http://hdl.handle.net/11189/5535 2026-08-12T12:53:02Z http://hdl.handle.net/11189/5163 Title: Antioxidant and inhibitory effect of Maillard reaction products (MRPs) against enzymatic browning of Granny smith apples Authors: Vhangani, Lusani Norah; Van Wyk, Jessy; Thys, Xabisa; Silabele, Sonia Abstract: Maillard reaction products (MRPs) were prepared from glucose-casein (1:2 w/w) model systems at pH 8 heated at 60 and 75 °C for 6, 12 and 24 h. Browning intensity (BI) and pH reduction were monitored throughout the reaction. The antioxidant activity (AA) of resulting MRPs was evaluated through metal chelation (MC), reducing power (RP) and 1,1-Diphenyl-2-picryl-hydrazyl radical scavenging (DPPH-RS). Inhibition of enzymatic browning was evaluated through colour change on the CIELab system and the polyphenolase assay. The pH of MRPs decreased (p < 0.05) as the reaction temperature and times increased, which in turn coincided with an increase (p < 0.05) in BI and RP. DPPH-RS of MRPs increased as the temperature increased from 60 to 75°C, although a sudden decrease (p < 0.05) was observed at 75°C for 12 and 24h. The loss in AA was attributed to the formation of high molecular weight melanoidins known to exhibit lower AA. MRPs which resulted in a pH drop below 5 were effective in inhibiting polyphenolase activity. This enzyme has an optimum pH in the range of 5-7. Most MRPs produced after 6 h did not show a pH reduction of less than 5, however, these were able to inhibit the polyphenolase activity. This inhibition was attributed to MC properties of MRPs on the active site of the enzyme for all model systems. 2016-01-01T00:00:00Z http://hdl.handle.net/11189/4936 Title: Antioxidant activity of Maillard reaction products (MRPs) in a lipid-rich model system Authors: Vhangani, Lusani Norah; Van Wyk, Jessy Abstract: Ribose-lysine (RL), ribose-glycine (RG), fructose-lysine (FL) and fructose-glycine (FG) Maillard models (whole mixture (WM) pH 4 and 9) were heated at 60, 80, 121 °C for 30, 60, 120 min, and dialysed into low (LMW) and high molecular weight (HMW) fractions. Reducing power (RP), DPPH and peroxyl radical scavenging (PRS) evaluated indirect antioxidant activity (AA). Direct AA in a water-in-oil emulsion was evaluated through peroxide value (PV), p-anisidine, TBARs inhibition and oxidative stability (OS). PRS and RP increased significantly with temperature and time from FL < FG < RL < RG and LMW < WM < HMW for all pHs. MRPs showed higher PRS and RP than BHA. DPPH decreased (p < 0.05) with temperature and time from LMW > WM > HMW. With DPPH, only MRPs at 121 °C exhibited higher AA than BHA. MRPs exhibited low PV, p-anisidine and inhibited the formation of TBARs. BHA showed the highest OS, with p-anisidine, PV and inhibition of TBARS similar to that of MRPs. 2016-01-01T00:00:00Z http://hdl.handle.net/11189/4935 Title: Antioxidant activity of Maillard reaction products (MRPs) derived from fructose–lysine and ribose–lysine model systems Authors: Vhangani, Lusani Norah; Van Wyk, Jessy Abstract: Maillard reaction products (MRPs) were prepared from aqueous ribose-lysine (RL) and fructose-lysine (FL) model systems at pH 9, heated at 60, 80 and 120 °C for 15, 60 and 120 min. Browning intensity (BI) and pH reduction were monitored throughout the reaction. 1,1-Diphenyl-2-picryl-hydrazyl (DPPH-RS), peroxyl (PRS), and hydroxyl radical scavenging (HRS) and reducing power (RP) measured their antioxidant activity. The pH of FL and RL system decreased (p < 0.05) as reaction temperatures and times increased. This reduction coincided with the increase (p < 0.05) in BI for all MRPs. With the exception of HRS activity, the antioxidant activity of FL increased (p < 0.05) with increased reaction temperature, while that of RL systems decreased (p < 0.05). Concerning sugar reactivity, RL systems exhibited higher (p < 0.05) reduction in pH, BI, DPPH-RS, PRS and RP than FL model systems, with no considerable differences (p > 0.05) in HRS activity. 2013-01-01T00:00:00Z http://hdl.handle.net/11189/4704 Title: Optimisation of vitamin B12 and folate production by Propionibacterium freudenreichii strains in kefir Authors: Van Wyk, Jessy; Witthuhn, Corli R; Britz, Trevor J Abstract: Vitamin B12 and folate deficiencies in poor Southern African communities lead to mandatory fortification of basic foodstuffs. Since folate- and B12-enriched kefir offers a cheaper alternative, regimes to include Propionibacterium freudenreichii (PAB) strains into kefir grains (KG) were investigated: two levels of PAB cell concentrations; freeze-drying KG to preserve PAB activity; and repeated PAB culture additions. Elevated B12 and folate levels and PCR results confirmed inclusion of PAB in all KG. Repeated inoculations with freeze-dried cultures (PAB concentration 1 × 108 cfu mL−1) delivered the highest B12 and folate production rate and concentration after 3 d. The best treatment (freeze-dried inoculum (5 × 107 PAB cfu mL−1) reacted once, followed by freeze-drying) delivered 186% Recommended Dietary Allowance (RDA) (B12) and 19% RDA (folate) per 200 mL serving. Freeze-drying preserved PAB activity in KG. 2011-01-01T00:00:00Z