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    The effect of calcium and milk formulations on biofilm formation of Geobacillus stearothermophilus : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Microbiology, the School of Food and Advanced Technology, Massey University, Palmerston North, New Zealand
    (Massey University, 2021) Wang, Tianyang
    G. stearothermophilus contaminates milk powder products from bacteria spores released from biofilms on product contact surfaces in dairy manufacturing plants. The dairy industry has observed that calcium-reduced milk protein powder is associated with reduced spore contamination of thermophilic bacteria during milk protein powder manufacture. Calcium, as a major component of milk minerals, was previously found to affect G. stearothermophilus biofilms grown on stainless steel exposed to milk formulations. Additionally, G. stearothermophilus cells cultured with additional calcium showed an increased attachment to stainless steel. The current study investigated the effect of calcium and milk formulations on cell attachment, biofilm formation, spore production and spore heat resistance of G. stearothermophilus to pinpoint the potential factors contributing to the reduced thermophile contamination in the calcium-reduced milk protein powder. The effect of calcium on biofilm formation of G. stearothermophilus dairy isolates A1, P3, and one reference strain 7953 in modified TSB media was studied to gain more insights into the role of calcium in biofilm formation of G. stearothermophilus. The presence of calcium increased biofilm cell numbers of strain A1, but reduced biofilm cell numbers of the reference strain and showed minimal effect on strain P3. Extracellular polymeric substances (EPS) quantity, in particular extracellular protein, varied between strains. Unlike the consistent biofilm promotive effect of calcium in milk formulations found in a previous study, the current findings suggest that a strain-to-strain difference exists in biofilm formation of G. stearothermophilus species in the presence of calcium. Calcium plays an important role in cell attachment by changing cell surface properties, cell physiology and metabolism, modifying cell surface structure and bridging between bacteria and substrata. In the dairy industry, milk formulations with different cation profiles may affect the cell attachment of a common contaminant G. stearothermophilus. The current study investigated the effect of calcium on cell attachment of G. stearothermophilus strains A1, P3 and 7953 to stainless steel and polystyrene substrata and characterized the cell surface charge, hydrophobicity and cell surface polymers. In addition, cell attachment in milk formulations on stainless steel was characterized. The presence of calcium increased the cell attachment of dairy isolates on polystyrene but not the reference strain, while calcium did not affect cell attachment of all strains on stainless steel. The presence of calcium changed the amount of cell surface polymers produced but not hydrophobicity. Although calcium affected the zeta potential, this did not correlate with the trend in cell attachment. It is assumed that the cell attachment of G. stearothermophilus is affected by the substrata, strain specific cell surface polymers, as well as calcium induced changes in cell surface polymers. Milk formulations (MF1 and MF2) with different cation profiles showed little effect on cell attachment of G. stearothermophilus on stainless steel, indicating a minimal effect of cell attachment to contamination levels of different cation-modified milk protein powder products in dairy manufacturing plant. The effect of total calcium concentration, total sodium concentration and bacteria growth history were investigated on biofilm formation of G. stearothermophilus in MF1 and MF2. The numbers of culturable biofilm cells of G. stearothermophilus strain A1, P3 and 7953 were compared in MF1 and MF2 over 18 h. MF1 and MF2 have similar protein, lactose and fat content except for cation profiles, where MF2 has relatively high sodium and low calcium concentrations. Biofilm formation of all three strains was lower in MF2 than in MF1, but the inhibition of MF2 was conditional and was highly dependent on the growth history of bacterial inocula. The inhibition of MF2 on dairy isolates A1 and P3 were further investigated by cation supplementation. Supplementation of MF1 with sodium decreased biofilm formation at 18 h for A1 and P3. Supplementation of MF2 with either 2 or 26 mM calcium increased biofilm formation of A1 at 18 h, and P3 at 10 h. High sodium and low calcium concentrations in the milk formulation seem to be required to inhibit biofilm formation of G. stearothermophilus. However, it is not known if the inhibitory effect of MF2 was due to the direct effect of