The effect of agitation on the nucleation of α-lactose monohydrate

Nucleation in an industrial crystallisation process determines how many crystals are formed and defines the final particle size distribution. This parameter plays a critical role in determining the success of an industrial lactose crystallisation, impacting yield, throughput and product quality. Previous studies on lactose crystallisation have reported that mixing can influence the nucleation kinetics of α-lactose monohydrate. This work looked to authenticate this by measuring the induction time required for nucleation of supersaturated lactose solutions across a range of agitation rates. Increasing agitation increased the rate of nucleation at a given supersaturation. The results show that this is a result of an increase in the frequency of activated molecular collisions and not a change in the critical nucleus size, which remains constant at a defined supersaturation.     

氯化钠晶体粒度对二次成核的影响

实验主要研究在悬浮状态下搅拌速率和母晶平均粒度对氯化钠成核速率特征、规律及新晶核粒度分布的影响.研究结果表明:在实验条件下,90%左右晶体仍保持原来粒度.在母晶平均粒度一定时,成核速率随搅拌速率的增大而增大,且母晶平均粒度较大时这种趋势更加明显.在搅拌速率一定时,成核速率随母晶平均粒度的增大而增大,且在母晶平均粒度较大时这种趋势更加明显.在母晶平均粒度一定时,搅拌速率增大,晶核的粒径分布中最大质量分率所对应的粒径呈减小趋势.在母晶平均粒度一定时,搅拌速率增大,破碎晶核的中值粒径逐渐减小.     

Effect of galacto-oligosaccharide concentration on the kinetics of lactose crystallisation

Galacto-oligosaccharides (GOS) are said to have a retarding effect on the nucleation and growth of α-lactose monohydrate crystallisation. Studies regarding the kinetic parameters for crystal growth and nucleation were conducted utilising gravimetric analysis and laser light scattering to follow the lactose crystallisation. A statistically significant retarding effect of GOS on lactose crystal growth at concentrations between 0.8 and 3% (w/w) GOS of total solution was observed. A significant nucleation suppressing effect was confirmed further. Studies concerning the change in crystal morphology revealed that as GOS concentration was increased, a transition from the common tomahawk shape to a shape with shortened crystal faces away from the apex to a needle like appearance occurred, strongly suggesting a growth retardation effect on the (010) face. Crystals subjected to high pressure liquid chromatography (HPLC) analysis did confirm an incorporation or inclusion of GOS into the crystal lattice.     

Evaluating the crystallization of lactose at different cooling rates from milk and whey permeates in terms of crystal yield and purity

The cooling rate of supersaturated lactose solution is one of the important parameters determining the yield and size distribution of lactose crystals. The influence of increasing cooling rate on lactose crystallization and quality of lactose crystals was evaluated in concentrated solutions prepared from deproteinized whey powder (DPW) and milk permeate powder (MPP). Concentrated permeates (DPW and MPP) with 60% (wt/wt) total solids were prepared by reconstituting permeate powders in water at 80°C for 2 h for lactose dissolution. Three cooling rates, 0.04°C/min (slow), 0.06°C/min (medium), and 0.08°C/min (fast) were studied in duplicate. A common rapid cooling step (80 to 60°C at 0.5°C/min) followed by slow, medium, and fast cooling rates were applied as per the experimental design from 60 to 20°C. After crystallization, the crystal slurry was centrifuged, washed with cold water, and dried. The dried lactose crystals were weighed to calculate the lactose yield. Final mean particle chord lengths were measured at the end of crystallization using focused beam reflectance measurement for slow, medium, and fast cooling rates, and observed to be not significantly different for DPW (27–33 µm) and MPP (31–34 µm) concentrates. Similarly, the lactose yield for slow, medium, and fast cooling rates in the DPW and MPP concentrates were in the range of 71 to 73% and 76 to 81%, respectively, and no significant difference between the 3 cooling rates was found. Qualitative analysis of dried lactose crystals exhibited no noticeable differences in the crystal purity with increasing cooling rate. This study evaluated the possibility of reducing the crystallization times by 8 h compared with current industrial practice without compromising the crystal yield and quality.     

