Skin layer stratification in drying droplets of dairy colloids (original) (raw)
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Morphology development in single drop drying for native and aggregated whey protein dispersions
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2019
Native and aggregated whey proteins (WP) are used in food and pharmaceutical applications as stabilizers, thickeners and carriers. For increasing shelf-life and facilitating transportation, WP are transformed into powders by spray-drying. Powder functional properties strongly depend on the final particle morphology. Focusing on colloidal aspects of drying, the goal of this work is to: (i) investigate morphology development during single drop drying of native and aggregated whey protein dispersions; (ii) use structure-mechanical parameters to predict the final morphology. Results showed evaporation rate and morphology development characteristic times are not affected significantly by colloidal size. However, the final morphology of particles depends on WP colloidal size. For small colloids, particles are shriveled, while their shape is cup-like for larger colloids. Structure-mechanical parameters allowed predicting a buckled/shriveled morphology in agreement with experimental observations. Specifically, predictions anticipated the formation of a solid shell at the particle surface, which is compressed during drying, as a result of colloidal interactions being dominated by van der Waals forces. This work provides a rationalization of morphology development of WP particles. In addition, the work suggests that the elasticor gelformation, that is governed by the permeation, may be very different depending on the permeability of the proteins gel. The collapse from a dispersion to an elastic gel may be responsible for the shriveled to buckled transition. The work shows that diverse final morphologies can be achieved using same drying conditions and composition, while only changing the degree of colloidal aggregation.
Shape, Shell, and Vacuole Formation during the Drying of a Single Concentrated Whey Protein Droplet
Langmuir, 2013
The drying of milk concentrate droplets usually leads to specific particle morphology influencing their properties and their functionality. Understanding how the final shape of the particle is formed therefore represents a key issue for industrial applications. In this study, a new approach to the investigation of droplet−particle conversion is proposed. A single droplet of concentrated globular proteins extracted from milk was deposited onto a hydrophobic substrate and placed in a dry environment. Complementary methods (high-speed camera, confocal microscopy, and microbalance) were used to record the drying behavior of the concentrated protein droplets. Our results showed that whatever the initial concentration, particle formation included three dynamic stages clearly defined by the loss of mass and the evolution of the internal and external shapes of the droplet. A new and reproducible particle shape was related in this study. It was observed after drying a smooth, hemispherical cap-shaped particle, including a uniform protein shell and the nucleation of an internal vacuole. The particle morphology was strongly influenced by the drying environment, the contact angle, and the initial protein concentration, all of which governed the duration of the droplet shrinkage, the degree of buckling, and the shell thickness. These results are discussed in terms of specific protein behaviors in forming a predictable and a characteristic particle shape. The way the shell is formed may be the starting point in shaping particle distortion and thus represents a potential means of tuning the particle morphology.
Coalescence and agglomeration of individual particles of skim milk during convective drying
Journal of Food Engineering, 2016
This work presents a methodology, which combines experiments and modelling, for investigating the coalescence and agglomeration ability of a product and to support the modelling of product properties during spray drying. Two particles were dried simultaneously and contact tests were performed along the drying time. A validated mathematical model describing the drying kinetics of milk particles was used to predict surface conditions during contact tests. Three major mechanisms were observed, coalescence, stickiness, and non-stickiness, which were related to adhesion and cohesion forces. The simulation model allowed evaluation of the surface Ohnesorge dimensionless number and surface glass transition temperature, which showed to be good parameters for predicting contact mechanisms. The model was also used to predict shell formation in drying particles. Wet and dry shell formation was simulated over the drying time, to improve understanding of observed contact behaviour.
Exploring drying kinetics and morphology of commercial dairy powders
Journal of Food Engineering, 2015
Understanding the effect of the initial composition of a liquid feed on the spray drying process and morphology of powders is important in order to reduce the time and costs for process design, and ensure the desired properties of the final product. In this work, seven commercial dairy products with different fat content were selected. The effect of initial composition on drying time during single drop experiments was studied. The morphology of powder particles and the influence of morphology changes on the drying rate were investigated in order to assess the effect of fat content on the effective diffusivity of water in dairy products. Results show that fat content influences drying time and morphology of powder particles. The higher the fat content the longer the drying time and particles appear to be less shrivelled. Changes in morphology and the drying rate seem to be related. Two falling drying periods were observed for most of the products. During the first period the drops shrink spherically, while during the second period shrivelling occurs. The effective diffusivity of water shows that high fat contents lead to a lower diffusivity of water in the products.
How surface composition of high milk proteins powders is influenced by spray-drying temperature
Colloids and Surfaces B: Biointerfaces, 2010
High milk proteins powders are common ingredients in many food products. The surface composition of these powders is expected to play an essential role during their storage, handling and/or final application. Therefore, an eventual control of the surface composition by modifying the spray-drying temperature could be very useful in the improvement of powder quality and the development of new applications. For this purpose, the influence of five spray-drying temperatures upon the surface composition of the powders was investigated by X-ray photoelectron spectroscopy. The major milk proteins were studied: native micellar casein and native whey, both more or less enriched in lactose.