cations on biofilms or the change of milk protein structure on biofilms due to calcium removal. Cations such as calcium and sodium were shown to affect sporulation and spore heat resistance of G. stearothermophilus. MF1 and 2 have distinctive cation profiles and therefore the sporulation and spore heat resistance of G. stearothermophilus A1, P3 and 7953 in milk formulations were investigated. MF2 effectively reduced the total biofilm spore numbers of A1 and P3 over the 18 h culture period compared to MF1, but the effect was not observed in 7953. The sporulation percentage of A1 was higher in MF1 than MF2 at 14 and 18 h, and a similar effect was observed for P3 at 10 and 14 h. Supplementation of calcium to MF2 increased the sporulation percentage of A1 at 14 and 18 h, as well as P3 at 14 h. Supplementation of sodium to MF1 decreased the sporulation percentage of A1 at 18 h. No significant difference in spore heat resistance of A1, P3 and 7953 was observed between spores produced from MF1 and MF2. Overall, the reduced biofilm formation and sporulation percentage of G. stearothermophilus in MF2 provided evidence to the reduced thermophile contamination in the calcium-reduced milk protein powder.
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    A study of the abiotic factors influencing the biofilm and spore formation of dairy isolates of Geobacillus stearothermophilus and characterisation of spores based on their heat resistance : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Safety & Microbiology at Massey University, Palmerston North, New Zealand
    (Massey University, 2021) Kumar, Murali
    Geobacillus stearothermophilus, a Gram-positive thermophilic bacterium is an important contaminant in the dairy industry during milk powder manufacture. This bacterium is capable of survival and growth in sections of the milk powder manufacturing plant where higher temperatures (40-65 °C) prevail. They form biofilms on industrial processing surfaces and their spores are heat resistant. Although, these bacteria are not pathogenic, their presence is an indicator of poor plant hygiene and diminishes the quality of end products thereby incurring economic loss. Previous studies have focused on designing control strategies to prevent the biofilm and spore formation during milk powder manufacture but with limited success. This is largely attributed to the limited knowledge we possess on the biofilm and spore forming capacity of these thermophiles and the impact of abiotic factors. This study focussed on the role of abiotic factors viz. incubation temperature and total dissolved solids on the biofilm and spore formation of Geobacillus stearothermophilus A1, D1, P3 and ATCC 12980. The heat resistance of the spores was characterized and the effect of phosphate on the heat resistance of spores was discussed. The effect of temperature and total dissolved solids on the biofilm and spore formation was evaluated using a Centre for Disease Control (CDC) reactor in a milk environment. The results from the CDC study demonstrated that 65 °C is the most preferred temperature for biofilm and spore formation. Milk proteins influence the attachment of bacteria to the stainless steel surface with caseins having the greatest effect. The three strains were characterized based on their heat resistance and the effect of phosphate on the heat resistance of these spores was determined using atomic absorption spectroscopy (AAS). AAS demonstrated that phosphate when present in the heating medium causes release of cations from within the spore thereby lowering their heat resistance. In addition, the Calcium and Dipicolinic acid (DPA) content of these spores was determined and the DPA content was found to be associated with the heat resistance of spores. The heat resistance of spores obtained by a milk biofilm system under continuous flow was determined and compared with previous values obtained from spores prepared in a static laboratory medium. There is no significant difference (p ≤ 0.05) in the heat resistance of spores obtained by the biofilm in milk and by a sporulation medium. The results from spore formation under different temperatures and total solids will aid in the design of thermal processing steps aimed towards controlling the biofilm and spore formation of G. stearothermophilus during milk powder manufacture. The effect of phosphate on the heat resistance of spores, will aid in the design of a phosphate based cleaning agent to minimise product spoilage caused by the presence of spores of G. stearothermophilus. The results showing the similarity between the heat sensitivity of spores prepared in sporulating medium and a continuous flow CDC reactor using milk provide confidence in results on heat resistance taken from laboratory sporulating medium in terms of their relevance to the dairy industry.
