite the proven economic importance of potatoes, some challenges tend to outweigh its importance. Key among them is bacterial wilt, which reduces potato yields by about 50 to 100%.²² The disease can, however, be controlled through induced mutation breeding of the tubers.²² Generally, induced mutation using gamma rays has been applied by exposing tubers to cobalt-60 rays at about 150 Gy to produce varieties with superior processing attributes compared to the parents.^23,24 For over 50 years, data indicates that seven mutant potato tubers have been developed and approved by the International Atomic Energy Agency (IAEA) across the globe from 1968 in Belgium to 2016 in Turkey using this technology.²⁴ Besides producing seed potatoes with desired agronomic advantages, gamma irradiation, unlike chemical treatment, can inhibit sprouting^23,25 and help prevent fruit ripening.²⁶

Whereas there are numerous positive contributions of gamma irradiation on improving shelf life and preventing tuber susceptibility to pests and diseases, it is also linked with developing unfavourable characteristics to the tubers by interfering with some physicochemical properties. For example, it has been reported that tubers treated with gamma irradiation often have reduced weight, reduced starch crystallinity, and low amounts of resistant starch when treated with doses above 150 Gy.²³ This is corroborated by Sarkar & Mahato,²⁶ who noted that tubers irradiated with 200 Gy significantly differed in colour and weight from those irradiated with 100 Gy. Similarly, high dosages of gamma irradiation have been reported to decrease the total protein and carbohydrate content.²⁷ Conventionally, gamma irradiation has been used over time as a post-harvest handling strategy to confer these characteristics in Irish potatoes. However, this can be expensive and inaccessible to most small-scale farmers. Studies have suggested that irradiation can be used to treat potato seeds for propagation, which makes farmers access irradiated Irish potato seeds with desired agronomic traits and increased shelf life, as well as better physicochemical properties as outlined by Chepkoech et al.²² In their study, Chepkoech et al. developed several tuber varieties using gamma irradiation, and three varieties were selected for continuous production due to their superior agronomic performance and better storability. However, the effects of irradiation on the physicochemical and functional characteristics of the three selected varieties have not been fully documented. Therefore, this study aimed to evaluate the physicochemical and functional properties of the three improved Irish potato varieties developed by gamma irradiation.

Materials and methods

Materials

Irish potatoes’ samples

Three improved tuber varieties (IP1, IP2, IP3) and two parent tubers (Asante and Sherekea) were used. IP1 variety was developed from Asante by exposure to gamma rays (15 Gy) from a Cobalt-60 source with a low dose rate of 2 grays per minute (Gy/min). IP2 was developed from Sherekea by exposure to 30 Gy, while IP3 was developed from Sherekea by exposure to 20 Gy at rates of 2 Gy/min.²² These were selected after screening several tubers exposed to different irradiation levels for agronomic studies.²² The tubers were collected from the Mau-Narok Agriculture Development Corporation (ADC) farm and sent to a laboratory at the Kenya Agricultural and Livestock Research Organization (KALRO) horticulture centre in Njoro, Nakuru. Mau-Narok ADC farm is located at 0°35’58.704’’ South and 36°0’25.092’’ East, while KALRO is located at 16°24’59.004’’ North and 120°35’35.015’’ East. The collection and handling of the tubers were guided by the regulations and guidelines of the Kenya Plant Health Inspectorate Service (KEPHIS).¹¹¹ The tubers were sorted, cleaned, and dried under ambient conditions (20°C and 30% Relative Humidity) in a naturally ventilated area for one week. Analytical grade sodium metabisulphite (96.5%, CAS No. 7681-57-4) was obtained from Science Lab Limited, Kenya.

Methods

The study adopted a Completely Randomized Design model with three replicates. It entailed comparing three potato varieties relative to their parent tubers by assessing their physicochemical and functional properties. The study was carried out between August 2023 to March 2024. Irish potato tubers for each analysis were randomly selected from each sack for each variety.

Potato flour preparation

To prepare potato flour, 1 kg of each potato sample was selected and peeled manually using a kitchen knife. The peeled tubers were dipped in 20 litres of water containing 7 g of sodium metabisulphite (96.5%) as described by Singh et al.²⁸ The potatoes were shredded using a kitchen shredder and spread uniformly on a tray to dry at 50 °C in an oven (Memmert-854, Germany) for 48 h to constant weight. Dried samples were allowed to cool in a desiccator (Top-180 mm, China) and then ground to pass through a 3 mm mesh sieve, and the resulting flour was packed in air-tight containers and kept at −20 °C for subsequent analyses.

Physical characteristics of the tubers

Twenty (20) tubers of each variety were randomly selected for physical analysis. The eye depth and number were determined using methods described by Kajunju et al.²⁹ Tubers’ shape and size were also determined using methods described by Ahangarnezhad et al.,³⁰ with slight modifications. The shape index of the tubers was determined using the formula below;

Shape index (I) = (L (mm))/(√DT) Where L = tuber length, D = tuber width, and T = tuber thickness as described by Githieya et al.³¹ Tuber dry matter content was estimated using the gravimetric method outlined by Yang et al.,³² with minor changes. The specific gravity was determined according to the weight underwater method described by Kajunju et al.²⁹

Chemical properties of the tubers

The moisture content, crude proteins, crude fat, crude fibre, ash content, total carbohydrate content, and specific mineral contents were determined following AOAC-approved methods.³³

Flour functional and rheological characteristics

Bulk density was determined using the gravimetric method described by Manupriya et al.³⁴ Water and oil absorption capacity were also determined as described by Manupriya et al.³⁴ with slight modifications. The pasting properties of the flour were analysed using Rapid Visco Analyzer (RVA) (Newport Scientific RVA-4, Australia) as described by Singh et al.³⁵ and Ahmed et al.³⁶

Statistical analysis

Data of the physical characteristics and laboratory tests carried out in triplicates were analysed using a one-way ANOVA test at a 95% confidence interval. Tukey Honest Significant Difference (HSD) multiple comparison was used to separate means in R-software version 4.3.2 for significant differences. Pearson's correlation analysis was also conducted to determine the correlation between functional properties (final viscosity, bulk density, OAC, and WAC) and physicochemical properties (dry matter content, specific gravity, moisture, lipids, and fibre) of the Irish potato flours.

