106 research outputs found
Good £uck
No full textGood £uck was an exhibition I curated for the Glasgow International Festival Supported Programme in April 2018. Five Glasgow-based artists made new works that were installed in four independent shops across the city, in response to the specific character of each location. The artists involved were Jack Cheetham, Erica Eyres, Beagles & Ramsay, David Sherry and Clara Ursitti. The shops were Bill’s Tool Store, Fabric Bazaar, Garnethill Stores, and Tam Shepherd’s Trick Shop
Characterization and classification of New Zealand unifloral honeys
No full textIn the honey industry it is very important to define the distinctive characteristics of unifloral honeys and to find efficient methods for their verification which allow them to be competitive in the market. The present work was undertaken on different types of New Zealand unifloral honeys and honeydew honey to characterize the honey floral origin and to differentiate various types of honey using five different approaches including physicochemical analysis, investigation of rheological and thermal behaviour of honeys, determination of phenolic compounds and antioxidant activities, evaluation of plant-derived toxic compounds as markers and volatile analysis. The most discriminating parameters were selected from each analytical approach and were applied to discriminate the pollen-identical manuka and kanuka honeys.
The results of physicochemical analysis were subjected to principal component analysis (PCA), artificial neural networks (ANN) and a combination of PCA and ANN. PCA explained 64% of the variations between sample types. In order to find the best ANN model with the least number of parameters for distinguishing the honey types, a stepwise elimination technique was applied to the data. The optimum model with the most efficient classification and the least number of parameters was achieved with 5 parameters including conductivity, colour, glucose, fructose and sucrose levels. The combination of PCA and ANN provided the most efficient discriminatory model with the accuracy of 0.92 and regression coefficient of 0.96.
The thermal and rheological analysis demonstrated that rheology values, glass transition temperatures and thermal decomposition events were influenced by the origin of honey samples. Discriminant analysis was conducted to determine the discriminating power of thermal and rheological properties of honey and to determine the most effective parameters for differentiation of honey samples according to their floral origin. The first discriminant function accounted for 78.7% of total variance while the second accounted for 13.7%. Samples of thyme, honeydew and manuka were clearly differentiated from other samples. The most powerful classifying parameters were viscosity at 10˚C, 0˚C, and 20˚C according to linear discriminant analysis.
Total phenolic content (TPC), antioxidant activity (DPPH and FRAP assays), colour and the profile of flavonoids and phenolic compounds were also affected by honey origin. A linear discriminant analysis (LDA) using the 10 most discriminating parameters successfully discriminated the honey types; explaining 73% variation by the first two functions. In the LDA discriminant model, seven variables were eliminated and ten variables remained (syringic acid, quercetin, benzoic acid, kaempferol, chlorogenic acid, colour, p-coumaric acid, myricetin, gallic acid and FRAP).
The incidence of echimidine as a specific floral marker of Echium vulgare spp. was evaluated in New Zealand honey samples. The presence of echimidine, a major hepatotoxic pyrrolizidine alkaloid produced by E. vulgar was significantly present in 67% of viper’s bugloss honey samples in the range of 2.6-52. µg.kg-1. However, neither the floral samples nor the honeydew honey contained echimidine at detectable levels.
One hundred and sixteen volatile compounds were identified in 29 honey samples by analysis of volatile compounds. In order for differentiation of the samples, PCA and partial least square discriminant analysis (PLS-DA) were performed. The first three components of PCA accounted for 57.3% of variation and indicated differentiation between honey types with some overlaps between confidence intervals of the honey clusters. PLS-DA demonstrated total variance of 45.1% using components 1 to 3 and showed clear separation between honey samples. Results suggested that the analysis of volatile compounds followed by multivariate data analysis was the most efficient method for differentiation of the honey samples according to their origin.
As manuka and kanuka honeys are indistinguishable by melissopalynology, selected discriminating parameters, including conductivity, pH, colour and FRAP values, rheology analysis and volatile analysis were applied to attempt discrimination of manuka from kanuka honey. Using physicochemical parameters, FRAP and viscosity values were not successful in discriminating and clustering of manuka and kanuka honey samples. However, the presence and level of volatile constituents showed high discriminating capacity between the pollen-identical manuka and kanuka honey samples
Characterisation of Volatile Organic Compounds in New Zealand Honey
No full textHoney is a naturally sweet, carbohydrate-rich, viscous fluid consisting of mainly fructose and glucose produced by honeybees (Apis mellifera) mainly from the floral nectars of plants. New Zealand honey is some of the most desired in the world with substantial export value to New Zealand producers. With this growing industry, a greater understanding of the volatile organic compounds (VOC’s) in New Zealand honey could be valuable in developing and promoting the benefits in regards to flavour and quality. The aim of this research was to characterise the VOC composition of three varieties of New Zealand unifloral honeys using three different analytical methods, namely gas chromatography mass spectrometry (GC-MS), GC-olfactometry (GC-O) and proton transfer reaction mass spectrometry (PTR-MS).
