Efficient yoghurt powder production: The effect of feed composition and spray drying conditions onstickiness

Research output: Book/ReportPh.D. thesisResearch

Spray drying is considered a suitable method for the commercial production of yoghurt powder dueto its high throughput and fast drying rate. The main challenges encountered during yoghurt powderproduction are related to a high product deposition on the walls of the drying chamber, commonlyreferred to as stickiness. Such deposition leads to a lower yield, increases cleaning frequency andmay compromise the quality of the final product. The presence of lactic acid in yoghurt has beenconsidered a primary cause of high product deposition. This PhD project is focused on efficientyoghurt powder production, where a small pilot plant experimental platform was used to provide adeeper understanding of stickiness and resulting wall deposition occurring during spray drying.The feed characteristics, primarily feed composition, and spray drying conditions (temperature,atomization), will influence the degree of wall deposition during spray drying. Hence, their effectswere investigated by manipulating the inlet and outlet air temperature and nozzle air flow rate andusing acidified skim milk solutions with varying lactic acid concentrations (0.5 -3 % w/w) as feed.The results showed that higher temperature difference between inlet and outlet air temperatureresulted in an increased feed flow rate. Consequently, this significantly affected process yield,moisture content and water activity of the powder, increasing all three responses. Glass transitiontemperature was only affected by the outlet air temperature. Lactic acid concentration significantlyinfluenced the moisture content and water activity of the powders as well as wall deposition duringspray drying. On the other hand, it showed no significant effect on glass transition temperature andyield within the tested design range, possibly due to the combining effect of high lactose contentand the presence of the proteins.Furthermore, the effect of feed composition on wall deposition during spray drying was alsoinvestigated in terms of varying lactose and lactic acid concentration (0-2 % w/w) in skyr basedfeed. The study showed that increasing the lactic acid concentration significantly reduced the glasstransition temperature of the powders, while an increased level of lactose had the opposite effect.Likewise, an increased level of lactic acid in the feed caused a significant increase in walldeposition. Interestingly, the addition of both lactose and lactic acid in feed resulted in a similarglass transition temperature compared to powder with no added ingredients, whereas substantiallydifferent wall deposition was observed during spray drying. These results indicate that stickinessand resulting wall deposition can not only be observed in terms of the glass transition temperaturebut also powder surface composition needs to be considered. Low-field nuclear magnetic resonance(LF-NMR) analysis of water mobility demonstrated that the presence of lactic acid makes powdersmore susceptible to sticking, hence causing an inter-particle binding. Consequently, this led to ashifting of water protons from a more mobile fraction to a tightly bound fraction, where the onsettemperature increased with the addition of lactose.Furthermore, stickiness, being a surface property, will depend more on the surface composition ofpowder particles. Therefore, particle surface composition was observed by Raman microscopy andX-ray photoelectron spectroscopy (XPS) which showed that the surface of the powders was mainlycovered by proteins. The addition of lactose and lactic acid in the feed resulted in an increasedproportion of both components on the particle surface, thus making particles more prone to stickingduring spray drying and resulting in higher wall deposition.Likewise, the potential of utilizing vibrational spectroscopic technique(s) to analyse the molecularstructure and composition of acidified milk powders was evaluated. The results showed that thesetechniques provided qualitative measurements that can be used in process controls. Raman imagingrevealed the distribution of components on the surface of individual powder particles that may beused in understanding the product deposition during spray drying. Near-infrared spectroscopy andattenuated total reflection (ATR), which can potentially be implemented in online and atline processcontrol, respectively, provided information on the composition and molecular changes occurringduring the prolonged exposure of powder adhered to the walls of the drying chamber to the highdrying temperature.
Original languageEnglish
PublisherDepartment of Food Science, Faculty of Science, University of Copenhagen
Number of pages123
Publication statusPublished - 2023

ID: 347808205