Results

Irish potatoes physical characteristics

Results for physical characteristics are presented in Table 1. The tubers’ thickness ranged from 48.41 ± 4.45 mm (Asante) to 69.85 ± 4.68 mm (Sherekea), with Asante differing significantly from the others. The tubers’ width ranged from 41.76 ± 3.87 mm (Asante) to 57.53 ± 4.59 mm (IP2). Asante and IP1 widths were significantly lower (p < 0.05) than those of Sherekea, IP2 and IP3. Tuber length ranged from 70.69 ± 10.15 mm (IP2) to 82.5 ± 11.17 mm (IP1). Generally, the eye depths of Asante and IP1 were significantly (p < 0.05) lower than those of IP2, IP3, and Sherekea. The shape index for the tubers ranged from 1.17 ± 0.10 for IP3 to 1.58 ± 0.14 for Asante. Across the tubers, measures for the shape index differed significantly (p < 0.05) between Asante, IP1, Sherekea, and IP2 and IP3.

Table 1.

Physical characteristics of Irish potato tubers developed through gamma irradiation.

	Asante	IP1	Sherekea	IP2	IP3
Thickness(mm)	48.41 ± 4.45^a	63.16 ± 9.25^b	69.85 ± 4.68^c	68.36 ± 6.62^bc	65.94 ± 6.13^bc
Width (mm)	41.76 ± 3.87^a	48.87 ± 7.60^b	55.91 ± 4.92^c	57.53 ± 4.59^c	57.14 ± 5.12^c
Length (mm)	70.75 ± 7.99^a	82.5 ± 11.17^b	75.81 ± 7.71^ab	70.69 ± 10.15^a	71.39 ± 5.01^a
Eye Depth (mm)	0.73 ± 0.20^a	0.73 ± 0.22^a	1.68 ± 0.32^b	1.79 ± 0.26^bc	1.99 ± 0.58^c
No of eyes	8.00 ± 1.75^a	8.50 ± 1.93^ab	8.15 ± 0.99^ab	9.40 ± 1.43^bc	10.25 ± 1.65^c
Shape index	1.58 ± 0.14^b	1.51 ± 0.27^b	1.23 ± 0.14^b	1.13 ± 0.11^a	1.17 ± 0.10^a

Values are means ± standard deviation of three replicates. Means with different superscript letters along the same row are significantly different at p < 0.05, as assessed by Tukey's HSD. IP1 variety was developed from Asante by exposure to 15 Gy of gamma rays, while IP2 and IP3 were developed from Sherekea by exposure to 30 and 20 Gy, respectively, all at 2 Gy/min.

Measures for the specific gravity showed significant differences between IP1, IP2, Asante, and Sherekea, as shown in Table 2. There was a significant increase in the specific gravity of all daughter tubers relative to their parents. However, the dry matter content of the tubers did not differ significantly (p < 0.05) for all varieties.

Table 2.

Specific gravity and dry matter content of Irish potato tubers developed through gamma irradiation.

Variety	Specific gravity	Dry matter
Asante	1.15 ± 0.10^c	25.52 ± 4.68^a
IP1	1.20 ± 0.00^d	25.18 ± 3.77^a
Sherekea	1.04 ± 0.10^a	20.52 ± 1.06^a
IP2	1.07 ± 0.10^b	22.46 ± 2.48^a
IP3	1.06 ± 0.10^b	19.59 ± 1.01^a

Values are means ± standard deviation of three replicates. Means with different superscript letters along the same column are significantly different at p < 0.05, as assessed by Tukey's HSD. IP1 variety was developed from Asante by exposure to 15 Gy of gamma rays, while IP2 and IP3 were developed from Sherekea by exposure to 30 and 20 Gy, respectively, all at 2 Gy/min.

Irish potatoes proximate and micronutrient composition

The results of the proximate composition of the tubers are reported in Table 3. The moisture content of all the tubers did not differ significantly. The protein content for the tubers differed between some varieties and ranged from 1.53 ± 0.12 (IP3) to 2.20 ± 0.00 g/100 g (Sherekea). The crude fibre content for the varieties ranged from 2.45 ± 0.10 (IP1) to 6.63 ± 0.45 g/100 g (Asante), and it significantly (p < 0.05) decreased for most of the varieties relative to their native parents except for IP3 and the parent Sherekea. Crude lipid content for the tuber varieties significantly (p < 0.05) increased for IP2 and IP3 relative to the parent Sherekea.

Table 3.

Proximate composition of the Irish potato tubers developed through gamma irradiation (g/100 g).