Eighty-seven volatile compounds were identified (out of 124 peaks detected) in 11 samples of New Zealand honey from three floral origins (thyme, rata and manuka) with analysis by headspace solid phase micro-extraction gas chromatography mass spectrometry (HS-SPME-GC-MS), of which thirty three were common across all three types of honey. However, the level that some of these compounds were present in each honey varied significantly.
Examination of this data by principal components analysis (PCA) explained 57% of the variation in the data set for the first two principal components (33% PC1 and 24% PC2). Manuka honey was found to have the greatest variety of VOC’s while rata honey was found to have the least. A range of compounds were found to occur in all three honey types, but unique compounds were also identified, with 11 found in manuka (e.g. dimethyl disulfide and pinocarveol), 3 in thyme (2-heptanone, lilac alcohol D, dihydrojasmone lactone) and 1 in rata (p-cresol). Some honeys showed discrimination by compound class with the VOC’s in thyme honey being mainly acids and terpenes, whereas the majority of VOC’s in manuka honey were terpenes, alcohols and ketones.
Gas chromatography mass spectrometry and olfactometry (GC-MS-O) was used to identify odour active compounds using a detection frequency method with a panel of six assessors. Thyme honey was found to have the most odour active compounds with 48, followed by manuka with 40, and rata honey had the least with 29. Some odour active compounds were found in all three honeys, such as linalool and phenylethyl alcohol, whereas other odour compounds were distinct, being only identified in specific samples, such as hexanoic acid in thyme, (Z)-linalool oxide in rata and acetoin in manuka.
The third objective of this study was to evaluate discrimination between 19 honey samples from four floral origins (thyme, rata, manuka and kanuka) by means of proton transfer reaction mass spectrometry (PTR-MS). Using a rapid headspace method, 130 mass ions (m/z) were found to have significant differences between the samples. It was found that the first two principal components (PCs) explained 60% of the variation in the data set (PC 1 38%; PC 2 22%). The honey samples of thyme and rata were clustered together in easily defined groups with thyme showing the greatest discrimination. However, there was no clear grouping for manuka or kanuka and the samples for both of these honeys tended to cluster together with considerable overlap, so that they could not be discriminated from each other in a robust manner.
Results suggested that the analysis of volatile organic compounds is an effective method for the characterisation of unifloral New Zealand honeys. The results obtained using GC-MS were better than PTR-MS for discrimination of honey samples. However, PTR-MS might still prove useful as a fast, online screening method. GC-O provided complementary information specifically on the odour active compounds to identify compounds that contribute to the flavour character. These findings could be useful for improving the export value of New Zealand honey by providing a greater understanding of how odour compounds and their sensory aspects relate to honey quality. This information could also be useful for monitoring changes in flavour during processing and storage for quality assessment and shelf life prediction
Characterisation of Volatile Organic Compounds in New Zealand Honey
No full textHoney is a naturally sweet, carbohydrate-rich, viscous fluid consisting of mainly fructose and glucose produced by honeybees (Apis mellifera) mainly from the floral nectars of plants. New Zealand honey is some of the most desired in the world with substantial export value to New Zealand producers. With this growing industry, a greater understanding of the volatile organic compounds (VOC’s) in New Zealand honey could be valuable in developing and promoting the benefits in regards to flavour and quality. The aim of this research was to characterise the VOC composition of three varieties of New Zealand unifloral honeys using three different analytical methods, namely gas chromatography mass spectrometry (GC-MS), GC-olfactometry (GC-O) and proton transfer reaction mass spectrometry (PTR-MS).
Eighty-seven volatile compounds were identified (out of 124 peaks detected) in 11 samples of New Zealand honey from three floral origins (thyme, rata and manuka) with analysis by headspace solid phase micro-extraction gas chromatography mass spectrometry (HS-SPME-GC-MS), of which thirty three were common across all three types of honey. However, the level that some of these compounds were present in each honey varied significantly.
Examination of this data by principal components analysis (PCA) explained 57% of the variation in the data set for the first two principal components (33% PC1 and 24% PC2). Manuka honey was found to have the greatest variety of VOC’s while rata honey was found to have the least. A range of compounds were found to occur in all three honey types, but unique compounds were also identified, with 11 found in manuka (e.g. dimethyl disulfide and pinocarveol), 3 in thyme (2-heptanone, lilac alcohol D, dihydrojasmone lactone) and 1 in rata (p-cresol). Some honeys showed discrimination by compound class with the VOC’s in thyme honey being mainly acids and terpenes, whereas the majority of VOC’s in manuka honey were terpenes, alcohols and ketones.
Gas chromatography mass spectrometry and olfactometry (GC-MS-O) was used to identify odour active compounds using a detection frequency method with a panel of six assessors. Thyme honey was found to have the most odour active compounds with 48, followed by manuka with 40, and rata honey had the least with 29. Some odour active compounds were found in all three honeys, such as linalool and phenylethyl alcohol, whereas other odour compounds were distinct, being only identified in specific samples, such as hexanoic acid in thyme, (Z)-linalool oxide in rata and acetoin in manuka.