Influence of temperature on crystallization of lactose in ice-cream

Sandiness in ice-cream due to lactose crystallization can still be a problem in many circumstances. Lactose crystallization occurs in ice-cream as the unfrozen phase becomes supersaturated. However, the effects of storage temperature and temperature fluctuations on lactose crystallization have not been very well quantified. In this work, an accelerated storage apparatus was used to determine the effects of thermal fluctuations (from ±0.01°C to ±2.0°C), at several mean storage temperatures (from −5.0 to −20.0°C), on the onset of lactose nucleation and subsequent crystal growth in a standard vanilla ice-cream. the induction time for nucleation initially decreased as temperature was lowered (for temperature oscillations of ±1.0°C), until a minimum induction time of 3 h was found between −10.0 and −12.0°C. Further decreases in storage temperature caused the induction time to increase. the induction time for nucleation increased as the extent of temperature fluctuations increased, from 0.01 to 2.0°C, while initial lactose crystal growth rate showed the opposite trend. the initial growth rate increased as temperature decreased between −5.0 and −10.0°C, but then decreased for temperatures below −10.0°C. At −20.0°C lactose crystals grew very slowly. At −10.0°C the rate of growth decreased with increasing amplitude of temperature oscillations.     

Laminar flow continuous settling crystalliser. Part 1. Initial exploration

Conventional lactose crystallisations give large spans that are not suitable for direct use in inhaler grade lactose. The factors causing large spans are successive nucleation events and growth rate dispersion. This paper (Part 1 of 2) explores a novel lactose crystallisation technique, laminar flow continuous settling crystallisation, proposed as a method for directly producing a narrow particle size distribution with a span of less than 1. A theoretical model was developed that modelled the growth and settling of individual crystals with growth rate dispersion within a column full of lactose solution flowing upwards in laminar flow. The model predicted a d₅₀ of 73.2 ± 0.9 μm and a span of 0.47 ± 0.01. In an experimental crystalliser crystals obtained had a d₅₀ between 50 and 90 μm but the span was greater than 1. The difference has been attributed to agglomeration and flow variations from true laminar flow, which is reported in Part 2.     

Influence of temperature on accelerated lactose crystallization in dulce de leche

The aims of the present work were to study the crystallization of lactose in dulce de leche and to evaluate the influence of temperature on this process. Samples of approximately 1 g were placed in plastic containers and stored at –5, 5, 20 and 35°C for 25 days. Lactose crystal and lactose crystal agglomerate numbers and size were measured by using an optical microscope. Results suggested that stirring and placing a small sample of dulce de leche in a small container induced lactose crystallization. Lower storage temperatures resulted in an increase of crystal and agglomerate number but in a decrease of crystal and agglomerate size. The effect of temperature on crystal and agglomerate growth rates could be attributed to the effect of temperature on viscosity, as lower temperatures resulted in decreased crystal mobility in dulce de leche and therefore reduced crystal and agglomerate growth rate.     

APPLICABILITY OF ALGINATE ENTRAPPED YEAST CELLS FOR THE PRODUCTION OF LACTOSE-HYDROLYZED MILK

To overcome the problem of enzyme extraction and poor permeability of cell membrane to lactose, permeabilized Kluyveromyces marxianus NCIM 3465 cells as a source of β‐D‐galactosidase were employed for the production of lactose‐hydrolyzed milk. In view of the advantages of an immobilized cell system over a free cell system, the yeast cells were entrapped in alginate gel for their subsequent use in lactose hydrolysis. Different process parameters (alginate concentration, bead size, biomass load, temperature, agitation and incubation time) were monitored to enhance lactose hydrolysis in milk. Maximum lactose hydrolysis (87.9%) was observed with yeast cells immobilized in 2% (w/v) alginate concentration with a bead size of 2.90 mm at 30C under agitation (80 rpm) after 150 min of incubation. The developed system was highly stable and the alginate entrapped yeast cells can be recycled up to the eight cycle without any marked change in their ability to carry out the lactose hydrolysis.     

