Browsing by Author "Ye, Aiqian"

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    Behaviour of fat globules and membrane proteins under different processing environments as related to milk powder manufacture : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology
    (Massey University, 2003) Ye, Aiqian
    The objective of the first part in this study was to gain a better understanding of the protein components of the milk fat globule membrane (MFGM). In the second part, the influence of processing factors on the fat globules and the MFGM during the manufacture of whole milk powder were examined. Relationships between the state of the MFGM in whole milk powders and their reconstitutions properties were also explored. The MFGM proteins, isolated from early-, mid- and late-season fresh whole milks, were characterized using one- and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) under reducing and non-reducing conditions. SDS-PAGE under reducing conditions showed the presence of about 40 protein bands, ranging in molecular weight from 15 to 200 kDa. The major MFGM proteins e.g., xanthan oxidase, butyrophilin, PAS 6 and PAS 7 constituted 60-70% of total MFGM proteins while 20-30% were minor proteins. Two-dimensional SDS-PAGE indicated that xanthine oxidase and butyrophilin might be complexed via intermolecular disulfide bonds in the natural MFGM. The examination of MFGM proteins heated at > 60 °C in the absence of skim milk proteins (caseins and whey proteins) showed that xanthine oxidase and butyrophilin interacted further to form very high molecular weight protein complexes, whereas PAS 6 and PAS 7 were relatively heat stable and did not form complexes. Heat treatment of fresh whole milk in the temperature range 65-95 °C caused incorporation of β-lactoglobulin (β-1g) into the MFGM. Small amounts of α- lactalbumin (α-la) and κ-casein were also observed in the MFGM material of heated milk. The amounts of β-lg and α-la that associated with the MFGM increased with an increase in temperature up to 80 °C, and then remained almost constant. The maximum values for β-lg and α-la association with the MFGM were ~1.0 mg/g fat and ~0.2 mg/g fat, respectively. Association of β-lg and α-la with the MFGM was described by a first-order reaction (65-85 °C for β-lg and 70-80 °C for α-la) in the low temperature range and by a second-order reaction in the high temperature range (85-95 °C for β-lg and 80-95 °C for α-la). Arrhenius plots showed an abrupt change in temperature dependence of the rate constants at 85 °C for β-lg and 80 °C for α-la. Of the major original MFGM proteins, xanthine oxidase and butyrophilin were not affected by the heat treatment of whole milk, whereas PAS 6 and PAS 7 decreased during heating. Interestingly, this behaviour is in contrast to that shown by these proteins in systems containing no skim milk proteins. The changes in fat globule size and MFGM proteins during the manufacture of whole milk powder were determined using light scattering, SDS-PAGE, confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM). Heat treatment of whole milk by direct stream injection (DSI) prior to evaporation caused a decrease in the fat globule size and an increase in the MFGM protein, through the association of caseins and whey proteins with the MFGM material. Evaporation of milk by a multiple-effect falling film evaporator caused a gradual decrease in the fat globule size and an increase in the MFGM protein after each effect. Caseins dominated the total MFGM protein, indicating the adsorption of casein micelles to the newly formed surface of the fat globules during evaporation. When whole milk was preheated (95 °C for 20 s) before evaporation, the amounts of total MFGM protein were higher (~6 mg/m2 compared to ~4 mg/m2 for the non-preheated whole milk) because of association of whey proteins with the native MFGM proteins and casein micelles. The average fat globule size decreased further upon homogenisation of the concentrated milk. The amount of MFGM protein (mg/m2) of concentrated milk also increased after homogenisation, the extent of the increase being dependent upon the temperature and pressure of homogenisation. Furthermore, heat treatment of concentrated milk to 79 °C either before or after homogenisation also increased the amount of MFGM protein. However, at the same homogenisation temperature and pressure, the amounts of whey proteins in the MFGM of the concentrated milk that was heated after homogenisation were higher than the concentrated milk that was heated followed by homogenisation. The amounts of the major native MFGM proteins did not change during homogenisation, indicating that the skim milk proteins did not displace the native MFGM proteins but adsorbed onto the newly formed surface. The fat globule size of homogenized concentrated milk decreased after spray drying, while the amount of MFGM protein (mg/m2) decreased slightly. Some "uncovered fat" was observed on the surface of powder particles. It is possible that the proteins do not adsorb to all newly formed fat surfaces during spray drying. The reconstitution properties of whole milk powders produced using different processing treatments were determined. High homogenization pressure and temperature used before spray drying resulted in poor reconstitution properties of the powder, particularly when the heat treatment was carried out after homogenization. It is suggested that the proteins adsorbed at the fat globule surfaces during homogenisation of the concentrated milk and their subsequent aggregation during heat treatment play a key role in determining the reconstitution properties of whole milk powders.