The third objective of this study was to evaluate discrimination between 19 honey samples from four floral origins (thyme, rata, manuka and kanuka) by means of proton transfer reaction mass spectrometry (PTR-MS). Using a rapid headspace method, 130 mass ions (m/z) were found to have significant differences between the samples. It was found that the first two principal components (PCs) explained 60% of the variation in the data set (PC 1 38%; PC 2 22%). The honey samples of thyme and rata were clustered together in easily defined groups with thyme showing the greatest discrimination. However, there was no clear grouping for manuka or kanuka and the samples for both of these honeys tended to cluster together with considerable overlap, so that they could not be discriminated from each other in a robust manner.
Results suggested that the analysis of volatile organic compounds is an effective method for the characterisation of unifloral New Zealand honeys. The results obtained using GC-MS were better than PTR-MS for discrimination of honey samples. However, PTR-MS might still prove useful as a fast, online screening method. GC-O provided complementary information specifically on the odour active compounds to identify compounds that contribute to the flavour character. These findings could be useful for improving the export value of New Zealand honey by providing a greater understanding of how odour compounds and their sensory aspects relate to honey quality. This information could also be useful for monitoring changes in flavour during processing and storage for quality assessment and shelf life prediction
Monoglyceride-based emulsifier technology to enhance emulsion stability in milk coffee beverages
No full textMilk coffee beverages are a protein-stabilised emulsion and consist of dispersed oil droplets in the aqueous phase. Casein, whey proteins and mono- and diglycerides (MDGs) are surface-active agents that adsorb at the oil-water interface to form and stabilise emulsions. Protein is an important natural emulsifying agent in emulsion systems and interact with MDGs at the oil-water interface to modify oil droplet size, zeta potential, flow behaviour, physical stability and oxidative stability. Many studies have investigated the interaction between milk protein and MDG and their effect on droplet properties, flow behaviour and food structure in whippable emulsions. However, there is very little knowledge around the interaction of MDG and milk protein in beverage emulsions. MDG is an oil-soluble emulsifier and has low solubility in water, making it not suitable for direct application in many food formulations. Encapsulation of MDG could be a potential solution to convert it into a stable water-dispersible powder; however, there is very little information published about the encapsulation of emulsifiers as a functional ingredient.
This project had two main objectives. The first objective aimed to gain fundamental understandings on the effect of MDG and milk protein compositions in a protein-stabilised oil-in-water emulsion system. The second objective investigated the preparation and characterisation of an encapsulated emulsifier system, including its physicochemical properties, stability after reconstitution and functionality in model emulsions. Five experimental studies were carried out to meet these objectives.
A model emulsion prepared by microfluidisation to mimic the characteristics of a beverage emulsion was used to investigate the effect of the composition of mono- and diglycerides (unsaturation of fatty acids; monoglyceride content) and co-emulsifier (sodium stearate) content on the physical properties and stability against creaming in protein-stabilised emulsions. The experimental results demonstrated that emulsions with 0.2% MDGs produced 15-30% smaller oil droplets and 17-27% lower polydispersity indices compared to the control (no MDG). Sodium stearate (6% w/w of MDG) increased the negativity of zeta potential by 12.6-17.3 mV in emulsions containing saturated MDGs and 1.8-5.0 mV in unsaturated MDGs. Unsaturated MDGs showed better creaming stability than the control after 28 days of ageing with no improvement observed for saturated MDGs. Glycerol monooleate (GMO) demonstrated the best creaming stability among the unsaturated MDGs.
The next study investigated the effect of milk protein compositions (different ratio of sodium caseinate to whey protein concentrate) on physicochemical properties, creaming stability and oxidative stability of protein-stabilised emulsions containing GMO. The experimental results showed that the emulsion with only sodium caseinate produced smaller droplets (174.7 nm), a more negative zeta potential (-50.8 mV) and a more viscous emulsion (1.89 mPa s) compared to the emulsion with only WPC (191.4 nm; 38.8 mV; 1.65 mPa s). Protein composition had no significant effect on creaming stability. Eleven volatile compounds were identified as lipid oxidation markers, and six compounds (2-pentylfuran, octanal, nonanal, 3-octen-2-one, 2,4-heptadienal, 3,5 octadien-2-one isomers) demonstrated that emulsions with mixed protein types (sodium caseinate and WPC) had better oxidative stability than emulsions with a single protein type, i.e. either caseinate or WPC alone.