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    Studies on the stability of probiotic bacteria during long term storage : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand
    (Massey University, 2019) Nag, Arup
    According to the Food and Agriculture Organization (FAO) and the World Health Organization (WHO), probiotics are defined as ‘‘live microorganisms which when administered in adequate amounts confer a health benefit for the host’’ (FAO/WHO, 2001). Lactobacilli and bifidobacteria are two major group of organisms considered to have probiotic properties. The primary objective of this project was to develop a novel stabilization technology for probiotic bacteria, through which a range of probiotic bacterial strains could potentially be delivered to the host through shelf stable dry and intermediate moisture foods. For preliminary experiments (reported in Chapter 4.0), Lactobacillus casei 431, a commercial strain from Chr Hansen, Denmark, was chosen as the experimental strain and milk powders (both skimmed and full-fat) were chosen as the principal supporting agent while stabilizing the bacterial cells. Stabilization efficiency in terms of long term ambient temperature storage viability was compared using freeze and fluidized bed drying techniques. Fluidized bed drying was able to retain 2.5 log cfu/g higher viability after 52 weeks of storage at 25 °C. A combination of fluidized bed drying and osmotic stress adaptation to the probiotic cells yielded further improvement of 0.83 log cfu/g higher viability compared to the unstressed cells. The findings were validated with other two lactobacilli and two bifidobacterium strains with probiotic characteristics and significant improvements in storage stability over freeze-dried samples were observed. Fortification of vitamin E in the stabilization matrix as an antioxidant improved the stability by 0.18 log cfu/g during 20 weeks storage period at 25 °C, whereas any similar benefit of fortifying inulin as a prebiotic was not observed. Incubation in simulated gastric fluid and intestinal fluid (in vitro) revealed that the L. casei 431 cells were better protected within the stabilized matrix than in the free form. The survival of the stabilized cells were 5.0 and 2.1 log cycles higher than free cells in gastric juice and bile salt solution respectively. Physical characterization of the probiotic ingredient showed very good flow-ability and solubility, with 470 Kg/m3 bulk density, water activity of 0.27 and agglomerated particles of 125.6 μm mean diameter. Thereafter, the project aimed to understand the underlying mechanism of the processes responsible for gradual decay in cell viability of another probiotic strain (Lactobacillus reuteri LR6) during long term storage at 37 °C (Chapter 5.0 onwards). Vacuum drying of sorbitol- or xylitol-coated Lactobacillus reuteri LR6 cells and fluidized bed drying of the same coated cells with different excipients were compared for the cell viability post drying. LR6 cells coated with xylitol and desiccated in unsupported form or together with skim milk powder as an excipient were found to be better protected when exposed to moderate as well as high drying temperatures. In Chapter 6.0, a closer examination of the protein and polypeptide components of the cell envelopes (amide regions) via Fourier transform infrared spectroscopy revealed different degrees of structural deformation in individual samples, which correlated well with the residual cell viability. It was also important to understand the underlying mechanisms responsible for the loss of viability of stabilized probiotic cells when stored at non-refrigerated temperatures. In Chapter 7.0, the stabilized Lactobacillus reuteri LR6 cells were stored at 37 °C and at two water activity (aw) levels. Superior storage stability was recorded in a lower aw environment, supported by a stronger glassy matrix when skim milk powder was used as the excipient. Fourier transform infrared spectroscopic examination of the cell envelopes revealed substantial dissimilarities between samples at the beginning and at the end of the storage period. In milk powder-based matrices, adjusting the aw to 0.30 resulted in a weaker or no glassy state whereas the same matrices had a high glass transition temperature at aw 0.11. This strong glassy matrix and low aw combination was found to enhance the bacterial stability at the storage temperature of 37 °C. During storage of the stabilized cells for 121 days at 37 °C, the measured Tg for all the samples was slightly lower than what was recorded at the beginning. Scanning electron microscopy revealed the formation of corrugated surfaces and blister-type deformations on the cell envelopes during the stabilization process whereas the freshly harvested cells were found to be with a smooth surface and undamaged membrane. Inspection of the cell bodies via transmission electron microscopy showed freshly harvested cells with normal shapes with no damage in the inner membrane structure. An almost intact but slightly waved outer membrane structure was observed. The findings emphasize the importance of protecting the integrity of the