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    Food structure modification in the gastrointestinal environment and its impact on the delivery of lipophilic bioactive compounds : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology, Riddet Institute, Massey University, Palmerston North, New Zealand
    (Massey University, 2023-11-10) Qazi, Haroon Jamshaid
    Lipophilic bioactive compounds such as curcumin, polyphenols, etc. are often encapsulated as lipid-based delivery systems before adding into various foods The food matrix and its structural reorganizations under the influence of gastric and intestinal conditions affect their release, and uptake and utilization by the human body. Understanding the digestion behaviour of different food matrices is mandatory to modulate the release kinetics of bioactive ingredients. This thesis focuses on understanding how microstructural rearrangement of the dairy and starch-based foods during gastric digestion influences the bio-accessibility of curcumin. Initially a curcumin nanoemulsion (CNE) was optimized with high encapsulation efficiency (~ 94%) along with acceptable shelf stability. Further, these CNEs were incorporated into milk, milk gels or corn starch gels. Using the human gastric simulator (HGS), these food systems were digested before being subjected to intestinal digestion in a static in vitro intestinal model. The results showed that milks reconstituted from low-heat, medium-heat, and high-heat skim milk powders exhibited significantly distinct crud structures and disintegration behaviours in the stomach due to the varying degrees of casein/whey protein interactions that occurred during milk powder manufacture. The reconstituted milk made using high-heat milk powder formed soft curd under dynamic gastric conditions, resulting in a faster outflow of proteins and entrapped curcumin nanoemulsion droplets. Thus, the changes in the gastric digesta profiles influenced both the rate of lipid hydrolysis and the bioaccessibility of curcumin during intestinal digestion. Milk gels were formed using rennet enzyme or acid with similar rheological and compositional profiles. The gastric emptying was significantly impacted by the way these gels disintegrated during dynamic gastric digestion. The curd particles from the acid- gel were digested much faster than that from the rennet gel. The composition of the digesta was affected by these changes to the curd structures and stomach emptying rates during the gastric phase, which altered how the oil droplets were released from the stomach. This in turn affected the related lipophilic curcumin's bioaccessibility during the intestinal phase. Furthermore, the digestion behaviour of corn starch gels made from waxy, native, and high amylose corn starches with added CNE were investigated. The physicochemical properties of the gels were drastically altered by the addition of curcumin nanoemulsion. Because of the waxy gel's adhesive character, most of the oil droplets were held inside the gel fragments throughout the dynamic gastric phase. This resulted in the delayed breakdown and emptying of gels from the stomach. This variation in the compositional and structural characteristics of the gastric digesta was further connected to the varying rates of starch hydrolysis, the release of free fatty acids, and the associated proportion of bioaccessible curcumin. These findings in this thesis highlight how the release of health-promoting bioactive compounds from food matrix can be manipulated by understanding the complex dynamic processing behaviour of the food materials within the gastrointestinal tract. This can further help in designing novel functional foods for various populations.