GMO demonstrated excellent creaming stability in protein-stabilised emulsions but was unsuitable for direct aqueous applications due to its low solubility in water. In order to use GMO as a functional ingredient in beverage emulsions, the feasibility of spray-drying to encapsulate oil-soluble MDG using appropriate wall materials to produce a stable water-dispersible powder with good reconstitution properties and extended shelf life was assessed. In addition, the effect of emulsion formulation (GMO concentration (33.6%, 47.0%); dextrose equivalent (DE) values of maltodextrin (DE 10, 18)) on emulsion properties, powder properties and oxidative stability was investigated. Results showed that all homogenised emulsions were suitable for spray-drying due to their high emulsion stability against phase separation, monomodal droplet size distributions (150-180 nm) and low viscosity (20-65 mPa s). All instantised powders exhibited good dispersibility (65-90%) in water and greater oxidative stability than bulk GMO. The instantised powder with low GMO and maltodextrin DE 10 showed both good dispersibility and low lipid oxidation, demonstrating that spray-drying can successfully produce an instantised GMO powder with a longer shelf life for food applications.
The application of two selected instantised GMO powders (low GMO content with maltodextrin DE 10 or DE 18) were investigated in protein-stabilised emulsions in terms of physicochemical properties, creaming stability and oxidative stability. Model emulsions with bulk GMO, two instantised GMO powders and two controls (contain either maltodextrin DE 10 or DE 18 with no GMO) were prepared using microfluidisation. The emulsion physicochemical properties were characterised by droplet size, zeta potential, viscosity and creaming index, while the oxidative stability was assessed by the formation of volatile secondary lipid oxidation products during storage (28 days at 45 °C) using gas chromatography-mass spectrometry (GC-MS). Experimental results revealed that all three emulsions with GMO had smaller average droplet sizes (180.0 nm) and narrower size distribution (polydispersity index of 0.161) compared to the two controls (197.6 nm, 0.194). All emulsions with GMO also had greater creaming stability than the control emulsions. Principal component analysis of the volatiles revealed that storage time had the greatest influence on lipid oxidation. Three lipid oxidation markers, 3-octen-2-one, 2,4-heptadienal isomer 2 and 3,5 octadien 2-one isomer 1, showed that controls had the same oxidative stability as instantised GMO, but were more stable than bulk GMO. Therefore, GMO powders can form stable protein-stabilised emulsions with good physicochemical properties and oxidative stability.
The different concentrations of bulk GMO and instantised GMO powder (low GMO with maltodextrin DE 10) was evaluated in an application of a model coffee beverage emulsion in terms of physicochemical properties, creaming stability and volatile profile at different storage time. The increasing GMO level formed fresh coffee emulsions with smaller droplet sizes and narrower size distribution that resulted in greater emulsion stability against creaming compared to the control (no GMO). However, emulsions prepared with 0.1% and 0.2% GMO were not stable at pH near to the isoelectric point of casein during storage and resulted in the growth of droplet size. These emulsions at 28 days of storage showed the presence of flocculated oil droplets due to protein aggregates when visualised using an optical microscope. The visible sediment particles in the emulsions with 0.2% GMO were associated with protein aggregate induced flocculation and could be explained by having a zeta potential below the critical level for stability ( 30 mV). The emulsions with 0.03% GMO powder demonstrated greater creaming stability than the control emulsions and had stable droplet size, zeta potential and viscosity. The chemical stability was similar to the control emulsions demonstrated by the very similar volatile concentrations.
In conclusion, the results from this thesis provide new insights into the relationship between milk proteins and a MDG-based emulsifier system and their effects on emulsion properties and volatile profile in model protein-stabilised and coffee emulsions. The knowledge from this study is useful to formulate a ready-to-drink coffee beverage with the desired emulsion properties and shelf stability. This study also presents an innovative application of spray-drying to design an emulsifier system that is not only in the right format for beverage emulsions but achieves the same functionality in the products at a lower application rate. Further development may be required to determine optimum dose rates for various food and beverage applications; but the instantised GMO powder will give better control over the dosage to ensure the desired functionality is obtained
Characterization and classification of New Zealand unifloral honeys
No full textIn the honey industry it is very important to define the distinctive characteristics of unifloral honeys and to find efficient methods for their verification which allow them to be competitive in the market. The present work was undertaken on different types of New Zealand unifloral honeys and honeydew honey to characterize the honey floral origin and to differentiate various types of honey using five different approaches including physicochemical analysis, investigation of rheological and thermal behaviour of honeys, determination of phenolic compounds and antioxidant activities, evaluation of plant-derived toxic compounds as markers and volatile analysis. The most discriminating parameters were selected from each analytical approach and were applied to discriminate the pollen-identical manuka and kanuka honeys.
The results of physicochemical analysis were subjected to principal component analysis (PCA), artificial neural networks (ANN) and a combination of PCA and ANN. PCA explained 64% of the variations between sample types. In order to find the best ANN model with the least number of parameters for distinguishing the honey types, a stepwise elimination technique was applied to the data. The optimum model with the most efficient classification and the least number of parameters was achieved with 5 parameters including conductivity, colour, glucose, fructose and sucrose levels. The combination of PCA and ANN provided the most efficient discriminatory model with the accuracy of 0.92 and regression coefficient of 0.96.