Analytical Techniques for Nucleation Studies in Lipids: Advantages and Disadvantages

ABSTRACT: Crystallization is generally considered a 2-step process. The 1st step, nucleation, involves the formation of molecular aggregates with a critical size great enough to become stable. During the 2nd step, nuclei grow and develop into crystals. Distinguishing between nucleation and growth constitutes a major challenge in lipid crystallization studies. Thus, it is of great importance to discuss the information obtained from the different techniques that are usually used to study nucleation behavior such as nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), rheological techniques, light-scattering techniques such as turbidimetry and scanning diffusive light scattering (SDLS), polarized light microscopy (PLM), and laser polarized optical sets such as laser polarizedlight turbidimetry (LPLT). Techniques to describe the nucleation process must be very sensitive to disregard growth. When crystallization is followed by methods such as DSC, NMR, and rheological measurements, at times, small amounts of crystals are present in the melt before any solids are detected. Clearly, at this stage, well beyond the induction time for nucleation (τ), these methods are measuring crystal growth. Techniques of low sensitivity for solid fat contents lower than 0.1% must not be used to evaluate nucleation effects. Sensitive turbidimeters with detectors that saturate below 0.3% solid fat content give good results as do scanning diffusive light-scattering equipment. Although the PLM technique is sensitive enough for these kinds of studies, an understanding of important basic concepts is essential. Laser optical sets are the most appropriated methods to study nucleation in fats systems.     

Isothermal batch crystallization of alpha-lactose: A kinetic model combining mutarotation,nucleation and growth steps

A kinetic model combining first-order differential equations of the three consecutive steps of lactose crystallization, i.e., mutarotation, nucleation and crystal growth rate, was developed. Numerical solutions successfully fitted the variations of crystal mass growth rate as a function of lactose concentration during unseeded isothermal batch crystallization, at different initial lactose concentrations and temperatures. The model allowed the induction phase and the influence of seeding to be simulated by taking into account the dependency of crystal growth rate on total crystal surface area and, consequently, on nucleation rate. The proposed model highlights the large influence of the mutarotation step, even in unseeded crystallization kinetics. The effects of high lactose supersaturations that prevail at industrial scale during whey powder manufacturing and the effects of alkaline pH (9.5) on lactose crystallization kinetic were successfully predicted.     

Application of in-line monitoring for aiding interpretation and control of dextrose monohydrate crystallization

Two in-line monitoring probes were inserted into a 3.2 L batch cooling crystallizer to monitor the progression of a dextrose monohydrate crystallization and how it was influenced by process variables including impurities, seed size and initial dextrose concentration. The probes used were a K-patents refractometer and a Lasentec FBRM probe. These probes were applied to continuously monitor dextrose concentration in solution and crystal chord length distribution throughout the duration of the crystallization. The influence of process variables on crystallizer performance, as determined by final crystal yield and chord size, may not be intuitive and some unexpected results occurred. The continuous in-line measurement data proved very useful for interpreting how variations in the process variables influenced the crystallizer performance. Supersaturation is a key variable in the control of the crystallization process as it influences both nucleation and crystal growth. In-line monitoring of solution dextrose concentration can be applied to continuously evaluate supersaturation and this can then be applied to control supersaturation so as to achieve a desired crystallizer performance.     