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    Influence of calcium chloride addition on the properties of emulsions formed with milk protein products : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology
    (Massey University, 1999) Ye, Aiqian
    The objective of this study was to investigate the effects of added CaCl2 on (i) the adsorption behaviour of caseinate and whey protein concentrate (WPC) at the oil-water interfaces and (ii) the stability of emulsions formed with caseinate or WPC. The relationship between aggregation state of protein, due to Ca2+ binding, and emulsifying properties is discussed. The effects of addition of NaCl to the emulsions containing various concentrations of CaCl2 were also explored. Protein solutions and 30% soya oil, at pH 7.0, were mixed and homogenized at 207/34 bar and 55°C to form emulsions. CaCl2 was added to protein solutions prior to emulsion formation or to the emulsions after they were made. The average particle size (d 32 or d43 ), the surface protein concentration, the composition of protein adsorbed layer at the interface and the creaming stability of emulsions were determined. The microstructure of emulsions was observed using the confocal laser microscopy. The droplet sizes of emulsions made with sodium caseinate or WPC were similar and were independent of the protein concentration at concentration above 0.5%. The surface protein concentration of emulsions made with sodium caseinate, WPC or calcium caseinate generally increased with increase in the protein concentration, although the trends were different. The emulsions made with calcium caseinate had higher (d32 and surface protein concentration than that of sodium caseinate or WPC. In emulsions made with sodium caseinate at low protein concentrations, the adsorption of β-casein occurred in preference to αs -casein, whereas αs- (αs1- + αs2 -)casein was found to adsorb in preference to β-casein at high protein concentrations. In calcium caseinate emulsions, the αs -casein was adsorbed in preference to β-casein at all concentrations. In emulsions made with WPC, β-lactoglobulin adsorbed slightly in preference to α-lactalbumin. In emulsions made with mixtures of sodium caseinate and WPC (1:1), the adsorption of whey proteins occurred in preference to caseins at low concentrations (< 3%), whereas caseins were adsorbed in preference to whey protein at high concentrations. In emulsions made with calcium caseinate or WPC, the creaming stability of emulsions followed mainly the changes in particle size of emulsions. However, the creaming stability of emulsions made with sodium caseinate decreased markedly as the caseinate concentrations were increased above 2.0%. This was attributed to depletion flocculation occurring in these emulsions. Whey proteins did not retard this instability, due to depletion flocculation, in emulsions made with mixtures of caseinate and WPC When CaCl2 was added prior to or after emulsion formation, the (d43 and surface protein concentration increased with increasing CaCl2 concentration in emulsions made with 0.5 and 3.0% sodium caseinate. The adsorption of αs -casein increased with increase in the concentration of CaCl2 , with a corresponding decrease in the adsorption of β-casein. The creaming stability of emulsions made with 0.5% caseinate decreased with increasing CaCl2 concentration. However, the creaming stability increased with CaCl2 concentration in 3.0% caseinate emulsion. The destabilising effects of CaCl2 in emulsions made with sodium caseinate were reduced by the addition of 200 mM NaCl. Addition of CaCl2 to protein solutions prior to emulsion formation increased the d43 and surface protein concentration in emulsions made with 0.5 or 3.0% WPC. In this case, the adsorption of β-lactoglobulin occurred slightly in preference to α-lactalbumin. The creaming stability of emulsions decreased with increase in the concentration of CaCl2 . The addition of CaCl2 to emulsions after emulsion formation also resulted in increases in and surface protein concentration of emulsions made with 0.5% WPC and formation of gel-like network structure at high CaCl2 concentrations. However, the stability of emulsion made with 3.0% WPC was not affected by the addition of CaCl2 . Different aggregation mechanisms are involved depending upon whether Ca2+ is added to protein solution before emulsification or to the emulsion after its formation. Addition of Ca2+ to protein solution may lead to decrease in emulsifying capacity and subsequently result in protein bridging flocculation between emulsion droplets. Ca2+ bridging flocculation between emulsion droplets may be formed in emulsions that have Ca2+ added. The change in aggregation state of caseinate due to Ca2+ binding could retard the instability of emulsion due to depletion flocculation. The protein unfolding at the surface of emulsions made with low whey protein concentrations may promote the protein-Ca-protein bridges forming between protein-coated emulsion droplets, consequently forming gel-like network structure in emulsions.