The thermal and rheological analysis demonstrated that rheology values, glass transition temperatures and thermal decomposition events were influenced by the origin of honey samples. Discriminant analysis was conducted to determine the discriminating power of thermal and rheological properties of honey and to determine the most effective parameters for differentiation of honey samples according to their floral origin. The first discriminant function accounted for 78.7% of total variance while the second accounted for 13.7%. Samples of thyme, honeydew and manuka were clearly differentiated from other samples. The most powerful classifying parameters were viscosity at 10˚C, 0˚C, and 20˚C according to linear discriminant analysis.
Total phenolic content (TPC), antioxidant activity (DPPH and FRAP assays), colour and the profile of flavonoids and phenolic compounds were also affected by honey origin. A linear discriminant analysis (LDA) using the 10 most discriminating parameters successfully discriminated the honey types; explaining 73% variation by the first two functions. In the LDA discriminant model, seven variables were eliminated and ten variables remained (syringic acid, quercetin, benzoic acid, kaempferol, chlorogenic acid, colour, p-coumaric acid, myricetin, gallic acid and FRAP).
The incidence of echimidine as a specific floral marker of Echium vulgare spp. was evaluated in New Zealand honey samples. The presence of echimidine, a major hepatotoxic pyrrolizidine alkaloid produced by E. vulgar was significantly present in 67% of viper’s bugloss honey samples in the range of 2.6-52. µg.kg-1. However, neither the floral samples nor the honeydew honey contained echimidine at detectable levels.
One hundred and sixteen volatile compounds were identified in 29 honey samples by analysis of volatile compounds. In order for differentiation of the samples, PCA and partial least square discriminant analysis (PLS-DA) were performed. The first three components of PCA accounted for 57.3% of variation and indicated differentiation between honey types with some overlaps between confidence intervals of the honey clusters. PLS-DA demonstrated total variance of 45.1% using components 1 to 3 and showed clear separation between honey samples. Results suggested that the analysis of volatile compounds followed by multivariate data analysis was the most efficient method for differentiation of the honey samples according to their origin.
As manuka and kanuka honeys are indistinguishable by melissopalynology, selected discriminating parameters, including conductivity, pH, colour and FRAP values, rheology analysis and volatile analysis were applied to attempt discrimination of manuka from kanuka honey. Using physicochemical parameters, FRAP and viscosity values were not successful in discriminating and clustering of manuka and kanuka honey samples. However, the presence and level of volatile constituents showed high discriminating capacity between the pollen-identical manuka and kanuka honey samples
Triumph Of Zero
No full textI co-curated this group exhibition with John Beagles. The exhibition featured works by Dennis & Debbie Club, Erica Eyres, Beagles & Ramsay, John Russell and Andy Warhol
Understanding the Impact of Pulsed Electric Field (PEF) Processing on Onions
No full textThis thesis aimed to obtain an extensive understanding of the mechanism by which pulsed electric fields (PEF) at low electric field strengths (E ≤ 2 kV/cm) affect the structure and functionality of intact onions. A few studies have investigated the effect of PEF treatment on intact tubers, but little information is known for a complex and multicellular plant organ, made up of layers. Therefore, in this study, onion bulbs and spring onions were chosen as a model system, to understand the PEF induced changes in the tissue integrity and some physiological responses.
The impact of PEF treatment on onion tissue integrity was studied based on ion leakage measurements as well as cell viability staining techniques. PEF treatment (0.3 – 1.8 kV/cm, 0.5 kJ/kg) significantly (P < 0.05) increased the ion leakage (conductivity) in the incubation medium as electric field strength increased. The ion leakage measurements determined in the individual scales of bulb and spring onions indicated that the outer tissue scales were more susceptible to PEF treatment compared to the inner tissue scales due to the positional differences between the scales, with outer scale protecting the inner scales. A similar result was obtained using a cell viability assay. Onion cells present in the outer tissue scales showed more damage as visualised by the amount of cell death, compared to the cells present in the inner tissue region. This indicates that the application of PEF treatment on onion bulbs results in non-homogenous and complex changes in onion structure and cell viability.
