Xiao Dong Chen

Recent advances in spray drying relevant to the dairy industry: A comprehensive critical review

ABSTRACT Milk is extremely perishable, and yet it has to be preserved for later consumption. In this view, membrane filtration, vacuum concentration lactose crystallization, homogenization, and spray-drying dehydration are valuable techniques to stabilize most dairy ingredients. Considering the increasing development of dairy trade, there is a need for the dairy industry to improve its understanding of how these concentration and spray-drying processes affect the quality of the resulting dairy powders, so to control it. However, the residence time of the droplet and the powder in the spray dryer is so short that it is very difficult to implement studies on the mechanisms of the structural changes in the protein without fundamental research into the process/product interactions. Moreover, several authors have reported the crucial and specific role of dairy components in the mechanisms of water transfer during drying and rehydration. The aim of this paper is to review the present and recent advances in knowledge and innovations, on the properties of spray-dried dairy products, on the modeling and simulation of water transfer processes (drying and rehydration), and on spray-drying equipment and energy consumption.

Effects of Co-spray Drying of Surfactants with High Solids Milk on Milk Powder Wettability

Dairy industries often spray lecithin, a food-grade surfactant, to spray-dried whole milk powders while fluidisation to produce instant powders. Though adding surfactant to milk feed was often reported to improve the wettability of dried powders, this approach was not favourably used. The present study investigated the effects of surfactant addition into high solids milk feed before drying on the wettability of whole milk particles. Adding either 0.1xa0wt.% Tween 80 or 0.1xa0wt.% lecithin to un-concentrated whole milk led to a significant wettability improvement of spray-dried powders. At higher feed solids levels of 23 and 33xa0wt.%, the wetting process of pure milk powders was comparatively rapid, but the surfactant-added powders showed similar wettability to the pure milk powders. The development of powder wettability as drying progressed was investigated using single droplet-drying technique for 32 and 43xa0wt.% whole milk in the presence and absence with surfactants. The technique captured an advanced shell formation during drying of higher solids milk. The wettability of surface shells formed by surfactant-added milk was similar to those formed by pure milk throughout drying, from initial shell formation to final drying stage. By contrast, coating surfactants on the outer layer of particles being dried could substantially improve wettability. The rapid shell formation and the slow material diffusion owing to the high medium viscosity were considered the main factors limiting the migration of surfactant molecules towards droplet surface during drying.

Preparation and characteristic of gelatine/oxidized corn starch and gelatin/corn starch blend microspheres

Combinations of gelatin (G) and oxidized corn starch (OCS) were explored as a new microcapsule composite for single droplet spray drying. The blending solutions property, gel time, transparency and viscosity of G/CS (corn starch) and G/OCS blend solutions were compared at different ratios (10:0;9:1;8:2;7:3;6:4;5:5) and concentrations(1%wt; 3%wt; 5%wt). The drying and dissolution behaviors of composite droplet have been studied using the single droplet drying technique. Possible reaction mechanisms in the composite blend were elucidated by SEM and FTIR techniques. Blends solutions of G/OCS showed longer Gel time, higher transparency and lower viscosity; further displayed faster dissolution rate than that of G/CS under similar conditions. This was attributed to the formed Schiff base between the aldehyde group of OCS and amino group of G which improved the compatibility between G and OCS. All results indicated that the composites could be prepared with excellent properties by G/OCS (6:4) which would overcome some disadvantage such as thermodynamic incompatibility and phase separation by G/CS.

Surface-coating synthesis of nitrogen-doped inverse opal carbon materials with ultrathin micro/mesoporous graphene-like walls for oxygen reduction and supercapacitors

Hierarchical carbon materials are highly attractive for energy storage and conversion. To improve their performances in electrocatalytic oxygen reduction and supercapacitors, the construction of 3D ordered macroporous frameworks built from ultrathin micro/mesoporous nitrogen-doped graphene-like walls is proposed to enrich accessible active sites and shorten the mass and electron transport paths. A surface-coating hard-templating method is developed for this purpose. The synthesis involves packing silica nanospheres into an opal, designed surface coating with histidine, carbonizing and template removal. The strong adhesive binding between silica and histidine and cohesive interactions among histidine molecules direct the uniform coating and subsequent formation of ultrathin (∼2.8 nm) carbon layers. Various N-doped inverse opal carbons (N-IOCs) with ordered macropores, graphene-like walls, high surface areas up to ∼1365 m2 g−1, narrow micro/mesopores of 0.6–5.0 nm, and high N content up to ∼14.72 wt% are obtained. The optimized N-IOCs exhibit outstanding performances in the oxygen reduction reaction with extraordinary activities, up to 0.95 and 0.87 V for the onset and half-wave potentials, which are among the best for heteroatom-doped carbon materials reported thus far, and excellent methanol tolerance and stability, as well as showing high capacitances of up to 222 F g−1 and good rate capability and stability in supercapacitors. Control experiments reveal that the remarkable performance is ascribed to the ordered macropores for rapid mass transportation, the ultrathin carbon layers for fast electron transfer, and the high surface areas for hosting abundant active sites.