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    Interactions between hemp globulins and dairy proteins : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Riddet Institute, Massey University, Palmerston North, New Zealand
    (Massey University, 2021) Chuang, Chih-Chieh
    Industrial Hemp (Cannabis sativa L.) is a sustainable protein source and is easily digested. However, the hemp globulins (HG), which constitute around 70% of total hemp seed protein, have low solubility in water at neutral pH. The insolubility of HG limits its usage in many food systems. This work explored the interactions between hemp globulins (HG) and dairy proteins, and aimed to increase the functionality of HG. HG was extracted with a mild salt-extraction and heat treatment was avoided, so the native structure of HG was preserved. The composition of HG was studied and a phase diagram of HG solubilisation was obtained regarding pH and ionic strength. Two methods were used to increase the solubility/colloidal stability of HG by introducing interactions between HG and sodium caseinate (SC). The first method is by heating HG and SC together at 90 °C and the ionic strength of 0.5 M. SC exhibited the chaperone-like activity and inhibited the formation of large aggregates of HG. The addition of SC did not change the denaturation kinetics of HG, but rather changed its aggregation pattern. The second method is by pH-cycling. HG and SC formed colloidally stable nanoparticles (Z-average diameter ≈ 130 nm) after adjusting the pH to 12, reacted for 1 hour and neutralised back to pH 7. The solubility of HG increased from ~20% to > 80% after the pH-cycling. The mechanisms and molecular interactions of both processing methods (heating and pH-cycling) were proposed. During heating, SC interacted with HG via hydrophobic interactions and the aggregation regime of HG changed from diffusion-limited cluster aggregation to reaction-limited cluster aggregation, while the kinetics of HG denaturation was unaffected. During the pH-cycling, hydrogen bond was one of the driving forces for assembly of HG|SC nanoparticles. HG partially unfolded at pH 12 and interacted with caseins during the neutralisation and the stable HG–SC nanoparticles were formed. The pH-cycled HG-SC nanoparticles can be used to make Pickering emulsions. Concentrated emulsions were prepared, and the rheological properties of emulsions during storage can be tuned by controlling HG:SC ratio in the HG|SC nanoparticles, i.e. emulsions became more solid-like when there was more HG in the HG-SC nanoparticles. The internal structure and interactions within the emulsions were evaluated by fitting frequency sweep test data according to a co-operative theory of flow. The results suggested that the solid-like emulsion resulted from stronger short-range interactions between flocs of oil droplets, which developed during storage when there was more HG in the HG–SC nanoparticles. In conclusion, the findings in this study advanced our understanding of the interactions between plant seed globulins and caseins during processing. Such knowledge will help to increase the functionalities of plant proteins by mixing plant and dairy proteins.
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    Physico-chemical properties and stability of lipid droplet-stabilised emulsions : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Manawatū, New Zealand
    (Massey University, 2022) Cheng, Lirong
    It is known that the structure of the interfacial layer impacts the stability and the function of emulsions. Hierarchical emulsions, known as droplet-stabilized emulsions (DSEs), were made from nano-sized primary oil droplets that coated with protein particles for potentially advanced functionality. In this study, the primary droplets were made of either rigid (whey protein microgel, WPM) or soft protein (Ca²⁺-cross-linked caseinate, Ca-CAS) particles. The structure of the protein particles and primary droplets in solution and at the oil-water interface were characterised; the oil exchange process between the surface and core oil droplets were examined, using light scattering, microscopy, small angle scattering, ultra-small angle scattering techniques, etc. The emulsification capacity of the primary emulsion has been shown to be improved by using soft and flexible protein particles, resulting in small droplet sizes and smooth interfacial layers of the DSE. The droplet-stabilised interfacial layer has been shown to provide DSE a good stability against coalescence during gastric enzymatic hydrolysis, long-term storage, and heating, as well as improved functionalities in the rate of the lipolysis during simulated intestinal digestion and the rheological properties at high oil content. Overall, this research provided new information on DSE physical-chemical properties and stability as affected by the structure of emulsifiers (protein particles and the subsequent primary droplets), digestion destabilisation, pH, storage time and temperature. The outcomes have potential for designing functional foods with improved active compound delivery and mechanical strength.