Variety	Moisture	Protein	Fibre	Lipids	Carbohydrates	Ash
Asante	74.48 ± 4.68^a	2.07 ± 0.58^bc	6.63 ± 0.45^d	1.15 ± 0.20^ab	14.27 ± 2.04^bc	4.15 ± 0.10^c
IP1	74.82 ± 3.77^a	1.90 ± 0.00^b	2.45 ± 0.10^a	1.68 ± 0.18^b	15.52 ± 3.03^c	1.67 ± 0.08^a
Sherekea	79.41 ± 1.06^a	2.20 ± 0.00^c	4.80 ± 0.23^c	0.87 ± 0.10^a	9.63 ± 1.15^a	3.02 ± 0.30^b
IP2	77.54 ± 2.48^a	1.67 ± 0.06^a	3.68 ± 0.25^b	1.48 ± 0.30^b	14.56 ± 0.45^bc	2.80 ± 0.20^b
IP3	80.41 ± 1.01^a	1.53 ± 0.11^a	4.67 ± 0.10^c	1.65 ± 0.28^b	8.59 ± 0.94^a	3.15 ± 0.28^b

A significant increase in the carbohydrate content was recorded for IP2 relative to the parent Sherekea, but no significant differences were noted between Asante and the daughter IP1, as well as IP3 and the parent Sherekea post-irradiation. The total ash content for the varieties was lowest in IP1 (1.67 ± 0.08 g/100 g) but highest in Asante (4.15 ± 0.10 g/100 g), which differed significantly (p < 0.05) for the two varieties post-irradiation. However, the ash content for IP2, IP3, and Sherekea did not differ significantly, but it differed from that of IP1 and Asante.

The specific mineral composition of the tubers is presented in Table 4. The calcium content of the tubers ranged between 6.68 ± 0.61 and 18.29 ± 1.29 mg/100 g, potassium content about 14.71 ± 0.38 to 33.36 ± 1.03 mg/100 g, iron between 1.88 ± 0.08 to 2.91 ± 0.30 mg/100 g and phosphorus content ranging 0.06 ± 0.00 to 0.10 ± 0.01 mg/100 g.

Table 4.

Micronutrient composition of Irish potato tubers developed through gamma irradiation (mg/100 g).

Variety	Calcium	Potassium	Iron	Phosphorus
Asante	6.68 ± 0.61^a	33.36 ± 1.03^e	2.91 ± 0.30^c	0.08 ± 0.01^abc
IP1	18.25 ± 0.40^c	14.71 ± 0.38^a	0.83 ± 0.93^a	0.08 ± 0.00^ab
Sherekea	18.29 ± 1.27^c	18.40 ± 1.03^b	2.00 ± 0.02^b	0.10 ± 0.01^c
IP2	14.21 ± 0.47^b	21.03 ± 1.03^c	1.88 ± 0.08^b	0.06 ± 0.00^a
IP3	12.34 ± 0.41^b	23.66 ± 0.99^d	2.18 ± 0.03^b	0.09 ± 0.01^bc

Irish potatoes functional properties

The Irish potato flour bulk density, water absorption, and oil absorption capacity are presented in Table 5. The bulk density of the tuber flours was not statistically significant (P < 0.05) across the varieties and between the daughter and parents. The WAI measures were significantly lower (p < 0.05) for IP1 than all the other varieties. Significant reductions in the WAC were noted for IP1 and IP2 relative to their parents after irradiation, while an increase was recorded for IP3. OAC for the potato flours ranged from 81.80 ± 2.40 to 102.40 ± 0.53%, and variations were recorded between the developed tubers and their parents. There was a significant increase in the OAC for IP2 and IP3 compared to their parent (Sherekea), while there was a reduction for IP1 compared to the parent (Asante).

Table 5.

Bulk density, water, and oil absorption capacity of Irish potato tubers developed through gamma irradiation.

Variety	Bulk density (g/cm³)	WAI (g/g)	WAC (g/g)	OAC %
Asante	0.90 ± 0.02^a	2.38 ± 0.02^c	2.43 ± 0.06^d	98.43 ± 2.10^bc
IP1	0.91 ± 0.02^a	1.61 ± 0.03^a	1.83 ± 0.06^a	96.90 ± 1.45^b
Sherekea	0.89 ± 0.01^a	2.01 ± 0.15^b	2.17 ± 0.06^c	81.80 ± 2.40^a
IP2	0.93 ± 0.02^a	1.93 ± 0.01^b	2.00 ± 0.00^b	95.97 ± 0.80^b
IP3	0.87 ± 0.05^a	2.20 ± 0.01^c	2.57 ± 0.06^d	102.4 ± 0.53^c

Values are means ± standard deviation of three replicates. Means with different superscript letters along the same column are significantly different at p < 0.05, as assessed by Tukey's HSD. WAI: Water Absorption Index, WAC: Water Absorption Capacity, OAC: Oil Absorption Capacity. IP1 variety was developed from Asante by exposure to 15 Gy of gamma rays, while IP2 and IP3 were developed from Sherekea by exposure to 30 and 20 Gy respectively, all at 2 Gy/min.

The pasting properties of Irish potato flours are presented in Table 6. The peak temperature for the tuber varieties ranged from 81.64 ± 0.16 (IP3) to 86.69 ± 0.28 °C (Asante) and differed significantly (p < 0.05) between most varieties. Notably, there was a reduction in the peak temperature for IP1 compared to the parent Asante after irradiation. The pasting time ranged between 6.20 ± 0.16 (IP1) and 7.55 ± 0.18 min (Asante), while the peak viscosity for the tubers was between 265.87 ± 1.20 (IP1) and 308.99 ± 4.07cP (Asante). The peak time was significantly reduced post-irradiation for IP1 relative to the parent, unlike for IP2 and IP3 relative to their parent tuber. Generally, the peak, breakdown, and setback viscosities for the flour of IP1 were significantly reduced after irradiation compared to the parent tuber Asante. Nonetheless, a significant increase in the trough viscosity was recorded for IP1 relative to the parent tuber. On the other hand, no major significant differences were noted between Sherekea and its daughters except for the peak viscosity of IP2 relative to the parent.

Table 6.

Flour pasting properties Irish potato tubers developed through gamma irradiation.

