Protective carriers that encapsulate probiotics in spray drying could improve the survival ratio of dried cells through different mechanisms. Unveiling the protective mechanism of each carrier will ...contribute to a rational design of high performance carrier formulation. This study utilized single droplet drying (SDD) technique to investigate the effects of calcium cation in varied carrier formulation. Inactivation histories of Lactobacillus rhamnosus GG (LGG) in different carriers were compared, and cellular injury history of probiotics during droplet drying was studied for the first time. Adding 1mM CaCl2 to lactose carrier protected cell viability, mitigated cellular injuries, and enhanced regrowth capability as drying progressed, demonstrating the positive effect of Ca2+ with possible mechanism of stabilizing sub-cellular structures. At later drying stages, cell survival in Lac/Ca carrier was increased by 0.5–1.5 log on selective media compared to lactose carrier. Supplementing calcium-binding agents lowered the protective effect, shortening the initiation of rapid cell inactivation down to 120s of drying. Adding CaCl2 to trehalose carrier barely improved cell survival, indicating that the protective effect could be influenced by carrier formulation. Pure trehalose carrier exerted excellent protection on LGG, supporting cells to regrow in liquid rich medium even after 180s of drying. The protection of trehalose may stem from stabilization of sub-cellular structures, which possibly overlap the effect of Ca2+. The findings suggested that high performance carrier formulation might be developed by combining carrier materials with different protective mechanisms, for maximizing the survival of active dry probiotics in industrial spray drying operation.
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•The injury history of a probiotics strain was studied as droplet drying progressed.•Adding Ca2+ protected cell viability and cellular structures in lactose carrier.•The protection was lost upon addition of a chelating agent, e.g. dipicolinic acid.•Trehalose well stabilized cellular structures regardless of the addition of Ca2+.•Carrier formulation is crucial to maximizing protection for drying of probiotics.
Spray-drying is expected to be a cost-efficient way to produce probiotic powders. Indeed, a novel simplified process was recently reported, using concentrated sweet whey (30 wt %) as a sole medium ...for both growth and spray drying of probiotics. The feasibility of scaling up this process was validated in the present work with a semi industrial pilot scale spray dryer. A multi-stage mild-conditions drying process, coupling spray-drying with belt drying and fluid-bed drying, was also applied in this work, in which the final probiotic survival was improved to approximately 100% (>109 CFU g−1). The change of probiotic viability in the powders was monitored during a 6-month storage, which indicated that storage temperature and moisture content of powders play crucial roles in the stability of probiotic powders. Moreover, spray-drying afforded a strain-dependent enhancement of bacterial tolerance in simulated intestinal fluid, in comparison with fresh cultures.
•Concentrated sweet whey was used for both probiotic growth and spray-drying.•Drying was conducted at semi industrial pilot scale (500 L level).•Multi-stage drying coupled spray-drying, belt drying and fluid-bed drying.•High probiotic survival (∼100%) was obtained in multi-stage drying process.•Enhanced storage stability and protection towards digestion was observed.
The concept of Metzner and Otto (1957) was initially developed for correlating power measurements in stirred vessels for shear-thinning fluids in the laminar regime with regard to those obtained for ...Newtonian liquids. To get this overlap, Metzner and Otto postulated and determined an “effective shear rate” which was proportional to the rotational speed of the impeller Although it was not based on a strong theoretical background, it was rapidly admitted as a practical engineering approach and was extended for seeking out a “Newtonian correspondence” with non-Newtonian results (i.e. different classes of fluids). This was applied in a variety of tank processes even for predicting heat transfer or mixing time, which stretches far away from the frame initially envisaged by Metzner and Otto themselves. This paper aimed to show how dimensional analysis offers a theoretically founded framework to address this issue without the experimental determination of effective quantities. This work also aimed to enlarge the underlying questions to any process in which a variable material property exists and impacts the process. For that purpose, the pending questions of Metzner and Otto concept were first reminded (i.e. dependence of the Metzner-Otto constant to rheological parameters, physical meaning of the effective shear rate, etc). Then, the theoretical background underlying the dimensional analysis was described and, applied to the case of variable material properties (including non-Newtonian fluids), by introducing in particular the concept of material similarity. Last, two examples were proposed to demonstrate how the rigorous framework associated with the dimensional analysis is a powerful method to exceed the concept of Metzner & Otto and, can be adapted beyond the Ostwald-de Waele power law model to a wide range of non-Newtonian fluids in various processes, without being restricted to batch reactor and laminar regime.