As(V) and Sb(V) co-adsorption onto ferrihydrite: synergistic effect of Sb(V) on As(V) under competitive conditions

Competitive adsorption of As(V) and Sb(V) at environmentally relevant concentrations onto ferrihydrite was investigated. Batch experiments and XPS analyses confirmed that in a binary system, the presence of Sb(V) exhibited a slight synergistic effect on As(V) adsorption. XPS analyses showed that As(V) and Sb(V) adsorption led to obvious diminishment of Fe–O–Fe and Fe–O–H bonds respectively. At pH of 9, a more significant decrease of Fe–O–Fe was observed in the binary system than that in a single system, indicating that As(V) displayed an even stronger interaction with lattice oxygen atoms under competitive conditions. Basically, ionic strength demonstrated a negligible or positive influence on As(V) and Sb(V) adsorption in binary system. Study of adsorption sequence also indicated that the presence of Sb(V) showed a promotion effect on As(V) adsorption at neutral pHs. Considering that co-contamination of As and Sb in waters has been of great concern throughout the world, our findings contributed to a better understanding of their distribution, mobility, and fate in environment.

Understanding the alkali cold gelation of whey proteins with NaCl and SDS

The alkali cold gelation of whey proteins is studied due to its fascinating rheological gelation profiles, as well as to investigate the existence of an alkali dissolution threshold for protein hydrogels with the protein concentration. Alkali cold gelation is achieved by first producing soluble protein aggregates followed by a sudden increase of the pH. At pH > 11.6 there is a de-gelation step with time following an initial quick gelation step. This dynamic transition involves only the formation of non-covalent interactions between the initial covalently crosslinked aggregates; first they are formed but later on they are destroyed. This mechanistic hypothesis is verified here by adding NaCl and SDS in addition to alkali. A sharp transition of the system modulus can be achieved due non-covalent interactions on soluble disulfide crosslinked aggregates in a narrow protein concentration range.

Quantification of the Local Protein Content in Hydrogels Undergoing Swelling and Dissolution at Alkaline pH Using Fluorescence Microscopy

Wide-field fluorescence microscopy was used to quantify the evolution of the volumetric swelling ratio, Q, i.e., solids content, in a protein hydrogel undergoing swelling and dissolution. Heat-induced whey protein hydrogels labeled with Rhodamine B isothiocyanate (RITC) were used as a model system. Complications in the quantification of Q using fluorescence of proteins conjugated with RITC, arising from alkali destroying protein-dye interactions, were overcome using a reaction-diffusion numerical scheme. At pH 12–13, when the hydrogels dissolve readily, overlapping fluorescence intensity profiles were observed at different times, consistent with a system dissolving at a steady state. In stronger alkali (e.g., 1 M NaOH), when dissolution proceeds very slowly, we confirm that there is little swelling next to the gel boundary. These results present the first quantification of the solids distribution within protein hydrogels under reactive conditions.

Chemical imaging of protein hydrogels undergoing alkaline dissolution by CARS microscopy

Hydrogels swell, shrink and degrade depending on the solution they are in contact which, strongly affecting their performance. The minimum information needed to validate many published simulations would be the spatial quantification of the solute material with time. In this study we develop a simple methodology to quantify the protein content in heat induced protein hydrogels using a commercial Coherent anti-Stokes Raman Spectroscopy (CARS) microscope. The system is used to quantify the whey protein isolate (WPI) concentration in hydrogels undergoing dissolution at alkaline pH. Quantitative measurements were performed in hydrogels up to depths of ∼600u202fµm, with an average accuracy of ∼1u202fwt%. Results show that the protein concentration within the swollen layer is constant with time, confirming the existence of steady state conditions during dissolution. The methodology presented can easily be implemented to other biopolymer hydrogels and foods.