	Asante	IP1	Sherekea	IP2	IP3
Peak Viscosity (cP)	308.99 ± 4.07^d	265.87 ± 1.20^a	276.90 ± 0.59^c	271.45 ± 0.95^b	278.60 ± 0.06^c
Peak Time (min)	7.55 ± 0.18^c	6.20 ± 0.16^a	6.41 ± 0.12^ab	6.25 ± 0.11^ab	6.60 ± 0.14^b
Peak Temp. (°C)	86.69 ± 0.28^d	84.93 ± 0.74^c	82.17 ± 0.23^ab	83.04 ± 0.50^b	81.64 ± 0.16^a
Trough viscosity (cP)	134.36 ± 0.60^a	272.69 ± 2.85^c	266.17 ± 0.92^bc	262.69 ± 2.94^b	271.77 ± 6.36^bc
Break Down (cP)	68.69 ± 0.97^c	56.82 ± 0.42^a	58.73 ± 0.40^b	57.60 ± 0.23^ab	59.21 ± 0.69^b
Final viscosity (cP)	474.13 ± 14.19^d	425.90 ± 3.61^a	451.13 ± 1.61^bc	433.87 ± 2.46^ab	453.84 ± 4.68^c
Set Back (cP)	366.64 ± 0.19^c	317.70 ± 5.12^a	335.25 ± 0.64^b	331.53 ± 5.17^b	337.40 ± 1.32^b

Correlation between physicochemical and functional properties of the flour samples

The correlation between the physicochemical and functional properties of the tuber varieties is presented in Table 7. The final viscosity of the tubers flour showed a strong positive correlation with the fibre content (r = 0.91, p = 0.00) and water absorption capacity (WAC) (r = 0.80, p = 0.00) of the tubers. Similarly, the fibre content strongly correlated with the WAC (r = 0.76, p = 0.001) and the OAC and positively correlated with the lipid content (r = 0.67, p = 0.01). Further, the specific gravity was positively correlated to the dry matter content of the tubers (r = 0.60, p = 0.02). However, the moisture content was significantly negatively correlated with specific gravity (r = -0.59, p = 0.02) and dry matter content (r = -1.00, p = 0.00), and so was the bulk density, which was negatively correlated to the WAC (r = -0.55, p = 0.03).

Table 7.

Pearson's correlation (r) for the physicochemical and functional properties of Irish potato tubers developed through gamma irradiation.

	Final Viscosity	Dry matter	Specific gravity	Fibre	Lipids	Moisture	OAC	WAC	Bulk density
Final Viscosity	1
Dry matter	0.031(0.912)	1
Specific gravity	−0.186(0.507)	0.596* ( 0.019)	1
Fibre	0.911**(0.00)	−0.087(0.758)	−0.235(0.399)	1
Lipids	−0.397(0.143)	0.02(0.945)	0.344(0.21)	−0.509(0.053)	1
Moisture	−0.031(0.912)	−1.000**(0.00)	−0.596*(0.019)	0.087(0.758)	−0.02(0.945)	1
OAC	0.064(0.82)	0.14(0.619)	0.388(0.153)	0.007(0.981)	0.672**(0.006)	−0.14(0.619)	1
WAC	0.800**(0.00)	−0.277(0.318)	−0.392(0.149)	0.764**(0.001)	−0.111(0.694)	0.277(0.318)	0.3(0.278)	1
Bulk density	−0.413(0.126)	0.005(0.987)	−0.009(0.975)	−0.358(0.189)	−0.166(0.554)	−0.005(0.987)	−0.229(0.412)	−0.554*(0.032)	1

**Correlation is significant at (p < 0.01); * Correlation is significant at (p < 0.05).

Discussion

Irish potatoes physical characteristics

Gamma irradiation resulted in differences in thickness, width, and length between Asante and its daughter IP1 and the shape index for Sherekea and the daughters. However, more investigations on these geometric characteristics for similar tuber varieties need to be done to ascertain how they are influenced by gamma irradiation. Measures of length, width, and thickness are useful as tubers with a length greater than 50 mm are best suited for making French fries, while those with lengths between 40 and 60 mm and large width are preferred for processing crisps.^37,38 Thus, IP1 is suitable for making French fries, while IP2 and IP3 are best suited to make crisps due to their wider width compared to IP1.

Irradiation significantly increased eye depth between Sherekea and its daughters (p < 0.05). These could be due to varietal and shape differences between the tubers. Across the varieties, the number of eyes for IP1, Asante, and Sherekea differed significantly (p < 0.05) from that of IP2 and IP3. Irradiation of the parent tubers did not significantly affect the number of eyes compared to the daughters except between Sherekea and IP3, which had more eyes. Tuber eye depth is an important quality parameter since deeper and more eyes result in huge losses during tuber trimming and peeling of tubers.^39,40 Findings from this study indicate that IP1 and Asante are better suited to prepare French fries because products made out of these tubers will be uniform in shape as there will be minimal invasion into the tuber's flesh during peeling.

The tuber shape index values showed that Asante and IP1 were oval (>1.5), while Sherekea, IP2, and IP3 were spherical (<1.5). Sherekea had a significantly higher shape index value than its daughter tubers IP2 and IP3. IP1 and Asante are suited for processing French fries since long and oval tubers or those with an oblong shape are preferred, while those with spherical shapes are preferred for crisps processing.^39–41 The oval tubers (IP1 and Asante) can also be used to prepare dehydrated and canned potato products since they result in a fairly firm to mealy texture.⁴² The findings also show that gamma irradiation significantly increased the geometric measures of the tubers, especially for IP1, compared to the parent tuber, which conferred better processing qualities, including ease of peeling.