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    Structured emulsion gel systems for delivery of bioactive compounds : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Manawatū, New Zealand
    (Massey University, 2021) Luo, Nan
    The structure of solid/semi-solid foods greatly impacts on how the food is broken down and digested in the human body, which affects its sensory perception, and the bioaccessibility of nutrients. In this project, heat-set whey protein emulsion gel was used as a model system for solid/semi-solid foods for the delivery of capsaicinoids (CAP); the capsaicinoids were dissolved in the emulsion droplets. The aim was to investigate the effect of emulsion gel structure on the breakdown properties and sensory perception of the gel in human mouth and to understand how gel structure affects its digestion behaviour as well as the release of capsaicinoids during in vitro gastrointestinal digestion. Small and large deformation properties as well as the microstructure of the emulsion gel were evaluated. Eighteen human subjects were used to investigate in vivo oral processing behaviour and sensory perception. The Human Gastric Simulator (HGS) was used for in vitro dynamic gastric digestion and pH-stat for simulated intestinal digestion. Human intestinal epithelial cells Caco-2 were used to evaluate the irritation effect of gastric digesta by the quantification of human interleukin-8 (IL-8) production using enzyme-linked immunosorbent assay (Elisa). Based on the rheological properties, the gels were classified into three groups: semi-solid gel (whey proteins as emulsifier, 10 mM NaCl with d4,3 of ~ 0.2 µm); soft and elastic gels (whey proteins as emulsifier, 10 mM NaCl with d4,3 of ~ 4, 1 and 0.5 µm); hard and brittle gels (whey proteins as emulsifier, 100 mM NaCl with d4,3 of ~ 4, 1, 0.5 and 0.2 µm). Results from in vivo study indicated that the degree of gel fragmentation during mastication was positively correlated with gel hardness (represented by Young’s modulus). A higher degree of fragmentation led to a greater surface exposure during mastication and, therefore, a greater release of capsaicinoid molecules, resulting in greater mouth burn perception. Results from in vitro gastrointestinal digestion of CAP-loaded soft gel and CAP-loaded hard gel showed that the hard gel was disintegrated and hydrolysed slower than the soft gel during gastric digestion. The rate and extent of lipid digestion during intestinal digestion were affected by several factors, such as fat content, gel structure, gel particle size and initial oil droplet size of the gastric digesta. Generally, the soft gel had higher degree of lipid digestion, mainly because of its soft gel structure and lower fat content. The bioaccessibility of CAP was found to be positively correlated with the extent of lipid digestion. The effect of active (whey proteins as emulsifier) versus inactive (Tween 80 as emulsifier) filler particles of CAP-loaded emulsion gels was also investigated. CAP-loaded Tween-80-coated oil droplets were not bound within the whey protein matrix (i.e. emulsion gels containing inactive filler particles) and appeared to be flocculated and heterogeneously distributed in the gel matrix; this led to drastically decreased gel strength. On the other hand, the whey-protein-coated oil droplets had strong interactions with surrounding whey protein matrix contributing to gel strength, and the oil droplets were relatively evenly distributed in gel matrix in CAP-loaded whey protein emulsion gels (i.e. emulsion gels containing active filler particles). During in vivo oral processing, CAP-loaded Tween 80 emulsion gels were readily broken down into small fragments in the mouth at small deformations with less chewing and released large amounts of oil droplets from the gel matrix. In general, the mouth burn perception was positively correlated with degree of gel fragmentation. The large amounts of oil droplets released from the gel matrix during mastication and the inhomogeneous distribution of the oil droplets of the CAP-loaded Tween 80 emulsion gels also contributed to their greater mouth burn perception. During in vitro gastric digestion, the gel with inactive filler particles was disintegrated and emptied out faster than gel with active filler particles, due to its significantly smaller masticated particle size entering the stomach. Large amounts of oil droplets were released during gastric digestion from the gel with inactive filler particles while gel with active filler particles had minor release of oil droplets at the end of digestion. During intestinal digestion, the presence of Tween 80 in gel with inactive filler particles has slowed down the rate and extent of lipolysis, because Tween 80 had certain resistance against replacement by bile salts from the interface. Moreover, the Tween 80 molecules, once displaced by bile salts from the interface, would also participate in the formation of mixed micelles and help solubilize the released CAP molecules, therefore, leading to improved bioaccessibility of CAP. An in vitro method was developed to quantify the gastric irritation of CAP-loaded food formulations during gastric digestion. Results suggest that Caco-2 cells had immune responses to CAP-loaded samples by secreting significant amounts of IL-8, confirming that CAP molecules are inflammatory to Caco-2 cells. The emulsion gel structure was modified using different emulsifiers: whey proteins versus Tween 80. The gastric digesta from CAP-loaded Tween 80 emulsion gel was able to stimulate more IL-8 production than CAP-loaded whey protein emulsion gel. Tween 80 was found to be a proinflammatory factor to Caco-2 cells and could stimulate IL-8 secretion. Overall, this research provided new information on the use of solid/semi-solid systems for delivery of capsaicinoids and how food structure affects disintegration and digestion behaviour and eventually the release of capsacinoids. The outcomes have potential for designing functional foods containing capsaicinoids, with increased incorporation of capsaicinoids in the foods / pharmaceuticals, reduced irritation in the mouth and stomach and increased bioaccessibility in the intestine.