味精的起晶新方法

本文简要介绍了用超声波刺激味精溶液快速超晶制种的新方法，与传统的制税法相比，这种新方法具有制种快、制得晶种数目稳定且粒子均匀、表面较完整等特点，投种量可从原来的最高20％（对产品比）降低至千分之一左右，且在实验室条件下进行的有晶试验表明，用新法制得的晶核投种后，结晶生长过程平稳，伪晶较少，整晶次数亦减少，品形良好，值得在实际生产中推广应用．     

A comparative study of microbubble generation by mechanical agitation and sonication

As focus on the potential applications of microbubbles increases, information about the efficiency of generation methods and their effects on the properties and stability of microbubbles is crucial in the selection of an appropriate method to generate microbubbles with the desired properties. This paper evaluates the generation efficiencies of two commonly used methods, mechanical agitation and sonication, in two surfactant systems. The results demonstrated that sonication was more effective than mechanical agitation in the generation of microbubbles in terms of higher gas hold-up, smaller bubble size, and larger interfacial area. Analysis of the changes in bubble size over time revealed that the existence of a critical diameter for the shrinkage of microbubbles. The behavior of microbubbles and the critical diameter depended on the generation method employed and the surfactant used.

Industrial relevance

Microbubble technology has gradually become accepted as a cost-effective and environmentally friendly technology with great potential within almost every field of the food industry. Selection of a suitable method to generate microbubbles with the desired properties is crucial. Mechanical agitation and sonication are two commonly used methods for microbubble generation. However, systematic information on their generation efficiency and effects on the properties of microbubbles is not available. Thus, a comparative study was conducted in this paper.     

Secondary nucleation studies on alpha lactose monohydrate under stirred conditions

Secondary nucleation is the dominant source of nuclei at the industrial scale. Traditionally, secondary nucleation has been expressed empirically as a function of stirrer speed and magma density with little or no understanding about the role of individual mechanisms (attrition and contact). This study considers the various mechanisms of secondary nucleation of alpha-lactose monohydrate crystals and link them to either crystal–crystal or crystal-impeller collisions. An agitated system was used to conduct trials at various levels of stirring speed (400 and 550 rpm), seed size (150, 250, 357 and 502 μm) and seed loading (2%, 5% and 10%, v/v) at a constant supersaturation. Crystal-impeller contacts are proposed to be the major source of secondary nucleation. It was found that the kinetic energy and not the collision frequency govern the rate of secondary nucleation with a threshold kinetic energy value below which no secondary nucleation occurs.     

Crystallization Kinetics of Alpha-Lactose Monohydrate in a Continuous Cooling Crystallizer

The crystallization of alpha-lactose monohydrate in a continuous cooling crystallizer was investigated at various temperatures and super-saturations. The population balance model was used to analyze the product crystal size distribution as determined by the Coulter Multi-Sizer for each condition studied. Nucleation and growth values were thereby determined for each temperature and supersaturation. Kinetic models were then developed for both nucleation and growth to demonstrate the effects of the operating parameters. The results showed that the nucleation process had a temperature dependence with an activation energy of 17.0 kcal/mole and that nucleation increased with the 1.9 power of supersaturation and the 1.0 power of the suspension density (mass of crystals). The growth process was found to have a temperature dependence with an activation energy of 22.1 kcal/mole and to be a second order function of the supersaturation.     

PARAMETERS THAT DETERMINE TRIPALMITIN CRYSTALLIZATION IN SESAME OIL

The crystallization of tripalmitin (TP) in sesame oil (0.98, 1.80 and 2.62% w/v) was investigated under isothermal conditions using a cooling rate similar to the one achieved by industrial crystallizers (1 K/min) and utilizing a modification of the Avrami equation. It was concluded that z, the crystallization rate constant, and the associated energy of activation (Ea) values determined through Avrami's original equation yield erroneous results. Additional results indicated that the increase in supercooling at constant oil phase viscosity (e.g., 5.25–5.5 dynes/cm²) during crystallization, produced a higher degree of nucleation without an appreciable effect on TP crystal size. In contrast, as viscosity of the oil phase decreased at equal supercooling conditions (e.g., 22.0 - 22.5C) the main crystallization process evaluated through z and Ea was crystal growth. It was also concluded that the characteristic crystallization stages of induction time for nucleation, crystal nucleation and growth, and end of crystallization were evident in the viscosity profile of the solid fraction developed during TP crystallization. The predominance of any of these processes (i.e., nucleation or crystal growth) as a function of supercooling and viscosity of the oil phase determines, to a great extent, the rheological profile during crystallization and the final viscosity value of the crystal suspension.     