Development of stickiness of whey protein isolate and lactose droplets during convective drying
Chemical Engineering and Processing, 2007
The stickiness development of droplets of whey protein isolate (WPI), lactose and their mixture solutions was determined using an in situ stickiness testing device at 24, 65 and 80 • C. Stainless steel, Teflon, glass and polyurethane probes were used. At room temperature, the presence of 0.5-1% (w/w) WPI greatly lowered the observed tensile strength of water and lactose solutions due to surface adsorption and led to a weakening of the cohesive strength. At elevated temperatures, lactose droplets remained sticky showing cohesive failure until the surface was completely covered with a thin crystal layer. WPI droplets formed a thin, smooth skin immediately on coming in contact with drying air. This surface became non-sticky early in the course of drying due to the transformation of the surface to a glassy state. The skin forming and surface active nature of WPI was exploited to minimize the stickiness of honey in a pilot scale spray drying trial. Replacement of 5% (w/w) maltodextrin with WPI raised the powder recovery of honey solids from 28% to 80% in a pilot scale drying test. At elevated temperature the magnitude of stickiness on probe materials was in the order of glass > stainless steel > polyurethane > Teflon. The Teflon surface offered the lowest stickiness both at low and high temperatures making it a suitable material to minimize stickiness through surface coating.
Journal of Food Engineering, 2014
A micro-fluidic-jet spray dryer was used to fabricate spray freeze dried (SFD) and spray dried (SD) uniform microparticles with feed solid content >30 wt% of reconstituted skim milk. The effects of feed solid content on the particle size, morphology, surface composition, wettability and solubility were investigated. The surface composition of SFD and SD sample was characterized via an X-ray photoelectron spectroscopy. Fat and protein was found to be over-represented on the SD particle surface, while lactose significantly declined. Such ingredient segregation was quantitatively shown to occur during atomization and continue within the drying process, i.e. atomization-induced ingredient segregation (AIIS) and drying-induced ingredient segregation (DIIS). The wettability and solubility of the spray dried samples were quantified using scaled-down GEA Niro Methods in relation to the feed solid contents.
Dairy Science and Technology, 2011
This study analyzes the early stages of the acid coagulation behavior of milk containing homogenized fat globules. By addition of Tween 20 to homogenized milk, it was possible to create two similar colloidal systems with completely different interfacial properties. Control experiments using skim milk demonstrated that the presence of Tween did not overly affect the acid coagulation behavior of the casein micelles. The initial stages of aggregation were also similar for the two homogenized milk systems, indicating that the casein micelles were the main driving force behind early gel formation. For the case of homogenized milk without Tween, the fat globules were fully incorporated in the network. The stiffness of the gel was higher than the control and the overall spatial distribution of the fat droplets was not largely affected by the developing gel matrix. In contrast, the homogenized milk with added surfactant contained fat globules which did not interact directly with the casein micelles but rather became trapped inside the pores of the ensuing network. This gel showed a lower elastic modulus than the homogenized milk case and freediffusing fat globules. Although the presence of interacting and non-interacting droplets did not overly affect the coagulation kinetics of the casein micelles, the acid gels did show different final properties which fully reflected the presence of "active" or "inert" fillers. The knowledge derived from this work will be the first step towards developing novel textures for dairy gels, modulating the extent of the interactions between the fat globules and the protein network.
Journal of Agricultural and Food Chemistry, 2003
Properties of condensed milks prior to spray drying dictate to a large extent the functionality of the resulting milk powder. Rheological properties of concentrated skim milk, with total solids content of 45% but different mineral content, were studied as a function of shear rate and storage time at 50°C. These milks are proposed as a model to study the effects of minerals on rheology and age gellation of condensed milk prior to drying. During storage of the concentrated milk, the apparent viscosity, particularly after 4 h, increased markedly at all shear rates studied. The yield stress also increased steeply after 4 h of storage at 50°C. The changes in apparent viscosity of concentrated milk stored for up to 4 h were largely reversible under high shear, but irreversible in samples stored for longer time. The appearance of yield stress suggested the presence of reversible flocculation arising from weak attraction between casein micelles, with a transition from reversible to irreversible aggregation during storage. Particle size analysis confirmed irreversible aggregation and fusion of casein micelles during storage. Gradual reduction of mineral content of concentrated milks resulted in a marked decrease in the apparent viscosity and casein micelle aggregation during storage, while addition of minerals to milk had the opposite effect. The results demonstrated that the soluble mineral content is very important in controlling the storage-induced changes in the rheology of concentrated milks. (Key words: concentrated milk, yield stress, casein micelle aggregation, mineral environment)
Surface energy analysis (SEA) and rheology of powder milk dairy products
Food Chemistry, 2015
Results of inverse gas chromatography adsorption/desorption experiments using selected probes on skimmed milk, whey and demineralised whey powder materials are presented. The dispersive component of surface energy was found to be dominant, indicating low polarity character. Surface energy profiles of demineralised whey and skimmed milk showed a characteristic steep exponential decrease from approximately 170 mJ/m 2 to 60 mJ/m 2 and 140 mJ/m 2 to 45 mJ/m 2 , respectively, whereas whey powder exhibited a constant (non-öÓ¿²«-½®•°¬ Ý´•½µ ¸»®» ¬± ª•»© ´•²µ»¼ λº»®»²½»