Irish potatoes specific gravity and dry matter content

The general trend was that irradiation significantly increased the specific gravity between the parents and daughters. According to a study by Ndungutse et al.,⁵ specific gravity above 1.09 is classified as high, 1.08 to 1.09 intermediate, and less than 1.08 low. Based on these three classifications, Asante (1.15 ± 0.01) and IP1 (1.20 ± 0.00) had high specific gravities, while Sherekea (1.04 ± 0.01), IP2 (1.07 ± 0.01) and IP3 (1.06 ± 0.01) had low specific gravity. These findings align with those of other studies, including Mohammed,⁴³ who reported tuber specific gravity ranging between 1.061 to 1.095 among 17 tuber varieties. Soboka et al.⁴⁴ also reported tuber specific gravity ranging from 1.086 to 1.107 among tubers grown in the bale highlands of South-eastern Ethiopia, which agrees with the current findings.

The tuber's specific gravity helps to determine whether tubers can be used for making fried products, and it is considered the mealiness practical index by processors^.⁴⁵ Processors preparing fried and dehydrated potato products prefer tubers with high specific gravity since they have decreased oil absorption, lower energy consumption, and superior flavour in fried products.⁴⁶ Tubers with high specific gravity (>1.08) are also better suited for general applications in baking and mashing, while those with low specific gravity (<1.07) are suited for canning as they cannot disintegrate easily.^42,47 An increase in tuber specific gravity by 0.005 increases the yield of fried products by 1%, which indicates that specific gravity is a vital parameter of consideration when selecting tuber for different applications, especially for fried products⁴⁸ and this study reports that gamma irradiation is a potential way to increase specific gravity.

Gamma irradiation had no significant impact on the dry matter content of tuber varieties. Ekin⁴² noted that tubers with a high dry matter content (above 20%) are suitable for making crisps, dehydrated products, and French fries. This is because they absorb less oil, produce high yields, consume less energy, and result in products with a crispy texture. Tubers with <20% dry matter content are suitable for processing canned products since high content results in sloughing of the products after canning.⁴² Based on the dry matter content classification, IP1, Asante, and IP2 can easily produce French fries, crisps, and dehydrated potato products, while Sherekea and IP3 can be used to process canned potato products.

Irish potatoes proximate and micronutrient composition

Moisture content

The moisture content of the tubers did not differ significantly between the parent tubers and daughters. The range of moisture is in agreement with other studies that reported moisture ranges of 72.48–83.74%.^10,50–52 A low moisture value is desirable as it translates to a higher dry matter content for the tuber, giving fried potato products a crispy texture preferred by consumers. The low moisture also lessens the frying time of the tuber products and confers less oil absorption.⁴⁹ Study findings have also indicated a tendency of reduction in the moisture content of tubers when irradiated at high doses (>2 kGy) due to an increase in water's radiolysis and evaporation rates as the irradiation dosage increases.^53,54 However, most studies reporting moisture content reduction had the tubers irradiated as a post-harvest management practice. In contrast, the current study on irradiation was done as an agronomic management strategy, a possible reason for the lack of significant differences between the moisture of parent tubers and derivatives.

Crude protein

Protein content for the tubers differed between some varieties and ranged from 1.53 ± 0.12 (IP3) to 2.20 ± 0.00 g/100 g (Sherekea). Similar to our findings, Ooko⁴⁶ reported crude protein content of between 1.00 and 2.00 g/100 g among common tuber varieties in Kenya, while Van Niekerk⁵⁵ and Ngobese et al.⁵⁶ also recorded crude protein content ranges of 0.88–1.69 g/100 g and 1.57–2.87 g/100 g respectively among some tuber varieties. Mostly, irradiation led to the decreased protein content of the developed tubers relative to the parents (Table 3). The effect of irradiation on protein content is varied. Studies by Soares et al.,⁵⁴ Nouri & Toofanian,⁵⁷ and Costa et al.⁵⁸ found no significant differences between the total protein content of gamma-irradiated tubers relative to their parent tubers despite the different irradiation levels used in those studies. However, a study by Maltsev et al.⁵⁹ recorded a significant decrease in the protein content of the tubers after irradiation with gamma rays at doses of 25 and 50 Gy. In this study, irradiation resulted in significant decreases in the protein content witnessed between Sherekea (20 and 30 Gy) and its daughters but not in Asante and the daughter where 15 Gy was used. This shows that irradiation can reduce tubers’ protein content at higher dosages by triggering transient metabolic activation in the tubers and suppressing physiological processes like respiration, leading to changes in protein synthesis.⁵⁹

Crude fibre

The fibre content ranged between 6.63 ± 0.45 g/100 g (Asante) and 2.45 ± 0.10 g/100 g (IP1). For Sherekea, the daughter IP2 had a significantly lower fibre content, unlike IP3, which had no differences from the parent. This decrease could be due to the rate of irradiation since, as the rate of irradiation increased, there was a significant decrease in fibre content. However, there is a need for more investigations to establish the effects of gamma irradiation on the fibre content of Irish potatoes. The fibre content for IP1 agrees with those reported by Visvanathan et al.⁶⁰ and Liu,⁶¹ who noted crude fibre ranging between 1.4 and 2.7%. In this study, we recorded a significant decrease in the fibre content of the developed tubers relative to their parents, with IP1 recording the highest reduction, but the amount was higher than those of other studies.^40,62,63 Fibre plays an important role and is known for increasing oil and water holding capacity,⁶⁵ promoting faecal bulking and satiety and blood glucose regulation.^60,66

