Digital Knowledge Collection:

Cryogenic pre-treatment during winemaking practices: effect on white wine sensory and chemical profiles

Thu, 01 Jan 2026 00:00:00 GMT

Title: Cryogenic pre-treatment during winemaking practices: effect on white wine sensory and chemical profiles Authors: Van Breda, Valmary Michelle Abstract: Sauvignon blanc (Vitis vinifera), one of the most extensively cultivated white wine grape varieties globally, is renowned for producing wines with characteristic “grassy” and “tropical” varietal aromas. These aromas result from aroma compounds, such as methoxypyrazines and varietal thiols, present in the grape skin and pulp, the latter being released by yeast during the alcoholic fermentation process. Similarly, Chenin blanc, another popular white wine cultivar that shares a genetic origin with Sauvignon blanc (both originating from Savagnin blanc or Traminer), was also shown to possess varietal thiols. Several anthropogenic factors have been investigated to increase varietal aroma compounds, specifically the varietal thiols; however, the reported results varied. Alternative technologies, such as low-temperatures and cryogenic pre-treatment, have been researched and shown promise. Additionally, the impact of harvesting technique (hand versus machine-harvested) has been researched, reporting that grape juice and wines from mechanically harvested grapes had higher levels of varietal thiol precursors and varietal thiols in the final wines. Therefore, this research aimed to investigate the effect of pre-fermentative cryogenic freezing (-20 ºC and -4 ºC) at four production stages (whole grapes [WG], macerated grapes [MG], turbid must [TM] and clear juice [CJ]), immediately (T0) and for a four-month (T4) storage period on the standard physicochemical properties, varietal aroma compounds (volatile thiols and methoxypyrazines) and the sensory profiles of two popular South African (SA) white wine varieties (Sauvignon blanc and Chenin blanc). Following complete defrosting, a standard white winemaking protocol was followed. The control wines were not subjected to any cryogenic treatments. Subsequently, all grape musts were analysed for physicochemical properties, and wines were analysed for physicochemical properties, varietal aroma compounds, and sensory properties. Physicochemical parameters in the grape must were generally unaffected by the cryogenic pre-treatments (total sugar, glucose/fructose ratio, and total soluble solids), except for the pH (higher in cryogenically pre-treated grape must) and total acidity (TA) (lower in cryogenically pre-treated grape must) when compared to the unfrozen control. Furthermore, the yeast assimilable nitrogen (YAN) was higher in the unfrozen control and cryogenically pre-treated grape must for all Producers in 2020 compared to 2021, suggesting that the differences were vintage-related. The physicochemical parameters of the final wines were generally within the legal limits for SA white wines, except for pH, which was slightly higher (> 3.4) in the wines made from the cryogenic pre-treatments. The concentrations of the varietal thiol, 3-sulfanylhexyl acetate (3-SHA), detected in the Sauvignon blanc wines exceeded the aroma perception threshold (4 ng L -1 ) and the reported range for SA Sauvignon blanc wines (23-151 ng L -1 ), whilst the concentrations of 3-sulfanylhexan-1-ol (3-SH) were generally within the aroma perception threshold (60 ng L-1 ) and reported range (178-904 ng L-1 ). Moreover, the concentrations of 4-methyl-4-sulfanylpentan-2-one (4-MSP) were found to be below the aroma perception threshold (0.8 ng L -1 ) and the typically reported range (0-21.9 ng L -1 ). For the Chenin blanc wines, the concentrations of 3-SHA were found to be higher than the typical reported range for SA Chenin blanc (5-253 ng L-1 ) and above the aroma perception threshold (4 ng L -1 ), whilst the concentration of 3-SH were generally below its reported range (99-1124 ng L -1 ) and aroma perception threshold (60 ng L -1 ). Furthermore, the concentrations of 4-MSP were detected above the reported range, not detected (n.d.), but below the aroma perception threshold (0.8 ng L -1 ). Methoxypyrazines were detected at concentrations above the reported aroma perception threshold (2-16 ng L-1 ) and range for 3-isobutyl-2-methoxypyrazine (ibMP) (2-30 ng L -1 ) and 2-methoxy-3-sec-butylpyrazine (sbMP) (< 10 g L -1 ) in Sauvignon blanc wines, for most cryogenic treatments compared to the control wines. Differences were also observed between producers from different regions and between vintages (2020 and 2021), with overall methoxypyrazine levels higher in 2021. Moreover, from a sensory perspective, the wines made from WG and MG subjected to cryogenic pre-treatment technologies yielded wines with higher tropical, thiol-type, pineapple and banana aromas as well as higher body, general quality and overall intensity when compared to most wines made from the control grapes as well as the remaining cryogenic treatments. This study highlighted that the production stage at which the cryogenic treatment was applied had the most prominent effect, whilst the effects of the cryogenic temperatures and storage times were negligible. Furthermore, vintage and regional differences also influenced the final wine sensory profiles. Differences in varietal thiol concentrations for Sauvignon blanc wines resulted mainly from wine region and vintage for the T4 wines made from cryogenically pre-treated WG and MG (-4 ºC). Although no definite trends were observed in terms of which cryogenic temperature, stage of production, or storage time yielded the most favourable levels in final wines, indications are that the region from which the grapes originated, the harvesting method, and the vintage contributed the most. Therefore, the industry recommendation would be to apply cryogenic pre-fermentative treatments to WG or MG at -4 ºC (energy efficient) for T0 (more economical), considering the region and vintage to achieve the desired outcome. Description: Thesis (Doctor: Food Science & Technology)--Cape Peninsula University of Technology, 2026