membrane of probiotic cells by using suitable protecting agents to enhance their stability during long term storage. The stabilized cell matrix samples were segregated into 4 groups based on the average particle diameter by passing through sieves of different mesh sizes. The degree of agglomeration had a very important role in offering physical protections to the LR6 cells during the desiccation process. The viable cell populations in the higher particle size groups (above 500μm and 1000μm) were between 9.5 to 9.9 log cfu/g whereas the same for the lower particle size (below 500μm but above 250μm) group was only 7.8 log cfu/g. The minimum viable cell concentration was recorded (7.3 log cfu/g) in the finer particles having less than 250μm diameter but having the maximum mass fraction. In case of stored samples, it was found that the bacterial cells adhered to the finest particles suffered the maximum loss in viability (41.4%) whereas the minimum loss (14.9%) was within the particles with average diameter above 500μm. In order to assess the effect of stabilization and storage (12 weeks, 37 °C) on the common probiotic attributes of the LR6 cells, an in vitro study on acid, bile salts tolerance and surface hydrophobicity was conducted. The results showed considerable reductions in cell viability for the desiccated as well as stored cells when incubated in simulated gastric (acid tolerance) and intestinal (bile salts tolerance) environments. A coating of xylitol over the cell bodies during desiccation was found to be marginally protective against these stresses. High aw storage was found to be more detrimental to the cells in terms of their ability to survive in the acid or bile environments. The cell surface hydrophobicity towards various hydrocarbons was also found to be adversely affected due to desiccation and non-refrigerated storage. Considerable degradation in hydrophobicity was found to be occurring in the cells stored at aw 0.30, a trend similar to the acid and bile resistance properties.
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    The stickiness curves of dairy powder : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Bioprocess Engineering at Massey University
    (Massey University, 2004) Zuo, Jenny Y
    Powder stickiness problems encountered during spray drying are important to the dairy industry. Instantaneous stickiness is a surface phenomena that is caused by exceeding the glass transition temperature of the amorphous sugar in the powder, usually lactose in dairy powders. Instantaneous stickiness occurs at a certain temperature above the Tg of amorphous lactose and has been denoted as the critical "X" value. Whether powder particles are sticky or not depends on whether there is enough liquid flow on the surface between the particles. Two particles stick to each other when there is enough liquid flow to form a bridge between them after the contact. This project aimed to measure the instantaneous sticky point conditions for various dairy powders and to relate these to the operating conditions to give a commerical outcome for the dairy industry. The particle-gun rig was developed to simulate the conditions in the spray drier and the ducting pipe and cyclone. The stickiness of powder particles occurs after a short resident time in the particle-gun. Thus, stickiness is a surface phenomenon and the point of adhesion is the instantaneous sticky point. The amount of deposit on the plate was measured at a temperature, with increasing relative humidity. At a particular temperature and relative humidity, the powder stuck to the stainless steel plate instantaneously. This was observed by a sudden change in % deposition on a % deposition verse RH plot. The T-Tg plot and stickiness curve profile were developed to determine the critical "X" value for the dairy powders. The critical 'X' value is the temperature which exceeds the Tg of amorphous lactose when instantaneous stickiness occurs. The critical "X" values tor various dairy powders including WMP, SMP, MPC, whey protein, buttermilk, white cheese powder and GLUMP powder were found to be 33-49°C. 37-42°C. 42-51C. 50°C, 37-39°C, 28.5°C, and 40.7°C respectively. In addition, the slope of the trend line in the T-Tg plot, indicates how quickly the particular powder becomes sticky once the instantaneous sticky point has been exceeded. The particle-gun rig demonstrated that powders with greater than 30% amorphous lactose are more likely to cause blockage than powders with less than 30%. Both the critical 'X' value and the slope are unique to the powder. The stickiness curve was used to relate the powder surface stickiness condition with the drier outlet temperature and relative humidity. It was recommended to operate at conditions below the stickiness curve for a powder to avoid any chamber or cyclone blockages caused by stickiness. The slope enables a decision to be made about how close to the critical point a plant should be run for a particular powder. The inlet air temperature or concentrate feeding rate can be used to move the operating conditions towards or away from the stickiness curve, according to the operating situations.