Crude lipids

Gamma irradiation significantly increased the lipid content of IP2 and IP3 compared to their parent (Sherekea). In contrast, studies by Mondy & Gosselin⁶⁷ and Todoriki et al.⁶⁸ noted a decrease in lipids among harvested tubers irradiated at 1 kGy and 0.5 kGy, then stored at 20 °C for 26 weeks and 5 °C for 28 weeks respectively after irradiation. Investigations in the two studies were done on tubers treated at the post-harvest stage; hence, there is a need for more research on tubers developed through this technology for propagation to ascertain its effects on lipid content. The crude lipids contents of the current studied tubers are within the USDA ranges of 0.10–1.00 g/100 g⁶⁹ for Sherekea (0.87 ± 0.10 g/100 g). The results of the lipid content of these tubers align with those reported by Burke,⁶⁴ who noted a lipid content of 1 g/100 g. However, the lipid content for these tubers is relatively higher than those reported by Abong et al.,⁴⁰ Murniece et al.,¹⁶ and Liang et al.,⁶² who reported ranges of 0.38–0.53%, 0.03–0.19%, and 0.03–0.13% among Kenyan, Latvian and Chinese tuber varieties respectively. Owing to its low amounts in tubers, lipids do not significantly contribute to the total potato calories but enhance the sensory characteristics of cooked tuber products and help maintain cell integrity.^64,70,71

Total carbohydrates

The findings of total carbohydrates are comparable to those of other studies, including Jin et al.,⁵² who recorded carbohydrate content ranging from 15.14 to 16.0 g/100 g among Korean tuber varieties. The high carbohydrate content for IP2 compared to its parent could be due to a higher irradiation dose (30 Gy), unlike IP3 exposed to 20 Gy. However, more studies need to be done to investigate gamma irradiation's effects on Irish potato varieties’ total carbohydrate content.

Total ash content

Total ash content for the varieties was significantly (p < 0.05) high in Asante (4.15 ± 0.10 g/100 g) and lower in IP1 (1.67 ± 0.08 g/100 g), as shown in Table 3. Unexpectedly, gamma irradiation at lower doses might have reduced the ash content of IP1 (15 Gy) relative to its parent tuber, unlike for IP2 (30 Gy) and IP3 (20 Gy), which might be due to differences in the dosage. The differences in the ash content can also be attributed to the genotypic variations between the tubers since mineral composition is majorly dependent on the tuber's genotype and agricultural practices.^40,72 Corroborating these results, Abong et al.⁴⁰ also reported a total ash content of 2.12–3.69% among eight common Kenya tuber varieties. However, the current results are higher than the findings by Sato et al.,⁵¹ Jin et al.,⁵² and Leonel et al.,⁷³ who recorded total ash content ranges of 0.88–1.03%, 0.87–1.04%, and 0.81–1.38% for tuber varieties in Japan, Korea, and Brazil respectively. The total ash content in a food product comprises minerals essential in promoting major nutrients’ absorption and utilization and maintaining cell integrity.

Micronutrients content

The findings fall within the normal ranges for calcium and Iron for some varieties, according to the USDA⁶⁹ report, which indicates calcium ranges for potato tubers between 6 to 16 mg/100 g and iron between 0.47 to 1.24 mg/100 g. These results for mineral content support those by Ndungutse et al.,⁵ except for potassium and phosphorus, who reported the average mineral content of tubers as 400.29 to 593.74 mg/100 g for potassium, 41.19 to 74.83 mg/100 g for phosphorus, 4.41–10.21 mg/100 g of calcium and 0.53–1.06 mg/100 g of iron among six tubers and four clones in Rwanda. Potassium and phosphorus content differences might be due to varietal characteristics, soil types, and agronomic practices, including fertilizer application and seed potato tuber storage.⁷⁴ The study findings also align with those by Pal et al.⁷⁵ except for phosphorus since they reported 10.40 to 15.55 mg/100 g of phosphorus, 6.36 to 10.50 mg/100 g of calcium, 28.61 to 38.77 mg/100 g for potassium and 0.35 to 1.49 mg/100 g for iron among eleven potato genotypes in India. The findings for calcium content in the current study also complement Jin et al.'s⁵² results, which reported calcium content ranges of between 5.25 and 9.31 mg/100 g among tuber varieties in Korea.

Gamma irradiation was noted to have varied effects on the mineral composition, with a significant reduction in calcium recorded for Sherekea and its daughters and an increase in IP1 relative to its parent. The potassium level was noted to reduce for IP1 relative to the parent (Asante) but increased for IP2 and IP3 compared to their parent. A significant reduction in iron content was also recorded for IP1 relative to the parent after irradiation, but no significant differences were noted in the phosphorus content except for IP2 and its parent. The reduction in some mineral composition could be due to differences in the gamma irradiation dosage, which has been documented to reduce mineral contents for sodium, potassium, lead, and copper.⁷⁶ However, a study by Hamideldin & Hussien⁷⁷ documented an increase in the amount of calcium, iron, and sodium among tubers irradiated at 10 Gy, unlike those irradiated at 30 Gy, hence the need for more investigations on mineral contents of tubers irradiated before planting. The reduction might also be due to a high concentration of antinutritive factors that might have increased due to irradiation, which masks the minerals, preventing ease of extractability.³⁴

Potato functional properties

Bulk density of the potato flour

Generally, irradiation did not significantly alter the bulk density. The results are comparable to study findings of bulk density by Hasmadi et al.⁷⁸ in potato tuber flour (0.998 ± 0.016 g/cm³) but higher than that of Kulkarni et al.,⁷⁹ who reported densities between 0.62 to 0.862 g/cm³ among Mauritanian potato flours. Bulk density is essential in determining flour's packaging requirements since those with small particle sizes have a higher bulk density.⁸⁰ Generally, flours with high bulk density are preferred for application as thickeners since they have a better mixing quality, unlike those with low bulk density, which are often used in preparing complementary foods for infants (6–24 months) since they facilitate ease of intake.^81,82 From the findings of the current study, the bulk density of the potato varieties is high, and they can easily be used as thickeners in the food industry since they have superior mixing qualities.