Rheological and stability properties of Citrullus lanatus mucosospermus, lanatus citroides and moringa oleifera seed hydrocolloids in oil-in-water

Wed, 01 Jan 2025 00:00:00 GMT

Title: Rheological and stability properties of Citrullus lanatus mucosospermus, lanatus citroides and moringa oleifera seed hydrocolloids in oil-in-water Authors: Olubi, Olakunbi Abstract: When mixed with water, hydrocolloids create gel-like structures and have grown in popularity due to their wide range of applications in food, medicine, and other industries. The extraction of hydrocolloids from natural sources, such as seeds, is an exciting idea because of the potential diversity in composition and function. In a variety of sectors, the use of seeds from Citrullus lanatus mucosospermus (egusi, EG), Citrullus lanatus citroides (makataan, MA), and Moringa oleifera (moringa, MO) as a hydrocolloid is in line with the growing demand for sustainable and natural products worldwide. This study examined hydrocolloids derived from EG, MA, and MO seeds, highlighting their diverse physicochemical and functional properties. Hydrocolloids were extracted from the seeds and analysed for their proximate composition, particle size distribution, and interfacial tension using the hot water extraction method. The raw oilseed flours had varying amounts of protein. Hydrocolloids had a higher protein concentration than raw oilseeds, greatly improving the amino acid profile. Furthermore, the hydrocolloid ash concentration ranged from 4.09% to 6.52% w/w dry weight, accompanied by low-fat levels. Smaller particles in all hydrocolloid samples showed a more narrow and uniform size distribution, suggesting a better degree of homogeneity in particle size within this range, according to the examination of particle size distribution. This implies a lower chance of size variation for small particles, which may affect their rheological and functional characteristics in different applications. This study also investigated the rheological behaviour of three novel hydrocolloids: egusi seed hydrocolloid (EGH), makataan seed hydrocolloid (MAH), and moringa seed hydrocolloid (MOH) in semi-concentrated (20-50 wt) as well as concentrated (50-75 wt%) slurries, when subjected to a shear steady flow, to reversible minor strain in amplitude as well as frequency sweep modes deformation. The high protein content of these hydrocolloids (48.12%, 34.00%, and 35.00% for MOH, MAH, and EGH, respectively—reduced the interfacial tension. Regardless of the hydrocolloid type and process parameters (pH = 4– 9; temperatures =30-75 oC; mixing time = 1–10 minutes; concentration = 20–50 wt%), semi concentrated slurries were pseudoplastic materials that behave like viscous liquids with no yield stress; G” > G’ in the entire range of strain (0.1–200%). The storage modulus, yield stress, and slurry concentration correlation showed two deflection points/transitional points, 50 wt% and 67 wt%, respectively. The first transition point was present in all three hydrocolloids, whereas the second was only related to EGH and MAH slurries; MOH-based slurries did not display such a point. The first transition point (50 wt%) was associated with the onset of structure formation. The bottle test further confirmed that slurries containing more than 50% by mass hydrocolloids did not flow when inverting the vessels. The second transitional point marked the boundary between the region of the slow or rapid response of the