This study attempted to identify the potential of volatile compounds as markers of cell membrane damage, using gas chromatography-mass spectrometry (GC-MS) and proton transfer reaction–mass spectrometry (PTR-MS). The changes in the volatile concentrations corresponded to biochemical changes associated with PEF treatment and during storage time. PEF treatment (0.3 – 1.2 kV/cm, 5 kJ/kg) significantly increased the concentrations of propanethial s-oxide (PSO, lachrymatory factor), propenyl propane thiosulfinate (PrPthiosulfinate), 2-methyl-2-pentenal and the disulfides (dipropyl disulfide, propenyl propyl disulfide, methyl propyl disulfide and methyl propenyl disulfide), immediately following PEF treatment (T0), compared to the control (non-PEF treated) samples. In addition, the effect of PEF treatment on the volatile concentrations was much higher after 24 hr of storage at 4 °C (T24). The concentrations of volatile sulfur compounds (such as PrPthiosulfinate, disulfides and trisulfides) increased significantly (P < 0.05), compared to control and sample concentrations detected at T0. The increase in the concentrations of volatile compounds at T0 and T24 was found to be dependent on the applied electric field strength. The maximum concentration (Cmax) of volatile compounds produced in spring onions (Ishikura and Red Bunching) were obtained upon PEF treatment at 1.2 kV/cm whereas, in the Yellow sweet Spanish bulbs, the Cmax was obtained upon PEF treatment at 0.7 kV/cm. The dynamic changes in volatile concentrations were investigated in the spring onions as a function of time following PEF treatment using PTR-MS. The Cmax of the mass ions measured for target volatile compounds and the rate at which volatile concentrations reached steady state were found to be based on the applied electric field strength, significantly different to the control samples. The mass ions were found to reach steady state at different time points, reaching their maximum concentrations either at the start or the end of the analysis (120 min). The results demonstrated that the volatile kinetic trends were due to the relative position of the volatile compound in the onion enzymatic reaction cascade, where the mass ions measured for target compounds of interest were found to either increase or decrease over time to reach completion. Overall, it was found that the concentrations of volatile compounds were predominantly linked to the degree of enzyme – substrate mixing, which corresponded to the applied electric field strength. However, some exceptions were found in the products formed through a mixed chemical reaction cascades, showcasing a different trend. These results suggested that the changes in the onion volatile concentrations were not only correlated to the enzyme initiated reaction cascade but also to physiological responses.
Investigation of cell viability following PEF treatment indicated that there were differences in the degree of cell disruption across the onion tissue regions, based on the applied electric field strength. The cells present in the outer tissue region were dead, and the cells that make the scale of the central core were found to be viable. To evaluate the effect of PEF treatment intensities on the volatile compositions according to the cellular disruption, the volatile compounds were analysed in the inner and outer tissue regions of spring onions. The results showed three different trends based on the concentrations detected in the onion tissue regions, indicating the differences in the physiological response from the onion tissue regions. To further comprehend these volatile trends, several biomarkers related to oxidative damage and antioxidant markers were measured to understand the degree of cell damage.
The effect of PEF treatment at 0.3 kV/cm was found to damage the cells present in the outer tissue region, but the inner tissue region was found to be unaffected. In the outer tissue region, the levels of antioxidant enzymes (SOD, CAT, GPOX, and GR) activities were found to increase significantly. However, no significant changes in the levels of oxidative damage (protein carbonyls and lipid peroxides) markers were observed in the outer tissue region. Volatile compounds such as methyl propenyl disulfide (MPrDS), propyl propenyl trisulfide (PPrTS) and methyl propyl trisulfide (MPTS) were produced in higher concentrations in the inner tissue region, and are suspected to be associated with the physiological response from the viable cells. These results indicate that the metabolically active cells are synthesising new proteins to counteract the oxidative stress. These results suggest that upon PEF induced stress, living cells can change their metabolism to prevent oxidative damage to the cellular components caused by the reactive oxygen species (ROS). In contrast, application of PEF treatment at 0.7 kV/cm resulted in significant accumulation of damage markers and a significant reduction of antioxidant enzyme activities, in both inner and outer tissue regions. This result indicates that the PEF treatment has resulted in extensive cellular disruption in both onion tissue regions, causing oxidation of lipids and denaturation of proteins and enzymes. The volatile compounds such as propanethial s-oxide (PSO), dipropyl disulfide (DPDS), methyl propyl disulfide (MPDS), propyl propenyl disulfide (PPrDS) and dipropyl trisulfide (DPTS), which are associated with cell damage were detected in higher concentrations in the outer tissue region and relatively lower concentrations in the inner tissue region.
This study has demonstrated that the overall PEF induced changes in the structure and physiological function of intact onions could be assessed by evaluating the markers associated with cellular damage and biochemical analysis. Evaluation of the volatile compounds produced in onion tissues makes a unique contribution to the current knowledge in understanding the properties of PEF treated fruit and vegetable tissues. From an industrial point of view, understanding these complex responses will aid in tailoring the activities of antioxidant enzymes, enhanced recovery of phytochemicals, improved texture, flavour and bio-active properties of fruits and vegetables
Milk Fat Globule Membranes: extraction, characterisation, and impact on model emulsions and volatile profile of cheese
No full textMilk fat plays a vital role in determining the flavour of cheese. The milk fat globule (MFG) consists of a triglyceride (TG) core and a three-layer membrane termed the milk fat globule membrane (MFGM). The TGs within the MFG are the major lipid constituents in milk and they have been the subject of numerous investigations on the role of milk fat in flavour development in cheese. In cheese, the MFGM is known to be an additional source of lipids, mainly polar lipids (PLs) and specific proteins and enzymes like xanthine oxidase (XO). However, research investigating the influence of MFGM components, other than TG, on the flavour profile of cheese is limited. XO, the main enzyme in the MFGM, is capable of catalysing the oxidation and reduction of a wide range of substrates (aldehydes) and therefore it has been speculated that it may play a role in determining the oxidation-reduction potential (Eh) of cheese. This is of interest as it has been hypothesised that a negative Eh is necessary for the correct formation and stability of flavour in cheese. Thus, the activity and availability of the redox enzymes (such as XO) in the MFGM, may change the volatile compound profile of ripened cheese through its effect on the oxidation-reduction potential (Eh) of cheese products. This thesis firstly aimed to understand the relationship between the structure and composition of the MFGM and XO activity, and their impact on the Eh in recombined emulsion systems. Secondly, the thesis explored the role of MFGM composition, XO activity and Eh on the volatile profile of ripened model Cheddar cheese samples.
