Influence of the Emulsifier System on Breakup and Coalescence of Oil Droplets during Atomization of Oil-In-Water Emulsions

Abstract

Spray drying of whey protein-based emulsions is a common task in food engineering. Lipophilic, low molecular weight emulsifiers including lecithin, citrem, and mono- and diglycerides, are commonly added to the formulations, as they are expected to improve the processing and shelf life stability of the products. During the atomization step of spray drying, the emulsions are subjected to high stresses, which can lead to breakup and subsequent coalescence of the oil droplets. The extent of these phenomena is expected to be greatly influenced by the emulsifiers in the system. The focus of this study was therefore set on the changes in the oil droplet size of whey protein-based emulsions during atomization, as affected by the addition of low molecular weight emulsifiers. Atomization experiments were performed with emulsions stabilized either with whey protein isolate (WPI), or with combinations of WPI and lecithin, WPI and citrem, and WPI and mono- and diglycerides. The addition of lecithin promoted oil droplet breakup during atomization and improved droplet stabilization against coalescence. The addition of citrem and of mono- and diglycerides did not affect oil droplet breakup, but greatly promoted coalescence of the oil droplets. In order to elucidate the underlying mechanisms, measurements of interfacial tensions and coalescence times in single droplets experiments were performed and correlated to the atomization experiments. The results on oil droplet breakup were in good accordance with the observed differences in the interfacial tension measurements. The results on oil droplet coalescence correlated only to a limited extent with the results of coalescence times of single droplet experiments.

Influence of Hydrophilic Surfactants on the W1–W2 Coalescence in Double Emulsion Systems Investigated by Single Droplet Experiments

“…Previous studies suggested single-droplet experiments as an additional tool to evaluate the stability of formulations with more than one surfactant against coalescence [17][18][19][20][21]. The general idea is the simplification of a double emulsion to a measurement setup where it can be distinguished between different instability mechanisms.…”

“…When the coalescences times are too short (<1 s) the determination of the moment of contact is not accurate enough to achieve a reasonable measurement. When the coalescence times are too long (>30 min) the necessary repetitions of the measurement become very time consuming, or there is no coalescence observed at all within a defined maximum measurement time [19,22,23]. The coalescence time increases on a logarithmic rather than on a linear scale when increasing the concentration or changing the surfactant.…”

Spray drying of emulsions: Influence of the emulsifier system on changes in oil droplet size during the drying step

“…Thus, the oil droplets after atomization are evidently larger compared to the oil droplets after atomization with the other emulsifier systems. In our previous study, we demonstrated that these large oil droplets are the result of droplet coalescence directly after oil droplet breakup during atomization (Taboada et al, 2020). When considering the ODSD of the emulsion after spray drying it can be seen that the proportion of small droplets is reduced compared to the ODSD after atomization, with the smallest oil droplets being around 0.4 µm.…”

“…Oil-in-water emulsions were prepared for the investigations. Emulsions were prepared following the procedure described in (Taboada et al, 2020). Briefly, emulsion premixes (50 wt.% oil) consisting of an aqueous WPI solution and MCT oil with LMWE (lecithin or citrem or MoDi) were prepared and homogenized in a colloid mill (IKA magic LAB, IKA-Werke GmbH & Co. KG, Staufen, Germany) operated at a gap width of 0.16 mm and a circumferential speed of 26 m/s.…”

“…No study has been found in which the phenomena occurring during each step of the spray drying process, namely the atomization of the liquid followed by the drying of the spray droplets were studied separately. In our preceding study, the changes in ODSD occurring during the atomization step in dependence of the emulsifier system were investigated (Taboada et al, 2020). We focused on the effects of adding lipid-based LMWE (lecithin, citrem, MoDi) to whey protein stabilized emulsions.…”