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    Study on the interactions between milk protein and digestive enzymes (pepsin) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Manawatū, New Zealand
    (Massey University, 2023-11-22) Yang, Mengxiao
    Pepsin, as a main digestive enzyme in the gastric environment, plays a key role in milk protein digestion. Pepsin and κ-casein at pH > 5, leading to coagulation of casein micelles. The hydrolysis of κ-casein in skim milk was investigated using reverse phase high performance liquid chromatography (RP-HPLC). The coagulation behavior and coagulum structure were observed using oscillatory rheology, microscopy, small and ultra-small angle neutron scattering. The effects of important variables, including temperature, pH, divalent cations, ionic strength, whey protein concentration, and pre-heat treatment of milk, were studied on the hydrolysis kinetics and coagulation process. The pepsin-induced hydrolysis of κ-casein was described using a combined kinetic model of first-order hydrolysis and pepsin denaturation. The hydrolysis rate was independent of the divalent cations, ionic strength, and whey protein concentration, but was highly dependent on the temperature, pH, pre-heat treatment, and milk species. Coagulation of the casein micelles occurred when a critical amount of κ-casein had been hydrolyzed (Hct). The Hct was independent of pepsin concentration (ranging from 0.275 to 5.50 U/mL milk) at a given pH, but it depended on the pH, temperature and calcium concentration: a lower Hct was observed at lower pH (5.3); at higher temperature (43 °C); and at a higher addition of CaCl₂ 17.5 (mM). Sheep milk coagulated at a lower Hct and goat milk coagulated at higher Hct compared to cow milk. In addition, the rheological properties and the microstructures of the curd were significantly different depending on all the variables. A human gastric simulator (HGS) was used to investigate the in vitro dynamic gastric digestion of different processed milk. The 4 °C milk exhibited slower coagulation with a faster breakdown and hydrolysis of the caseins by pepsin, when compared to that at 37 °C and 50 °C. Addition of CaCl₂ enhanced coagulation of casein micelles and resulted in more cohesive curd, which decreased the emptying rate of caseins. Overall, this research provides fundamental information on the kinetics of hydrolysis of κ-casein and how it affects the coagulation behavior of casein micelles under various conditions in milk systems. The different gastric digestion behaviors of different processed milks are explained. The outcomes from this study have the potential to design dairy products with specific coagulation and digestion behaviors to meet the specific digestion requirements of consumers.
