Effect of pulsed electric fields on potato chips processing

Abstract

This thesis aims to investigate how pulsed electric fields (PEF) at low field strength (E < 1.5 kV/cm) influences potato processing. The impact of PEF and potato cultivars on the colour development during frying of potato chips was studied thoroughly. The impacts of PEF on the properties of starch granules located inside the potato tissue was also investigated.

Four potatoes cultivars – Russet Burbank, Nadine, Moonlight and Crop-77 – were chosen to study the effect of PEF on colour development during frying, due to their distinct differences in chemical composition such as dry matter content, total starch, amylose, glucose and total amino acids. PEF treatment was conducted on peeled potato at an electric field strength of 1 kV/cm, 20 µs pulse width, frequency of 50 Hz and different specific energy inputs of 50 kJ/kg (PEF Low) and 150 kJ/kg (PEF High). After being PEF-treated, potatoes were sliced (1.0 ± 0.5 mm thickness), blanched (boiling water for 3 min), pre-dried (75 °C for 10 min) and fried at temperatures of 150 – 190 °C for different time intervals.

It was found that PEF processing did not change the first-order kinetics of the colour (L*) change of potato chips made from the four cultivars. The inherent differences in reducing sugar and amino acids content are the important factors influencing the estimated kinetic rate constant (k) of changes in parameter L* during frying, showing Nadine having the fastest rate and Crop-77 having the slowest rate of darkening regardless of the frying temperature. Depending on the cultivars, PEF treatment combined with blanching affected the estimated k and Arrhenius activation energy (Ea) values. Combined PEF and blanching treatment decreased the estimated Ea values for Russet Burbank and Crop-77.

The intensity of Maillard reaction that occurred during frying was further studied using the volatile profile obtained in the headspace fraction of potato chips fried for 4.5 min in the temperature range of 150 and 190 °C. From a total of 76 volatile compounds identified across cultivars, it was found that the volatile compounds came not only from Maillard reaction but also from thermal degradation of fatty acids. (Alkyl) Pyrazines dominated the volatiles from Russet Burbank, Moonlight, and Crop-77, while Strecker aldehydes dominated those from Nadine. Alkylpyrazines are considered as precursors for acrylamide that is considered to be carcinogenic. Increasing frying temperature from 150 to 190 °C enhanced the occurrence of Maillard reaction to the final stage with increased pyrazine formation and decreased Strecker aldehydes, compared to thermo-oxidation.

Based on discriminant analysis of headspace volatiles of potato chips, Maillard reaction at the final stage (heterocycles formation) is hypothetically became less intense with increasing PEF energy for Crop-77, depended on PEF energy for Moonlight and Nadine, and became more intense for Russet Burbank. Strecker degradation is hypothetically became less intense after PEF for Crop-77, depended on PEF energy for Moonlight and Nadine, and became less intense for Russet Burbank. Oxidative degradation is hypothetically depended on PEF energy for Crop-77, became more intense for Moonlight, Russet Burbank, and Nadine.

Moreover, the effect of PEF (electric fields of 0.5, 0.7, 0.9, 1.1 kV/cm; 20 µs pulse width; 100 Hz and specific energy inputs of 50 and 150 kJ/kg) on the properties of potato starch was investigated on Agria potato. This study has shown that PEF had no effect on the native state of starch granules inside potato tissue as indicated by the presence of birefringence properties of starch after PEF treatment. The thermal stability, gelatinisation behaviour, susceptibility of starch in potato to hydrolysis by heat-stable α-amylase and digestive enzymes under in vitro human digestion conditions remained unchanged after PEF treatment. However, starch on the surface of potato that leached out into the medium was found to be more prone to PEF treatment effects, especially after treatment with high energy of PEF, as indicated by a narrow range of the gelatinisation temperature.

The impact of PEF on colour development and formation of volatile compounds through Maillard reaction and during frying of potato chips was shown in this study to be cultivar dependent. As hypothesised, potato cultivars having distinct different chemical properties responded differently to PEF when processed into potato chips. Concerning both colour and volatile parameters, the frying performance of potato cultivar was influenced by PEF when the glucose content was relatively low (as found in Crop-77 and Russet Burbank). Crop-77, a cultivar having a distinct low in glucose/amino acids ratio showed decreased levels of alkylpyrazines with increasing PEF, while Russet Burbank, a cultivar having a moderate glucose/amino acids ratio showed lowered activation energies of frying with increasing PEF.

However, the impact of PEF on the colour and volatile profiles was not affected by decrease in glucose content due to leaching. Probably, changes in amino acids was more influential in determining the effect of PEF on the colour and volatile profiles of the potato cultivars. Lastly, this study shows that PEF treatment does not influence the properties of starch found in potatoes. Based on this study, strategic selection of potato cultivar and optimisation of PEF processing parameters are possible and essential to achieve optimal benefits of retained starch properties, improved colour, and reduced levels of acrylamide-precursors in potato chips.