Water absorption Index and capacity of potato flour

The WAI and WAC increased significantly for the developed tuber varieties compared to their parents. Variations in WAC were noted by Anwar et al.⁸³ among flours derived from wheat grains treated with irradiation after harvest at different dosages of 3, 6, and 9 kGy, which ranged from 55.0 to 60%. Irradiation influences the interaction of amylose and amylopectin by reducing their swelling ability, which is responsible for holding flour's granular structure and affecting its WAC.³⁴ Irradiation also alters the protein composition through denaturation, which enhances starch gelatinization, promoting swelling of the flour's fibre and affecting its WAI and WAC.^34,84 In this study, irradiation significantly reduced the protein content for IP2 and IP3 (Table 3), which could be the possible reason for the increased WAC recorded for the two tuber varieties compared to the parent tuber (Sherekea). Hydrophilic carbohydrate groups (polysaccharides) that bind water molecules in food products and the gel-forming capacity also influence WAI and WAC.⁸⁵ Potato flour WAC is important in making extruded, smashed, and other baked food products. The relatively high WAC of flours from IP2 and IP3 indicates that they can be used for preparing mashed or potato-extruded foods.

Oil absorption capacity of potato flour

The variations in OAC for the daughters relative to their parents might be due to irradiation and differences in proximate composition, such as protein and lipids, which directly affect the flour's capacity to absorb and hold oils. The values are within those of Ndungutse et al.,⁵ who reported that the OAC of Irish potato tubers ranged from 99.21 to 123.14%. The OAC of flour is related to protein characteristics, including the types of amino acids, their hydrophilic and hydrophobic properties, and their formation.⁸⁶ Flours with a high concentration of hydrophobic amino acids are noted to have a higher oil absorption capacity since they easily interact with oil, which makes them suitable for flavour binding/retention, hence improving palatability and promoting a good mouthfeel of fried food products.^85,86 The potato tubers in this study have a relatively high OAC and can easily be suited for the preparation of different food products where flavour is of greater significance, such as French fries and crisps.

Pasting properties of the potato flours

The pasting properties of the tuber flour (peak, breakdown, setback, and final viscosity) were reduced considerably upon radiation. These differences may be due to exposure to varying rates of irradiation, which have been noted to interfere with the association of amylopectin and amylose molecules by breaking the long-chain amylopectin structures. Differences in peak viscosity signify the variation in the extent of polymerization of the leached amylopectin and amylose starch molecules due to irradiation.^34,89 During irradiation, the glycosidic bonds are broken down, reducing the starch's water-holding capacity, resulting in less swelling power of the starch granules and a reduced peak viscosity.⁹⁰ High peak viscosity in flours indicates strong gelling properties and better mixing ability at maximum temperatures,⁸⁸ weaker cohesive forces within their structure, and high phosphorus content.⁹¹ A significant increase in trough viscosity was recorded for IP1 and IP3 compared to their parent tubers. Trough viscosity is an essential measure as it helps compute breakdown viscosity. Trough viscosity is also critical in deriving setback, a proxy for retrogradation. A significant drop in breakdown viscosity was noted for IP1 relative to its parent (Asante). Breakdown, like the peak viscosity, displays the stability of a food material's starch resistance to disintegration when exposed to heating and cooling cycles. IP1 had the lowest values for breakdown, indicating that its starch granules are least susceptible to disintegration during retrogradation,⁸⁷ forming a more stable paste.

There was a significant decrease in final viscosity between Asante and IP1. However, the final viscosities between Sherekea, IP2, and IP3 did not decrease significantly. Low values for final viscosity indicate resistance in the starch to change viscosity during the heating and cooling cycle.^87,93 Tubers with high peak and final viscosity are preferred for the preparation of mashed products since they have a higher thickening capacity^,⁹² and fewer potatoes will result in a bulky product that confers a satiety feeling for the consumers as compared to those with a lower viscosity.⁹⁴ Tubers with high final viscosity are also suitable for producing fried products since they produce firm products with better textural characteristics.⁹⁵ From the current findings, Asante and IP3 (among the daughter tubers) had the highest values for peak viscosity; hence, they can suitably be used to prepare mashed tuber products (e.g. Mokimo) since fewer tubers or flours would be used to produce a denser product.

A significantly reduced setback viscosity was recorded for IP1 as compared to its parent. Asante's significantly high setback value compared to other varieties makes it more susceptible to retrogradation and suitable for producing noodles.⁹⁶ There was a significant reduction of peak time between Asante and IP1 upon gamma radiation. However, for Sherekea, there was no significant reduction as compared to its daughters. Similarly, a significant reduction in peak temperature was noted between IP1 and Asante, unlike for Sherekea and its two daughters (IP2 and IP3). This could be due to the differences in the irradiation levels and variations in the granular structures of each tuber's starch component. The peak temperature and time show the relative inverse sensitivity of the starch granules to heat and often vary with the type and form of starch used.⁸⁷ Extended peak time shows a lengthier time to cook.⁸⁸ From the current findings, the low peak time and temperatures recorded indicate that the starch granules are highly sensitive to heat treatment; hence, products prepared from these tubers will take a shorter time and lower temperatures to cook.

The reduction in pasting properties of the tubers relative to their parents can be attributed to exposure to gamma irradiation, as noted by several study findings, which also reported a decrease in peak, setback, and final viscosity of tubers starch after irradiation.^89,97,98 Gamma rays at lower doses (<20 kGy) induce active particles, including ions, free radicals, and electrons, which interact with starch molecules, resulting in oxidation and degradation of the starch constituents, which appear depolymerized and have reduced crystallinity.^89,99 The changes in the structure of the starch makeup resulted in decreased potato starch viscosity but increased stability and anti-digestibility of the paste.^23,99 It has also been noted that the starch fine structures, including helical, lamellar, pore, and channels, can also be altered through gamma irradiation, which all results in changes in the pasting properties.⁹⁹ Verma et al.⁹⁰ also noted that gamma irradiation reduced pasting properties such as peak viscosity, trough viscosity, setback viscosity, final viscosity, and pasting temperature as the dosage of gamma rays increased. Generally, these findings suggest that whereas there is a reduction in peak viscosity, gamma irradiation can be used to produce flours with stable pastes and lower setback viscosities.