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    Native milk fat globule membrane damage : measurement and effect of mechanical factors in milk powder processing operations : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology of Massey University
    (Massey University, 2005) Downey, Mark
    The goal of this work was to measure native milk fat globule membrane (NMFGM) damage in a number of processing operations within the milk powder manufacturing process. Analysis of the literature showed that NMFGM damage was not well understood, particularly as caused by processing operations within factories. Reliable methods of measuring NMFGM damage were not available: current methods had limited scope or were qualitative in nature. In the highly mechanised dairy industry, damage to the NMFGM can lead to serious quality and financial losses owing to consequences such as lipolysis and creaming. The aims of this work were to develop new techniques for measuring NMFGM damage, and to use these in assessing the effects of a number of operations within the milk powder process. The majority of time was spent on developing two new tests, the selective lipolysis (SL1) test and the particle size zoning (PSZ) test. The SL1 test measures a chemical consequence of NMFGM damage, that is the production of free fatty acids (FFAs). The PSZ test measures a physical consequence of NMFGM damage, that is the change in the fat globule size distribution. Controlled experiments were used to measure NMFGM damage in process operations including pumping, agitation, preheating and evaporation. For these operations, variables such as shear, time, temperature, air inclusion and cavitation were investigated. Surveys of two industrial milk powder plants were also conducted. The results showed that the SL1 and PSZ tests were reproducible, sensitive enough to detect NMFGM damage in a number of process operations, and, together, could give a reasonably comprehensive picture of NMFGM damage. The results of pumping and agitation experiments were consistent with previous research, but were more comprehensive. The effects on measured NMFGM damage of the presence of separated fat in foam or as churned fat have hardly been described by previous workers. Results for the effects of preheating and evaporation on NMFGM damage are new, and challenged the findings of previous research. The need to improve the flexibility and practicality of the SL1 and PSZ tests, so they can he used as widely as possible to gain a comprehensive picture of NMFGM damage across many dairy processes, was identified. Studies should be made to connect the results of the particle size zoning and selective lipolysis tests with product quality and process efficiency data from industrial sites.
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    Granulation of whole milk powder : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Process Engineering at Massey University
    (Massey University, 2002) Field-Mitchell, Gary
    High-shear granulation is an attractive alternative to spray drying for producing dried milk products. The capital cost of a granulation circuit is likely to be much less than a spray drying circuit which will reduce the manufacturing costs of milk powders. This work investigated the high-shear granulation of milk powder using milk concentrate as a binding agent in order to determine the feasibility of granulation as an alternative to, or and improvement on, the spray drying process. This research has laid the groundwork for further investigation into milk granulation by defining the conditions for which granulation is achieved and describing the effects of processing parameters on granulation for a pilot-scale mixer granulator. The technical feasibility of granulation is shown by proving that granulation does not affect the quality of the milk. Designs for perceived continuous granulation circuits are included to aid in further milk granulation research. Successful granulation occurs at a total moisture content of approximately 11% (±1%). This was found to be suitable using either reconstituted or evaporated milk concentrated binder at between 20 and 50% total solids. The time of granulation affects the size distribution of the granules and the granule yield at the end of the process. A narrower size distribution with increasing granule sizes and a reduction in the granule yield is observed for longer granulation times. Granules were found to have better handling qualities than spray dried milk powders. Granules performed better in many functional tests having a higher bulk density, less change in bulk density during handling, better flowability and less fines. Granulation does not affect the chemical quality of the milk providing the granules are dried immediately after granulation. However, it was found that extended exposure of dried milk solids to a moisture content of 11% results in an unacceptable amount of insoluble material forming. Granules are well suited as a product for reconstitution but did not perform adequately in wettability tests, suggesting that their use as an instantised product would require further study and improvement. Further research is required to understand the role of lactose crystallisation and the generation of insoluble material to ensure scaling up of granulation will be successful. An investigation into continuous granulation would be useful for further milk granulation work.