Drying usually results in various types of particle morphology that influence the properties and the functionality of powders. Understanding how the final shape of the particle is formed is therefore ...a key issue for industrial applications. However, it is difficult to carry out relevant studies and interpret them on an industrial scale, mainly because of the complexity and scale of the drying operation and the difficulty of monitoring drying parameters in situ. To overcome these difficulties, experiments are currently performed with the simplest system conceivable: drying of a single droplet in a controlled environment. Different approaches are available in the literature in order to study the drying of a single droplet, from a sessile droplet to a free-flying droplet. This review aims to illustrate results obtained with these experiments, and their key contributions to fundamental and applied understanding of drying (or evaporation) phenomena are discussed. It is shown that investigation of a single droplet may be a key approach to spray drying in order to improve understanding of the chemical and physical mechanisms involved in particle processing and governing the properties of the final particle.
Infant formulas (IFs) can be defined as substitutes for human milk, which are mostly based on cow milk proteins. For sustainability reasons, alternative to animal proteins in food have to be ...considered. Plant proteins offer interesting nutritional and functional benefits for the development of innovative IFs. However, the behaviour of these proteins during processing and storage must ensure the physical stability and ability to reconstitution of IF powders, and that needs to be tested. This work aimed to study how a partial substitution of dairy proteins by plant proteins may influence the functional properties of 1st age IFs. Three IFs were developed at a semi-industrial scale using two different processing routes. The IFs composition was identical, except that 50% of the proteins were whey proteins in the “reference IF” (RIF), and pea or faba bean proteins in the “plant IFs” (PIF and FIF, respectively). After reconstitution, the three IFs result in similar stable emulsions with equivalent free fat release. In comparison to RIF, PIF and FIF were difficult to disperse, conducting to remaining insoluble particles. Thus, the protein source greatly influences IFs properties, and process parameters need to be adapted for each formulation to meet IFs quality criteria.
•Plant protein dispersibility is the limiting factor all along IFs manufacturing.•Similar emulsion stability and free fat were found for plant and dairy IFs.•Homogenization partly improved dispersion of plant protein aggregates.•Protein source had a great impact on IF manufacturing and functionalities.
In general, milk powders are submitted to agglomeration in order to enhance their rehydration properties. This study evaluated the impact of six different methods of agglomeration of the same sweet ...whole milk powder on the physical and rehydration properties of the final product: non-agglomerated powder (control); agglomerated powder with fines returned to the top of the dryer, above the internal fluid bed (IFB) or above the cone; and agglomerated powder with fines returned above the IFB with a nozzle that injects steam or sprays water in the middle of the IFB, respectively. As expected, the biochemistry results showed no difference since the same concentrate was used in the whole set of experiments. The physical properties led to higher bulk densities, tapped densities and interstitial air for the control, and higher occluded air and particle size for agglomerated powders, regardless of the agglomeration process. All of these physical properties influenced the rehydratability. In fact, the wetting time and the dispersibility of sweet whole milk powder were significantly improved by the agglomeration method, with the return of fines and steam injection in the IFB. An experimental design of the agglomeration process was devised for this agglomeration approach.
•Various agglomeration steps to rehydrate sweet milk powders were studied.•The rehydration properties depended on the configuration of the dryer.•The optimal scheme consisted of steam injection in the internal fluidized bed.
•Soft-elastic baffles (SEBs) intensified mixing in a soft-elastic reactor (SER).•A best design of SER with SEBs was identified.•Mixing in SERs with large length to diameter ratios was intensified by ...SEBs.