Correlation between physicochemical and functional properties of the flour samples

The final viscosity of the tuber flours was significantly positively correlated with fibre content (r = 0.911, p = 0.00) and WAC (r = 0.800, p = 0.00). A strong positive correlation was also reported between final viscosity and fibre content (r = 0.965, P < 0.01) for three new Irish potato flours in Cameroon.¹⁰⁰ High fibre in flours increases the viscosity (peak, setback, and final viscosity) of flours due to gelatinization and retrogradation of the starch. Contrarily, Buzera et al.⁹² reported a negative correlation (r = -0.71, p < 0.01) between the fibre content and the final viscosity of potato flours derived from tubers exposed to different processing operations. The negative correlation was attributed to the effects of processing applications, including boiling, which results in gelling and retrogradation of starch granules; hence, a high fibre content restricted the swelling of starch granules, resulting in a negative impact on viscosity.¹⁰¹ The heating cycle disrupts starch structure by unfolding the double helix of amylose, which increases the intermolecular hydrogen bonds in starch, leading to increased viscosity due to decreased starch-water hydrogen bonds.^96,102 The interrelationship between the final viscosity and WAC can also be linked to the presence of fewer amylose-lipids complexes, which makes more starch interact with water, high polar amino acids, and high starch content,¹⁰³ thereby increasing viscosity.

The WAC of the tuber flours was significantly positively correlated to the fibre content (r = 0.764, p = 0.001). Ayo's¹⁰⁴ study also documented that an increase in the fibre content of amaranth flour resulted in an increase in its WAC under the same temperature due to the flour's uniform and unique granular structure. Bojňanská et al.¹⁰⁵ also noted that adding potato fibre to wheat flour resulted in an increased WAC of the flour, which shows the influence of fibre content on the WAC of flour. The OAC of the tuber flours showed a significant positive correlation (r = 0.672, p = 0.006) with the lipid content. The current results contradict a study by Garcia et al.,¹⁰⁶ which evaluated the OAC of sweet potatoes and biofortified sweet potatoes under a discontinuous frying process. In their study, Garcia et al.¹⁰⁶ design involved measuring OAC on tubers after frying, while in the current study, OAC was done on flours without prior exposure to the frying process. The lipid content of the flour samples in the current study was also significantly low, which might not have a profound effect on the OAC of the flours.

Moisture content of the flours also showed a negative correlation with the dry matter content (r = -1.000, p = 0.00) and the specific gravity (r = -0.596, p = 0.019). This is because the higher moisture content in potato flour is inversely proportional to the flour's dry matter, bulk, tap, true density, and porosity.¹⁰⁷ On the other hand, dry matter content was positively correlated (r = 0.596, p = 0.019) with the specific gravity of the tuber flours and is consistent with the findings of Mohammed⁴³ and Teye et al.¹⁰⁸ Another study examining the relationship between specific gravity and intercellular space for potato flour found that changes in intercellular space differ with varieties and may become large during storage, resulting in differences between dry matter content and specific gravity.¹⁰⁹ Wilson & Lindsay¹¹⁰ also examined the correlation between dry matter content and specific gravity in potatoes and concluded that the relationship is hyperbolic, but a linear approximation was adequate for most values of dry matter content.

Limitations of the study

Irish potatoes used in this study were sourced from Mau Narok region; hence, the findings can only be generalized in that region. Sample size calculation was also not performed for the current study since the tubers were all collected from the same field; hence, the limited number of samples may affect the statistical significance of the results.

Conclusions

From the results, gamma irradiation was noted to impact the positive attributes of the tubers in comparison with their parents. IP1 had a higher specific gravity, high lipid content, high carbohydrates, relatively higher sugar content, and lower ash content than its parent (Asante) after irradiation. On the other hand, IP2 and IP3 had low-value measures for shape index, higher specific gravity, low proteins, high lipid content, lower calcium levels, lower WAC, high OAC, and low peak and final viscosities, unlike their parent tuber after irradiation and multiplication. The results also showed that IP1 can be suited to preparing potato-fried products, while IP2 and IP3 can be used to make mashed or boiled products. Therefore, gamma irradiation can be recommended as a strategy to produce Irish potato cultivars for small-scale farmers with better processing characteristics since most of them cannot afford to access the technology at the post-harvest stage. More studies are recommended to investigate the storability characteristics, greening, economic feasibility analyses, and freeze-thaw stability of strips made from these improved tubers.

Footnotes

Acknowledgments

The authors are grateful to the staff at the food laboratory at KALRO Njoro and Jomo Kenyatta University of Agriculture and Technology for providing the necessary facilities and equipment for analysis purposes. We are also sincerely indebted to the invaluable support the University of Eldoret staff, students, and friends provided during the research period. The authors are also grateful to the 9^th International Interdisciplinary Conference held at the University of Eldoret, where an abstract of this manuscript was presented.

ORCID iDs

Victor Musitia

Emmanuel Ayua

Miriam Kinyua

Heka Kamau

Author Contributions/CRediT

Conceptualization of the research idea [Victor Musitia, Miriam Kinyua]; methodology [Victor Musitia, Emmanuel Ayua, Heka Kamau]; provision of study materials and payment of bench fee [Miriam Kinyua]; formal analysis and investigation [Victor Musitia]; writing of the original draft manuscript [Victor Musitia]; review and editing of the draft [Victor Musitia, Emmanuel Ayua, Heka Kamau]; supervision [Emmanuel Ayua, Miriam Kinyua, Heka Kamau].

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability

The data used to generate the results of this study is available upon reasonable request from the corresponding author.

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