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    Understanding the effect of processing and species on milk proteins during digestion : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Biochemistry at Massey University, Manawatū, New Zealand
    (Massey University, 2025-02-28) Maidment, Catherine Ann
    Milk is an important source of protein in a balanced human diet. Milk proteins not only have high nutritive value but also have biological properties. Milk composition and structure vary based on factors such as species, processing methods, and lactation stage. These differences are believed to affect digestion by influencing the breakdown of milk proteins, fats, and carbohydrates, as well as the rate and efficiency at which nutrients are absorbed in the gastrointestinal tract. The overall objective of this PhD thesis was to investigate how milk proteins from different species (cow, sheep, goat, and deer) are affected by digestibility under varying processing treatments (heating and homogenisation). Digestibility was assessed by the amount and types of bioaccessible peptides generated during gastrointestinal digestion. A dynamic in vitro digestion model (human gastric simulator (HGS)) was used for this study. Size exclusion chromatography was employed to measure the amount of peptides generated throughout digestion, with significant differences determined by a p-value threshold of 0.05. Mass spectrometry was used to analyse the types of peptides, requiring peptides to be present in at least two-thirds of the samples for inclusion. To assess the validity of the results obtained using the HGS model, comparisons were made with the peptide profiles generated using an in vivo (pig) digestion model. In addition, further work was undertaken looking into the protein composition of deer milk throughout the different lactation stages. This study investigating digestibility found differences in the amount and types of bioaccessible peptides generated throughout gastric digestion in milk from different species. Overall, deer milk produced the most peptides, while goat and sheep milk produced the least. Ruminant species also affected which regions of the parent protein were resistant to digestion as well as their bioactive properties. In contrast, processing treatment did not have as significant an effect on the amount and types of bioaccessible peptides but did affect the digestion kinetics. Differences were only observed during early digestion and appeared to be species dependent. Similarities were found in the peptides released throughout gastric digestion between the HGS model and the in vivo pig model, which suggests that the HGS model is suitable for the study of gastric digestion of protein-rich food. However, the peptide profiles differed during the intestinal stage indicating that the intestinal step attached to the in vitro model needs improving to fully mimic the dynamic nature of in vivo digestion. The study investigating deer milk proteins found that proteins related to transport e.g. apolipoprotein E and vitamin D-binding protein and immunity e.g. osteopontin, immunoglobulin J and lactotransferrin were found to change throughout lactation. This is thought to reflect the changing needs of the newborn as well as the development and protection of the mammary gland over lactation. Proteins were investigated using mass spectrometry, and significant differences throughout lactation were determined using simple linear regression calculations and log fold change calculations, comparing protein levels between week 3 and week 16 of lactation. The results from this thesis will contribute to the knowledge of how milk composition and structure impact protein digestibility throughout gastrointestinal digestion. The information gained from this study may have important consequences for developing dairy products that deliver superior digestive and nutritional outcomes to targeted consumer groups.
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    Understanding the impact of New Zealand milk seasonality on dairy product quality : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Manawatū, New Zealand
    (Massey University, 2020) Li, Siqi
    In seasonal calving countries like New Zealand, the seasonal variations in milk composition and properties are particularly pronounced due to the significant impact of the stage of lactation on milk characteristics. This study investigated the seasonal variations in the composition and properties of New Zealand milk, as well as the impact of seasonality on product systems including acid milk gel, yoghurt, whipping cream and UHT milk. In addition, this study also demonstrated the effect of processing on the physicochemical properties of milk and the quality of products. The study of milk composition and properties over two full milking seasons demonstrated that the seasonal variations in the composition and physicochemical properties of New Zealand milk were largely controlled by the stage of lactation. Consistent seasonal patterns found include increases in protein and fat and a decrease in lactose in late-season milk, decreasing proportion of α-Lactalbumin in milk protein during the season and increased glycosylation degree of κ-casein in late season milk. Both acid milk gels made by the addition of glucono delta-lactone and yoghurts made by bacterial fermentation showed significant decreases in firmness during the late season, despite the standardization of protein and fat content. Standardization was not sufficient in controlling the acid gelation properties of late-season New Zealand milk. The seasonal variation in the glycosylation degree of κ-casein might play an important role in affecting the acid gelation process by altering the electrostatic and hydrophobic interactions. An investigation into the use of ultrafiltration for standardization demonstrated that a higher proportion of ultrafiltration retentate in the milk improved the overall acid gelation properties of milk. The seasonal variations in the fatty acid composition and the melting behaviour of milk fat broadly followed the lactational trend that the high-melting fatty acids (e.g. C16:0) reached the maximum in the mid-season. The physicochemical changes during the storage of seasonal UHT milk were monitored. The age gelation of UHT milk was least pronounced in the early season. A new hypothesis for gelation mechanism that involves the interaction and sedimentation of κ-CN-depleted casein micelles was proposed.