Soft-elastic reactor (SER) has been demonstrated as an alternative approach to effective mixing of viscous liquid. Here, the mixing performance of Newtonian fluid—glycerin water solutions (1200±50 mPa⋅s) was experimentally investigated for the first time inside a simple cylindrical SER with soft-elastic baffle (SEB) aligned in parallel to the SER axis. Visualization based on an acid-base reaction was used to reveal the presence and the disappearance of isolated mixing regions. The mixing performance was characterized using the mixing curve and the mixing time. The effect of SEB positioning on mixing intensification was explored. Results show that a single SEB could improve mixing efficiency with the optimal positioning angle of 90° With the increase of the number of the SEBs, the mixing efficiency was found to be improved regardless of positioning angles. Compared with the SER without SEB, adding a single SEB could reduce the mixing time by about 34% at the probe penetration depth of 2.5 cm and the penetration frequency of 1.0 Hz. Efficient mixing for SERs of large length to diameter ratios was also achieved with the presence of SEB.
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•Homogenization of miscible viscous fluids with a soft-elastic reactor (SER).•Establishment of the set of dimensionless numbers governing mixing time of SER.•Comparison of SER mixing ...curve with those of conventional mixers.
A novel mixing device (Soft Elastic Reactor – SER) that induces mixing by vibration of the tank wall is presented and characterized using dimensional analysis. This type of mixing system, which does not have an agitator inside the tank, is interesting for specific mixing applications, although it is still seldom used due to the lack of knowledge about how it works. First, the set of dimensionless numbers governing the mixing time of the SER was established. Then, the shapes of mixing curves and the onset of different mixing regimes as a function of the fluid property (i.e., viscosity) and operating conditions (i.e., penetration depth and vibration frequency) were experimentally obtained. Results showed that a constant number of beatings is required to achieve the desired degree of homogeneity under a laminar regime, while a sharp decrease in the number of strikes can be observed when the laminar regime is over. The shape of the mixing curves for the SER was compared to those of conventional mixers in which agitation is ensured by the revolution around a vertical axis of a centered agitator. Similarities between the mixing curves are discussed: it appears that regardless of the mixer type, the key parameter governing the desired degree of homogeneity is the number of excitation cycles, i.e., either the number of agitator revolutions for classical mixers or the number of strikes for the SER.
Controlled agglomeration during spray drying offers several advantages for both powder manufacturers and consumers, and thus it is commonly implemented by industry. The implementation, however is ...largely based on experience, given the scarcity of comprehensive prediction tools. A resource-efficient approach to numerically treat agglomerates and yet provide an indication of their structures is desired to perform realistic simulations without the need for high-performance computing. In this work, a new numerical model for the treatment of coalescence and agglomeration was implemented and evaluated at two distinct scales with significantly different particle number densities within a Eulerian-Lagrangian CFD framework. The model could accurately predict the trends in the final particle size distributions and distinguish realistic agglomerate structures occurring under different conditions. Challenges were encountered as a result of how the underlying collision detection routine handles high particle number density. Several strategies are proposed to overcome these challenges. This work constitutes significant progress towards achieving an efficient prediction tool to estimate final powder properties and will prove useful in performing large-scale simulations to design and control agglomeration.
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•A rigorous modelling approach was developed for agglomeration in spray drying.•The proposed model was found to be resource efficient and yet useful and informative.•Coalescence and agglomeration of particles were distinguished via penetration depth.•Surface area reduction coined as a parameter indicated agglomerate compactness.•Experimental data obtained from lab- and large-scale dryer supported the assessment.
Probiotic bacteria have been reported to confer benefits on hosts when delivered in an adequate dose. Spray-drying is expected to produce dried and microencapsulated probiotic products due to its low ...production cost and high energy efficiency. The bottleneck in probiotic application addresses the thermal and dehydration-related inactivation of bacteria during process. A protective drying matrix was designed by modifying skim milk with the principle of calcium-induced protein thermal aggregation. The well-defined single-droplet drying technique was used to monitor the droplet-particle conversion and the protective effect of this modified Ca-aggregated milk on Lactobacillus rhamnosus GG. The Ca-aggregated milk exhibited a higher drying efficiency and superior protection on L. rhamnosus GG during thermal convective drying. The mechanism was explained by the aggregation in milk, causing the lower binding of water in the serum phase and, conversely, local concentrated milk aggregates involved in bacteria entrapment in the course of drying. This work may open new avenues for the development of probiotic products with high bacterial viability and calcium enrichment.
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