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    Incorporation of extracellular polysaccharide produced by Xanthomonas campestris into milk powders : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University
    (Massey University, 2003) Sharpe, Hamish
    The purpose of the research was to investigate the functional properties of milk powders following exopolysaccharide (EPS) addition to milk solutions and their subsequent spray-drying. The aim was to replace some of the milk proteins with polysaccharide in dairy products while maintaining or improving the functional characteristics. Both commercial xanthan EPS and ferment xanthan EPS were incorporated into whole milk powder (WMP), skim milk powder (SMP), and milk protein concentrate (MPC). Ferment EPS was produced from a by-product of the dairy industry, milk permeate, through the hydrolysis of the lactose and fermentation with a strain of Xanthomonas campestris. Ferment EPS had a characteristic and unpleasant odour. The main compound responsible for this odour was p-cresol which, in milk, is largely bound in the conjugate form. Xanthomonas campestris hydrolyses these conjugates releasing the odour compounds. Ultrafiltration (UF) of the ferment or passing the ferment through a bed of activated carbon was effective in reducing the odour. UF was proven to reduce the levels of p-cresol in the ferment from 138ppb to less than 5ppb after 98 concentration factors. Milk powders made with UF ferment were more acceptable to the consumer sensory panel than those made with untreated ferment. The incorporation of EPS into milk powders has beneficial effects on the product with small additions increasing the viscosity of reconstituted SMP and WMP considerably. The EPS addition could result in a thickened milk product or alternatively, substitute for some of the milk solids. Sensory testing showed that 13.3% WMP solution, containing 0.02% commercial EPS, was not detectably different from a 15% WMP solution. The addition of both commercial and ferment EPS into milk powders leads to the formation of separate flocculated casein and polysaccharide phases with reconstituted milk. Confocal microscopy showed that casein flocculation occurred at all EPS concentrations tested, but this only resulted in sedimentation at intermediate EPS concentrations. At high EPS concentrations of approximately 0.2% the high viscosity limited flocculation and prevented sedimentation. At low EPS concentrations of approximately 0.05% flocculation was insufficient to overcome Brownian motion. Fresh cheese (Panela) made from MPC containing either ferment or commercial EPS showed greatly decreased whey loss. This was attributed to (i) the increased viscosity of the continuous phase limiting the flow of liquid through the pores of the cheese, and (ii) diminished casein interaction in the presence of EPS leading to a looser curd and lower contraction forces. For example the incorporation of 0.161% ferment EPS decreased the whey lost by approximately 75%. Negative effects were also apparent. The addition of EPS led to a granular appearance, which became more apparent with increasing EPS concentration. Cheese firmness was also decreased by approximately 40% by the addition of the ferment EPS at 0.161%. This could also be attributed to the localised aggregation of protein during renneting and the increased heterogeneity of the network. Sensory testing of cheeses made with 15.6% MPC + 0.045% commercial EPS compared with cheese made with 17.37% MPC alone showed that the consumers had no significant preference for one cheese over the other, but did notice a difference in texture. For reasons of safety and health, the sensory testing of milk and cheese in this research was confined to commercial xanthan. Future sensory testing of milk and cheese should be conducted with ferment EPS after odour removal rather than commercial EPS, and use consumers familiar with these cheese and milk products. For commercial production of dairy powders containing UF ferment EPS it is vital that either the xanthan or casein micelle structure be altered to prevent casein flocculation. If this is not feasible then an alternative use of the product may need to be found. A potential option involves the addition of the powder containing UF ferment EPS into food products as a minor food constituent. This may limit the occurrence of phase separation while improving the functionality of the product. Commercialisation is also limited by the increasing costs caused by ferment EPS purification and the lower solids concentrations required for spray-drying. As such the viability of the powder production must be determined.