strength or rigidity of the structure of the slurry of EGH and MAH with changes in hydrocolloid concentration. The strength of the structure increased rapidly with the hydrocolloid concentration below this point, whereas a slow increase was observed above the critical point. Conversely, the rigidity of the structure of the slurries displayed an opposite effect. Interestingly, because of its high content of hydrophobic proteins, the dominating mechanism of structure formation within MOH slurries was the entanglement network of polymers (proteins and polysaccharides) whose yield stress originated from the presence of high concentrations of dispersed particulate material within the structure. This gave rise to a weak, predominantly elastic rather than viscous gel with a cohesive energy range of 0.2 – 0.7 kJ. On the other hand, due to their high hydrophilic protein content, the dominant mechanism of structure formation within EGH and MAH slurries could be cross-liking rather than entanglement. This cross linking generated a predominantly elastic gel with a relatively high cohesive energy of 2-7 kJ. The effects of extracted hydrocolloid on the stability and rheological behaviours of oil in-water (O/W) emulsions (MOH, MAH, and EGH) were investigated using plant-based emulsifiers from moringa, makataan, and egusi hydrocolloids. In this work, the mixture design was optimised for each level of limitation between hydrocolloid, oil, and water. Stability testing was used to experiment with and optimise the mixture. The droplet size distributions, morphology, creaming index, and polydispersity index were all measured on 11 emulsions, and the best emulsion with a stable profile was chosen for further investigation. The rheological characteristics and the microscopic morphology were acquired to understand the mechanism and interaction of droplets in the O/W emulsion. The results indicated that optimal for O/W emulsions was found in samples with 20-30% hydrocolloid, 37-40 % oil and 25-45 % water while using the three hydrocolloids. EGH and MAH, with the lowest hydrocolloid composition, showed the smallest droplet size and highest creaming index values. The study concludes by highlighting the promising potential of hydrocolloids generated from egusi (EG), makataan (MA), and moringa (MO) seeds. These plant-based hydrocolloids displayed different functional features, including increased protein content, considerable rheological behaviour, and excellent stabilisation in oil-water emulsions. Their capacity to produce gel-like structures, lower interfacial tension, and increase structural rigidity is useful in food systems, particularly for thickening, emulsifying, and texture modification. These findings add to the growing interest in using sustainable, natural ingredients to improve food formulation and product durability. Description: Thesis (Doctor: Food Science & Technology)--Cape Peninsula University of Technology, 2026

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

Sat, 01 Jan 2022 00:00:00 GMT

Title: The use of nano-encapsulated plant extracts in inhibiting non-enzymatic browning in fruit canned in juice Authors: Vhangani, Lusani Norah 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. Description: Thesis (Doctor: Food Science & Technology)--Cape Peninsula University of Technology, 2022

Rheological behaviour of Bambara groundnut starch-soluble dietary fibre nanocomposite for delivering active compounds in food systems

