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The use of nano-encapsulated plant extracts in inhibiting non-enzymatic browning in fruit canned in juice
Author(s)
Vhangani, Lusani Norah
Date Issued
2022
Type
Thesis
Publisher
Cape Peninsula University of Technology
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.
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.
Additional information
Thesis (Doctor: Food Science & Technology)--Cape Peninsula University of Technology, 2022
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