Buttermilk, was produced using various churning conditions at different temperatures (10, 15 or 20°C) and pH values (5.5 or 6.6) and used for MFGM isolation (Chapter 3). The isolated MFGM fractions were characterised for protein composition, and XO content. Milkfat (5%) emulsions (containing 2% MFGM) were prepared from the MFGM isolates and examined for XO activity and Eh. A greater proportion of non-membrane proteins, e.g., caseins and whey proteins, were found at higher temperatures and cream pH, and the XO content decreased significantly under these conditions. XO activity was greater at lower temperatures and pH, yielding a more positive Eh value for the emulsions. Storage at 4°C for 14 days significantly reduced the Eh of the emulsions to between -320 mV and -580 mV for low pH/low temperature, and high pH/high temperature emulsions, respectively.
Industrial sources of MFGM material, namely buttermilk powder (BMP), β-serum, and α-serum, were also used for MFGM isolation and emulsion preparation (Chapter 4). Caseins were the dominant proteins in the MFGM fractions isolated from BMP, whereas fractions from α- and β-serum contained higher amounts of the MFGM proteins. The XO content and activity was >70-fold and >700-fold higher in α- and β-serum samples, respectively, compared to the BMP fraction. The Eh values of the recombined emulsions were highest for α-serum (196 mV), and β-serum (169 mV), followed by BMP (131 mV). These positive values contrasted to the highly negative Eh of the emulsions characterised in Chapter 3 (-580 mV). From this study, no consistent trend was observed between the XO content, activity and the Eh of the emulsion system.
The polar lipid (PL) profiles of the MFGM fractions isolated from fresh raw cream and the commercial sources were subsequently investigated (Chapter 5). Decreasing the temperature and pH during churning increased the total PLs in the MFGM fractions, with temperature having a greater effect at pH 5.5 compared to pH 6.6, and pH have a greater effect as the temperature decreased from 20 to 15 to 10°C. The optimal preparative conditions to maximize the total PL content was determined to be churning at pH 5.5 and 10°C. BMP contained the lowest amount of total PL amongst all fractions. Phosphatidylinositol was the primary polar lipid component in all MFGM fractions, followed by phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, and sphingomyelin. The results provide evidence to support a trilayer MFGM model, where phosphatidylinositol and phosphatidylserine are located in the inner monolayer.
Finally the impact of MFGM composition and structure, and XO activity on Eh and the volatile profile of ripened model Cheddar cheese samples was investigated (Chapter 6). Recombined emulsions produced using the MFGM fractions isolated in Chapter 3 (at pH 6.6, and 20°C) and Chapter 4 (all fractions) were combined with reconstituted skim milk for Cheddar cheese manufacturing. Two reference cheese samples, one made using cream and skim milk (native MFGM) and one made with Tween 80 were used to evaluate the effect of the MFGM and its structure and composition on flavour of cheese. All six cheeses were ripened for 6 months at 8°C. The composition and structure of the recombined MFGs did not affect the redox potential of the cheese medium. A higher XO activity favoured the production of carboxylic acids, particularly short-chain fatty acids such as acetic, butanoic and hexanoic acid, whereas rearrangement of the MFGM layer (removing MFGM from the lipid interface and re-emulsifying it back onto the surface of reformed fat globules) negatively affected the acetoin / diacetyl production pathway. The rearrangement of the MFGM layer, or the XO activity did not affect the production of none of the detected esters during ripening.
The MFGM had a significant effect on development of volatile compounds in cheese, as the Ref 2-cheese containing recombined MFGs with Tween 80 had significantly lower concentration of the important volatile groups of mainly carboxylic acids, alcohol, and ketones.