Sat, 01 Jan 2022 00:00:00 GMT

Title: Rheological behaviour of Bambara groundnut starch-soluble dietary fibre nanocomposite for delivering active compounds in food systems Authors: Maphosa, Yvonne Abstract: This aim of this study was to assess the effect of Bambara groundnut (BGN) (Vigna subterranea (L.) Verdc) starch-soluble dietary fibre nanocomposite (STASOL) on the functional, physicochemical and rheological properties of orange oil beverage emulsions. STASOL was composed of 11.5% Bambara groundnut starch (BGNS) and 88.5% Bambara groundnut soluble dietary fibre (BGN-SDF). STASOL had a mean particle size of 74.01 nm and conductivity of -57.3 mV which qualified it as a nanocomposite and a very stable compound, respectively. STASOL and BGN-SDF were amorphous in nature while BGNS was crystalline, showing strong peaks at 15, 17 and 23° (2θ), thus classifying it as type C starch. STASOL, BGN-SDF and BGNS had functional groups in the regions 3600-2900, 1641.71, 1200-900, 1300-800 cm-1 which were attributed to the vibrations of C-H and OH, C=O and OH, C-C and C-H-O as well as C-O and C-C bonds, respectively. BGNS had smooth, oval structures while BGN-SDF and STASOL exhibited irregular, polygonal morphologies. STASOL was the most thermally stable biopolymer suggesting its suitability for high-temperature food applications. STASOL was high in carbohydrates (78.69%) and proteins (6.96%), low in fat (0.84) and had a considerable amount of ash (4.88%). BGNS, BGN-SDF and STASOL showed significant (p = 0.00) differences in solubility with BGNS being insoluble in water. The emulsion activity index (EAI) and emulsion stability index (ESI) of BGNS, BGN-SDF and STASOL were 23.25 and 23.33%, 85.71 and 87.13%, 90.65 and 87.49%, respectively. The significantly (p = 0.000) higher EAI and ESI of STASOL suggested its suitability as a stabiliser in emulsions. The oil binding capacities of BGNS, BGN-SDF and STASOL differed significantly (p = 0.000) and were 1.13, 3.78 and 1.61 g/g, respectively. STASOL had a substantial amount of antioxidant compounds with 1.45 μmol AAE/g ferric reducing antioxidant power and 0.46 mg GAE/g. Colour characteristics described all studied biopolymers as light (L*), reddish (+a*) and yellowish (+b*). The mean initial backscattering (BSAVO) of STASOL stabilised emulsions was in the range 50.73-70.47% for emulsions composed of 14:30:56 and 20:30:50 (STASOL:oil:water), respectively. The turbiscan stability index (TSI) of the emulsions ranged from 0.0005 to 0.1000 for formulation 11 (20:30:50 STASOL:oil:water) and formulation 5 (8:42:50 STASOL:oil:water), respectively. Low TSI values indicate a low probability of phase separation. The hysteresis loop area (HLA) of emulsions ranged from 2.04 Pas-1 [Formulation 10 (12:34:54 STASOL:oil:water)] to 43.09 Pas-1 [Formulation 2 (20:30:50 STASOL:oil:water)]. The first-order stress decay with a zero equilibrium stress value and Herschel-Bulkley models were the best predictors of time-dependent and time-independent rheological flow behaviour, respectively. The most stable emulsion system was characterised by the highest STASOL (20%), lowest orange oil (30%) and lowest water (50%) concentrations and had the highest BSAVO, lowest TSI and highest HLA. All emulsions were non-Newtonian, time-dependent, thixotropic, shear-thinning and possessed yield stress. Both temperature and time largely affected the extent of destabilisation, with emulsions stored at 5 and 45°C showing the least and most destabilisation over time, respectively. The viscosity of emulsions stored at 5°C started significantly (p = 0.000 decreasing after the 9th day while that of emulsions stored at 20 and 45°C significantly (p = 0.000) decreased after the 3rd day. Emulsions stored at 5 and 45°C for 20 days were the most and least stable, respectively. STASOL stabilised emulsions should be refrigerated to prolong their shelf life. Description: Thesis (Doctor of Food Science and Technology)--Cape Peninsula University of Technology, 2022