Impact of pulsed electric fields treatment on oat flour properties

Abstract

This thesis aimed to investigate the impact of Pulsed Electric Fields (PEF) treatment on oat flour properties. Two types of oat flour, namely raw (ROF) and thermally processed (TPOF), were characterised and subjected to PEF treatment at varying levels of electric field strength (2.0–2.2 and 4.0–4.5 kV/cm) and specific energy input (48–53, 200–249, 418–484 kJ/kg). The physicochemical and functional properties of PEF-treated whole flour and the resulting flour “fractions” were evaluated.

The commercial thermal treatment (kilning at 115 °C for 30 min and steam-cooking at 100–104 °C for 18 min) applied on oat flour (TPOF) highly influenced its physicochemical and functional properties. Compared to ROF, TPOF had significantly (p<0.05) higher β-glucan (3.88 vs. 1.04%), crude protein (11.51 vs. 10.60%), and crude fat (8.70 vs. 5.12%) contents, lower total starch content (62.80 vs. 76.83%), higher thermal transition temperatures (56.76–72.17 vs. 54.49–64.91 °C), and lower thermal enthalpies related to starch gelatinisation (4.27 vs. 7.74 J/g) and melting of amylose-lipid complex (0.59 vs. 1.68 J/g). TPOF also exhibited undetected lipase and peroxidase activities, larger component particle sizes, and aggregated starch granule clusters. Moreover, TPOF produced a more viscous paste that displayed higher resistance to disintegration compared to ROF. It is clear that the commercial thermal treatment applied caused drastic changes to the inherent properties of oats.

The effect of PEF treatment on oat flours with varying intrinsic characteristics prior to processing was investigated using ROF and TPOF. PEF treatment of ROF and TPOF produced oat flours with modified physicochemical and functional properties. Considerable changes in the properties of oat flours were exhibited after PEF treatment, with a greater impact observed on ROF compared to TPOF. Increased particle size (up to four-fold), damaged starch granules, and modified protein secondary structures were exhibited in both oat flours as a result of PEF treatment. Moreover, changes in the functional properties of oat, including decreased gelatinisation enthalpy (up to 80%), increased thermal transition temperatures (at least 3 °C), decreased overall viscosity profile, reduced pasting temperature (up to 12 °C), and increased susceptibility to digestive enzyme action were revealed with elevated specific energy input application. Modifications in the physicochemical and functional properties of PEF-treated ROF and TPOF were mainly driven by the level of specific energy input applied. Nonetheless, elevated electric field strength (2.1–2.2 vs. 4.2–4.4 kV/cm) application appeared to lower oat flour susceptibility to digestive enzyme action when ROF and TPOF were PEF-treated at 48–249 kJ/kg. With PEF treatment, the pasting stability of both ROF and TPOF improved. Moreover, the retrogradation property (reduced syneresis and hardness) of PEF-treated ROF was enhanced with lower temperature requirement and without affecting pasting time.

Owing to the heterogeneous nature of oat flour (in terms of composition and multimodal particle size distribution), PEF treatment effect on ROF and TPOF was further investigated though a systematic comparison of the physicochemical properties of flour “fractions” (referred as Supernatant, Top or S1, Middle or S2, Bottom or S3) obtained from untreated (referred as Control) and PEF-treated oat flours. PEF treatment of ROF and TPOF resulted in flour “fractions” exhibiting distinct percent compositional profile, in terms of β-glucan, crude protein, crude fat, and total starch contents. PEF treatment of ROF and TPOF also resulted in the release of β-glucan to the Supernatant. Moreover, PEF treatment of both types of oat flour resulted in “fractions” exhibiting varying physicochemical properties within (e.g. Top vs. Middle vs. Bottom “fractions” under similar PEF treatment intensity) and across (e.g. comparing S1 from different PEF intensities) treatments. For instance, it was found that S1 from PEF-treated ROF at 421–438 kJ/kg exhibited deformed and aggregated starch granules, and lower percent relative crystallinity and 1042/1019 FTIR ratio compared to S1 from Control. On the contrary, the S3 “fraction” from the same PEF treatment displayed intact starch granules, and demonstrated comparable percent relative crystallinity and 1042/1019 FTIR ratio with S3 from Control. Meanwhile, S1 obtained from PEF-treated TPOF exhibited an increase in particle size, while gelatinisation enthalpy, percent relative crystallinity, and 1042/1020 FTIR ratio were significantly lower compared to its corresponding Control. In particular, S1, from PEF-treated TPOF at 422–451 kJ/kg manifested severe changes in granule morphology. S2 and S3 fractions from PEF-treated TPOF exhibited the highest gelatinisation enthalpy and the least degree of observed changes in particle size, respectively. Amongst the different “fraction” samples (S1 vs. S2. vs. S3) of ROF and TPOF, S1 displayed the greatest extent of modification compared to S2 and S3.

Findings from this thesis demonstrated the potential of PEF treatment as a processing strategy to modify oat flour properties. Moreover, the uneven extent of PEF treatment effect on ROF and TPOF, which was most likely due to the heterogeneity of the starting material itself, provided an opportunity to produce “fraction” samples of diverse physicochemical properties, which can be utilised to tailor flour composition for specific applications, as well as for the production of a new line of food products and ingredients with targeted functional properties.