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    Development of new RSCM processes : a thesis presented in partial fulfillment of the requirements for the degree of Masters of Technology in Food Technology at Massey University
    (Massey University, 2007) Sawatdeenaruenat, Chanapha
    All concentrated milks thicken with storage time and the degree of thickening is highly dependent on the storage temperature. The aim of the current research project was to investigate this phenomenon in reconstituted concentrated milk (RCM) and recombined sweetened condensed milk (RSCM) and to investigate a method for overcoming the quality defect. RCM was initially investigated as this system had been extensively documented by previous works at Massey University, New Zealand. The RCM system was chosen to provide an opportunity of learning all about time dependent rheology. It was observed that reshearing of age thickened RCM samples destroyed the ability to age thicken again in subsequent storage. RSCM was then investigated to assess the effect of shear and temperature on age thickening during storage. Two shear levels of 900 and 31,000s-1 were applied during the recombination stage in the process of producing RSCM. Samples of RSCM produced using both shear rates were then stored at temperatures of 30, 40 and 50°C for a period of 12 weeks. Triplicate samples from each storage temperature were analysed weekly for apparent viscosity, particle size distribution and colour. The RSCM samples stored at 50°C gelled by the 7th week while RSCM samples stored at 30 and 40°C did not gel even by the 12th week. The results of particle size distribution were consistent with the age thickening results. The particle sizes of samples stored at 30 and 40°C almost did not change with storage time but the particle sizes of samples stored at 50°C increased with storage time until they gelled. The colour of RSCM became darker with increased storage temperature and time. This was particularly noticeable at 50°C. The study showed that the commonly observed quality defect in RSCM could be overcome for samples stored below 40°C for at least 12 weeks by the application of shear rates in excess of 900s-1 during the manufacture of the product.
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    Spray dried milk-protein stabilized emulsions with high oil content : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand
    (Massey University, 2016) Taneja, Amit
    This study explores the behaviour of oil droplets in milk protein-stabilized emulsions during spray drying. The impact of preheat treatment on the stability of oil droplets during drying in milk protein-stabilized emulsions in maltodextrin was also observed, using a variety of techniques such as particle size analysis, various microscopy techniques and sodium dodecyl sulphate polyacrylamide gel electrophoresis. In the last section of the study, the stability of the powdered emulsions was investigated against oxidative deterioration when soybean oil was replaced with fish oil in the emulsion formulation. The results showed that spray drying and redispersion of the powdered emulsions in water (at similar total solids content) caused a shift in the droplet size distribution to larger values for all emulsions made using low concentration of whey protein isolate or sodium caseinate (0.5-2.0%, w/w w.b.), in comparison with their respective parent emulsions. However, the droplet size distribution was affected only very slightly by spray drying when the protein concentration was above 2.0% (w/w). This minimum concentration of protein that was required to produce emulsions that were stable during the spray drying process was 3.0% (w/w) for the emulsions prepared using aggregated milk protein products as compared with 2.0% (w/w) for the NaCas- and WPI-containing emulsions. It was suggested that the amount of unadsorbed protein in the bulk phase of the parent emulsions play a crucial role in stabilizing the oil droplets during spray drying. When the surface of the oil droplet is saturated with protein molecules and the bulk phase of the emulsion has sufficient unadsorbed protein, the oil droplet is stable during drying. However, for emulsions with a low concentration of unadsorbed protein in the bulk phase (= 1.0% for WPI or NaCas emulsions), protein molecules could potentially migrate from the surface of the oil droplet to the air?water interface, causing “gaps” in the oil droplet interface and leading to coalescence