Effects of temperature and concentration on particle size in a lactose solution using dynamic light scattering analysis

Nucleation, as well as the processes affecting it, is a constant source of interest for scientists dealing with crystallisation. The majority of published material focuses on secondary nucleation because of the complexity of the crystallisation process. The influence of temperature and different levels of concentration, from highly supersaturated solutions down to lactose solutions below the supersaturated level, on lactose primary cluster was investigated using the dynamic light scattering (DLS) method. The minimum particle size in the solution was found; it varied from 0.89 to 1.17 nm and matched the size of a single lactose molecule (1.18 nm), which was estimated by using molecular modelling techniques. Using the DLS technique, it was confirmed that particle size increased with the decrease of temperature and increasing of concentration of the solution. Theoretical calculations were made to estimate a predicted number of molecules that form particles of primary clusters with different sizes.     

Induction Time of Crystallization in Vegetable Oils, Comparative Measurements by Differential Scanning Calorimetry and Diffusive Light Scattering

ABSTRACT: In this work we compared the induction time of crystallization (τ_i. measured by DSC (τ_iDSC) and scanning diffusive light-scattering (SDLS, τ_iSDLS) in a vegetable oil blend. Several crystallization temperatures and cooling rates (1,10, and 30 °C/min) were investigated. Additionally, rheograms obtained during the crystallization of the oil blends were obtained to differentiate the stages of nucleation and crystal growth. The results obtained showed that independent of cooling rate and crystallization temperature, SDLS provided an earlier detection of triacylglycerides'(TAGS) nucleation than DSC. Furthermore, τ_iSDLS was more consistent (that is, lower standard deviation) than τ_iDSC particularly at high crystallization temperatures and cooling rates. The rheograms showed that τ_iDSC involves a measurement of crystal growth, while τ_iSDLS is associated with the early formation of a solid phase in the oil blend (that is, TAGS nucleation). When the Fisher-Turnbull equation was therefore used to calculate the free energy of nucleation (ΔG_c), τ_iSDLS provided a better estimate of ΔG_c than that obtained by τ_iDSC     

乙醇-水溶剂中阿斯巴甜溶解度及介稳区的研究

在5～60 ℃温度范围内,研究阿斯巴甜在不同比例的乙醇-水混合溶剂中的溶解与超溶解特性,得到了阿斯巴甜的结晶介稳区,并推算出表观成核级数m,给出了成核速率方程。采用λh方程关联6 个体系的溶解度数据,并考察温度、溶剂组成、降温速率以及搅拌速率对结晶介稳区的影响。结果表明：阿斯巴甜的溶解度随着温度和乙醇质量分数的增加而增加,当质量比w=0.5时出现溶解度的协同效应;随着温度和乙醇质量分数的增加,阿斯巴甜结晶介稳区宽度减小;在相同条件下,搅拌速率的降低和降温速率的升高均会增大阿斯巴甜结晶介稳区的宽度。     

悬浮状态下氯化钠晶体二次成核过程研究

实验利用60 L流化结晶装置,研究了悬浮状态下晶种粒度、流体循环速率以及循环时间对碰撞成核速率特征、规律及晶核粒度分布的影响.发现二次成核过程中,破碎机理占主导作用,碰撞后的晶体碎片具有连续分布;母晶粒度和循环速率不仅影响颗粒的碰撞能,而且对颗粒的悬浮状态及分布状况有明显作用;成核速率随晶种粒度及流体速度的增大而增加,循环时间对粒度较大晶体颗粒的二次成核有明显作用.