The use of nano-encapsulated plant extracts in inhibiting non-enzymatic browning in fruit canned in juice

Abstract

Non-enzymatic browning (NEB) reactions occurring during processing and storage are critical to the quality of fruit and fruit-based products, particularly canned fruits. This PhD work aimed to obtain more insight into inhibiting NEB reactions occurring in ‘Golden Delicious’ apples canned in fruit juice during storage by applying β-cyclodextrin (β-CD) encapsulated extracts of green rooibos. The first approach was the optimisation of β-CD-assisted extraction of green rooibos. Extraction conditions of β-CD (0 – 15mM), temperatures (40 – 90°C) and time (15 – 60 min) resulted in optimal conditions of: 15 mM β-CD: 40°C: 60 min, yielding an extract with a total polyphenolic content (TPC) of 398.25 mg GAE.g-1, metal chelation activity (MTC) 92.95%, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) radical scavenging of 1689.70 μmol TE.g-1, ferric reducing antioxidant power (FRAP) of 2097.53 μmol AAE.g-1, oxygen radical absorbance capacity (ORAC) of 11162.82 TE.g-1 and aspalathin content of 172.25 mg.g-1. Strong positive correlations of TPC towards the antioxidant activity were observed R2 (0.929 – 0.978) at p < 0.001. The physicochemical properties of optimal extract (β-GRE) in comparison to an aqueous counterpart (GRE) revealed that no differences (p > 0.05) were observed between the moisture content (MC) of GRE and β-GRE. However, the aw of β-GRE was significantly (p < 0.05) lower at a value of 0.11 than that of GRE at 0.18. Regarding colour, β-CD resulted in increased lightness (L*) and reduced redness (a*) (p < 0.05), with no significant differences (p > 0.05) on the yellowness (b*) of green rooibos. Thermogravimetric analysis (TGA) thermograms of β-CD, GRE and β-GRE revealed an initial loss in weight of 11, 2 and 6%, respectively. This loss was attributed to the evaporation of surface and adsorbed water. The thermal degradation of β-CD was observed between 340 – 375°C, while the GRE decomposed around 180°C. The thermogram of β-GRE was a superposition of GRE and β-CD, thus confirming the formation of inclusion complexes and improved stability with the degradation of β-GRE observed at 260°C. FT-IR Absorption spectra of β-CD and β-GRE samples overlapped at specific regions and showed certain spectral differences compared to the aqueous extract (GRE). Similarities between GRE and β-GRE were observed at 578, 1025, and 1154 cm-1. When the β-GRE inclusion complex formed, most characteristic peaks of GRE and β-CD disappeared or shifted in the newly formed complex. Browning kinetics and activation energy (Ea) of AA-added canned apples was investigated at 5, 23, and 37°C for 24 and 60°C for 12 weeks, respectively. Brix (°B), pH, browning indices (A294 and A420 nm, lightness (L*value) and colour difference (ΔE*)), reactant consumption (reducing sugars (RS) and AA) and intermediate NEB reaction products (furfural and hydroxymethylfurfural (HMF)) were monitored. The initial total sugar content comprised 66% fructose, 22% glucose and 12% sucrose. The °B ranged between 19.58 – 27.00 and pH 3.37 – 3.72. Overall, an increase and decrease in °Brix and pH were observed as the storage temperature and time increased, respectively, with no differences (p > 0.05) observed between (+AA) samples and those without added ascorbic acid (-AA). On the other hand, simultaneous sucrose hydrolysis and progression of the MR and sugar degradation resulted in no observed changes (p > 0.05) in RS for all samples. In terms of browning indices, AA degradation, HMF and furfural formation, an increase (p < 0.05) in reaction rate constants (k0 and k1) was observed as the storage temperature increased. A clear indication that higher temperatures favour NEB reactions. Samples (+AA) exhibited faster progression browning, HMF and furfural content compared to -AA, as shown by higher (p < 0.05) reaction rate constants (k0 and k1). Ascorbic acid added samples (+AA) at 60°C after 12 weeks of storage exhibited the highest A294 nm (281.96), A420 nm (9.93) ΔE* (61.88), lowest L*-value (6.64), lowest AA (4.13 mg.L-1) content, highest HMF (26.19 mg.100g-1) and furfural (64.31 mg.100g-1). The furfural content was higher than HMF, and this was due to the high content of fructose in the sample. Regarding kinetics, A294 and A420 nm followed first-order kinetics at 5 – 37°C, and changed to zero-order at 60°C. The opposite was observed for L*-value; meanwhile, ΔE*, AA degradation, HMF and furfural were adequately described as zero-order for all temperatures. The anti-browning capacity of β-GRE and GRE was described as inhibition (%I) and reduction in k0 of canned apples added with 0.25 and 0.5% extracts stored at 23 and 37°C for six months. Overall, β-GRE samples demonstrated superior inhibitory (p < 0.05) effect compared to GRE, and higher inhibitory was observed for samples stored at 23°C. For instance, β-GRE 0.25 and 0.5 exhibited the highest %I against browning development via L*value (40.93 – 46.67%), β-GRE 0.25 for ΔE* (46.67%) and β-GRE 0.25 and 0.5 for HMF (59.55 – 67.33%). In terms of furfural, no significant differences (p > 0.05) were observed between all GRE and β-GRE, although inhibition of furfural was reported at a range between 62.69 – 72.29%. The control sample at 23°C exhibited a high (p < 0.05) (k0) compared to GRE and β-GRE for L*value, ΔE*, furfural and HMF. However, no significant differences (p > 0.05) were observed amongst all extracts, with the exception of HMF. Increased storage temperature of 37°C reduced (p < 0.05) the inhibitory efficacy of all extract types, resulting in comparable abilities between GRE and β-GRE. In some cases, β-GRE 0.5 exhibited less inhibition (p < 0.05) than GRE, and even exhibited pro-oxidant activity, i.e., -17.17% for ΔE*. Higher Ea further confirmed the browning inhibition capacity of β-GRE in terms of colour development and HMF; however, GRE 0.25 proved superior against furfural formation.