In conclusion, the results from this work have provided new insights into the influence of processing on the MFGM structure and its subsequent effect on the flavour profile of cheese. The knowledge taken from this study can be used as a platform to manufacture food products or ingredients with beneficial health effects or low-fat cheese products with enhanced flavor profiles
Milk Fat Globule Membranes: extraction, characterisation, and impact on model emulsions and volatile profile of cheese
No full textMilk fat plays a vital role in determining the flavour of cheese. The milk fat globule (MFG) consists of a triglyceride (TG) core and a three-layer membrane termed the milk fat globule membrane (MFGM). The TGs within the MFG are the major lipid constituents in milk and they have been the subject of numerous investigations on the role of milk fat in flavour development in cheese. In cheese, the MFGM is known to be an additional source of lipids, mainly polar lipids (PLs) and specific proteins and enzymes like xanthine oxidase (XO). However, research investigating the influence of MFGM components, other than TG, on the flavour profile of cheese is limited. XO, the main enzyme in the MFGM, is capable of catalysing the oxidation and reduction of a wide range of substrates (aldehydes) and therefore it has been speculated that it may play a role in determining the oxidation-reduction potential (Eh) of cheese. This is of interest as it has been hypothesised that a negative Eh is necessary for the correct formation and stability of flavour in cheese. Thus, the activity and availability of the redox enzymes (such as XO) in the MFGM, may change the volatile compound profile of ripened cheese through its effect on the oxidation-reduction potential (Eh) of cheese products. This thesis firstly aimed to understand the relationship between the structure and composition of the MFGM and XO activity, and their impact on the Eh in recombined emulsion systems. Secondly, the thesis explored the role of MFGM composition, XO activity and Eh on the volatile profile of ripened model Cheddar cheese samples.
Buttermilk, was produced using various churning conditions at different temperatures (10, 15 or 20°C) and pH values (5.5 or 6.6) and used for MFGM isolation (Chapter 3). The isolated MFGM fractions were characterised for protein composition, and XO content. Milkfat (5%) emulsions (containing 2% MFGM) were prepared from the MFGM isolates and examined for XO activity and Eh. A greater proportion of non-membrane proteins, e.g., caseins and whey proteins, were found at higher temperatures and cream pH, and the XO content decreased significantly under these conditions. XO activity was greater at lower temperatures and pH, yielding a more positive Eh value for the emulsions. Storage at 4°C for 14 days significantly reduced the Eh of the emulsions to between -320 mV and -580 mV for low pH/low temperature, and high pH/high temperature emulsions, respectively.
Industrial sources of MFGM material, namely buttermilk powder (BMP), β-serum, and α-serum, were also used for MFGM isolation and emulsion preparation (Chapter 4). Caseins were the dominant proteins in the MFGM fractions isolated from BMP, whereas fractions from α- and β-serum contained higher amounts of the MFGM proteins. The XO content and activity was >70-fold and >700-fold higher in α- and β-serum samples, respectively, compared to the BMP fraction. The Eh values of the recombined emulsions were highest for α-serum (196 mV), and β-serum (169 mV), followed by BMP (131 mV). These positive values contrasted to the highly negative Eh of the emulsions characterised in Chapter 3 (-580 mV). From this study, no consistent trend was observed between the XO content, activity and the Eh of the emulsion system.
The polar lipid (PL) profiles of the MFGM fractions isolated from fresh raw cream and the commercial sources were subsequently investigated (Chapter 5). Decreasing the temperature and pH during churning increased the total PLs in the MFGM fractions, with temperature having a greater effect at pH 5.5 compared to pH 6.6, and pH have a greater effect as the temperature decreased from 20 to 15 to 10°C. The optimal preparative conditions to maximize the total PL content was determined to be churning at pH 5.5 and 10°C. BMP contained the lowest amount of total PL amongst all fractions. Phosphatidylinositol was the primary polar lipid component in all MFGM fractions, followed by phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, and sphingomyelin. The results provide evidence to support a trilayer MFGM model, where phosphatidylinositol and phosphatidylserine are located in the inner monolayer.
Finally the impact of MFGM composition and structure, and XO activity on Eh and the volatile profile of ripened model Cheddar cheese samples was investigated (Chapter 6). Recombined emulsions produced using the MFGM fractions isolated in Chapter 3 (at pH 6.6, and 20°C) and Chapter 4 (all fractions) were combined with reconstituted skim milk for Cheddar cheese manufacturing. Two reference cheese samples, one made using cream and skim milk (native MFGM) and one made with Tween 80 were used to evaluate the effect of the MFGM and its structure and composition on flavour of cheese. All six cheeses were ripened for 6 months at 8°C. The composition and structure of the recombined MFGs did not affect the redox potential of the cheese medium. A higher XO activity favoured the production of carboxylic acids, particularly short-chain fatty acids such as acetic, butanoic and hexanoic acid, whereas rearrangement of the MFGM layer (removing MFGM from the lipid interface and re-emulsifying it back onto the surface of reformed fat globules) negatively affected the acetoin / diacetyl production pathway. The rearrangement of the MFGM layer, or the XO activity did not affect the production of none of the detected esters during ripening.
The MFGM had a significant effect on development of volatile compounds in cheese, as the Ref 2-cheese containing recombined MFGs with Tween 80 had significantly lower concentration of the important volatile groups of mainly carboxylic acids, alcohol, and ketones.
In conclusion, the results from this work have provided new insights into the influence of processing on the MFGM structure and its subsequent effect on the flavour profile of cheese. The knowledge taken from this study can be used as a platform to manufacture food products or ingredients with beneficial health effects or low-fat cheese products with enhanced flavor profiles
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