and/or bridging flocculation. Emulsions containing low levels of maltodextrin showed marked coalescence during spray drying and redispersion even at a WPI concentration of 10.0% (w/w). Above a critical concentration (12.0%, w/w), maltodextrin appeared to stabilize proteins at the interface and provide adequate rigidity to the matrix perhaps by forming a glass, under the drying conditions. In whey protein-stabilized emulsions made with preheat treated protein solution (above 70ºC), a shift was observed in average droplet diameter towards the larger size range, because of droplet coalescence as a result of spray drying. This was thought to probably be a result of protein aggregation in emulsions, which adversely affected the ability of proteins to stabilize the emulsion droplets during spray drying and further emphasized the crucial role of monomeric whey proteins. A reduction in the non-adsorbed monomeric whey proteins as a result of preheat treatment led to oil droplet coalescence during drying. The stability of the emulsion made with pre-heat treated whey proteins was noticeably improved when NaCas was added to the emulsion either before or after the homogenization step. This improved stability was believed to be a result of the steric effect of caseins that prevented large-scale aggregation of whey proteins. The stability of emulsions during drying as shown by the change in the average droplet diameter before and after drying showed a negative correlation with oxidative stability of these emulsion where soybean oil was replaced with fish oil. The protein content and preheat treatment also showed a positive impact on the oxidative stability of spray-dried emulsions. Overall, the finding from this systematic study has advanced the understanding of the mechanisms of the stability of oil droplet during drying as well as the impact of emulsions components and processing conditions. This may help to design emulsion formulations and processes and extend the applications of milk-protein stabilized powdered emulsions with high oil content.
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    The production of a high free-fat whole milk powder for the chocolate industry : the spray chilling technology : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University
    (Massey University, 1996) Kebakile, Martin M
    Whole milk powder containing down to 80 percent free fat was manufactured by spray chilling suspensions of skim milk in milk fat, using a modified laboratory spray drier. Also, the pure unmodified milk fat, and the soft (SBF23), medium (SBF27) and hard (SBF42) fractions of the same were transformed into powder by spray chilling the molten samples. The effect of chilling with air and nitrogen was investigated. The powders were satisfactorily stable at 5°C, but were relatively unstable at ambient temperatures. The powders' particle size distributions ranged from 4.44 to 215.56 μm. The powder characteristics were influenced by the size of the nozzle, the atomising gas pressure, the chilling temperature, the feed flow rate, and to a lesser extent, the feed temperature. The shelf life of the unmodified milk fat powders stored at 20°C, 5°C and -10°C was assessed. The powders chilled with air had excessively oxidised after one month of storage at all the temperatures, whereas powders processed with nitrogen were still usable after the same period of storage. Lower peroxide values were recorded for the powders stored at - 10°C and 5°C, while significantly higher values were obtained for the samples stored at 20°C. The powdered fats dry-blended successfully with skim milk and calcium caseinate powders at the ambient temperature. In comparison, the hard fraction mixed better than the other softer fractions. Up to 50 percent of the hard fraction, and just 30 percent of the softer fractions, could be blended with the skim milk powder. An upper level of 70 percent hard fraction, and of 50 percent for the softer fractions, were mixed with calcium caseinate. The repose angles of the skim milk and milk fat blends increased with the increasing fat content, and the blends containing up to 20 percent fat were free flowing. For the calcium caseinate and skim milk blends, the repose angle decreased with the increasing fat content, and all the blends were not free flowing. The bulk densities of the skim milk blends decreased with the increasing amount of fat, while those of the calcium caseinate blends increased with the increasing fat content.