Reducing stickiness in spray dried dairy emulsions
Graphical abstract
Introduction
Spray dried emulsions form the basis of powdered milk replacers tailored to meet the nutritional needs of children. The powders comprise milk proteins, lactose and a lipid component that is typically of vegetable origin such as palm, rapeseed, sunflower or coconut oil. Recently, demand for low protein powdered milk replacers has increased, firstly due to lower cost thereby increasing the accessibility of good nutrition to a broader cohort of children. Secondly, recent research on the early protein hypothesis suggests that in children under 5 years of age, protein intake amounting to greater than 15% of total energy intake was associated with greater weight (Axelsson, 2006; Jen et al., 2018; Pimpin, Jebb, Johnson, Wardle, & Ambrosini, 2015; Techakittiroj, Cunningham, Hooper, Andersson, & Thoene, 2005). As a result there is an interest in developing powdered milk replacers with a range of protein levels enabling parents to select a powder that best suits the needs of the child.
The manufacture of low protein spray dried emulsions presents a number of challenges including stabilisation of the initial emulsion and subsequently stickiness and caking in the spray dried powder. Stabilising the initial emulsion is essential as non-emulsified fat in the liquid emulsion will lead to free fat in the spray dried powder leading to a powder with poor hydration properties and subsequent caking (Tham, Xu, Yeoh, & Zhou, 2017). A recent study on whey protein isolate has shown the whey proteins can stabilise a 10 %w/w oil emulsion with 0.5% w/w protein, however, below this the emulsion destabilises (Schröder, Berton-Carabin, Venema, & Cornacchia, 2017). In the spray dried emulsion stickiness is a multifactor phenomenon influenced by (i) protein level (ii) lactose state (iii) free fat and (iv) powder surface composition (Jayasundera, Adhikari, Aldred, & Ghandi, 2009; Fitzpatrick et al., 2010; Hogan & O'Callaghan, 2010; Huppertz & Gazi, 2016; Munoz-Ibanez et al., 2016; Burgain et al., 2017; Chever et al., 2017; Nuzzo, Sloth Overgaard, Bergenståhl, & Millqvist-Fureby, 2017).
The glass transition point of a molecule is related to its molecular weight (Levine & Slade, 1986). Proteins, as large molecules have higher Tg values than lactose, as a result their presence decreases a powders susceptibility to stickiness. Furthermore, larger molecules such as proteins are less susceptible to the plasticizing effects of moisture and heat (Bhandari & Howes, 1999; Hogan, Famelart, O’Callaghan, & Schuck, 2010; Kelly, O’Mahony, Kelly, & O’Callaghan, 2016). Studies in model infant formulas have observed that lactose predominantly influences glass transition point rather than protein type (Kelly et al., 2016). However, studies have shown protein level to significantly impact powder glass transition point, under anhydrous conditions the Tg of powders containing whey protein isolate and lactose decreased as the protein to lactose ratio decreased (Maidannyk & Roos, 2017). Impurities in lactose have been shown to significantly increase moisture sorption and susceptibility to caking (Carpin et al., 2017). In the production of low protein powdered milk replacers manufacturers often partially substitute the protein with lactose leading to a powder with approximately half of its dry weight comprising lactose. As lactose is present in its amorphous state, the powder is susceptible to stickiness if the lactose transitions from a glassy amorphous state to a rubbery state, which will occur if the temperature of the powder exceeds the powders glass transition temperature. Studies on lactose in skim milk powder (SMP) have shown hydrolysis of the lactose leads to increased stickiness within the spray drying chamber and a lower Tg value (Shrestha, Howes, Adhikari, & Bhandari, 2007). The opposite effect on Tg is observed when the lactose is replaced with higher molecular weight maltodextrin. The Tg of SMP/maltodextrin mixtures increased with increasing maltodextrin content, however, the greater the degree of maltodextrin hydrolysis the smaller the increase in powder Tg (Silalai & Roos, 2011).
The lipid component of a spray dried emulsion also plays a critical role in a powder becoming sticky. Surface fat during manufacture can cause adhesion to processing equipment while elevated temperatures during transport can cause fat to melt and migrate within a powder causing uncontrolled powder agglomeration. In spray dried emulsions the surface composition of the powder plays a critical role in the powders flowability and solubility. Spray dried emulsions do not possess a homogeneous distribution of the protein, fat and lactose within the powder particles. Powders often contain an over representation of fat at the surface and display a trend of decreasing fat content and increasing protein content moving from the surface to the core (Murrieta-Pazos, Gaiani, Galet, & Scher, 2012). Fat content has a significant impact on surface fat content, in powders containing 5% fat over 45% of the surface can be covered in fat, while powders containing 20% fat have over 90% surface coverage by fat in the final powder (Kim, Chen, & Pearce, 2002). Investigations into the cause of this nonhomogeneous distribution of the fat have shown increasing drying temperatures leads to increased surface fat content while mechanical disruption of powder particles in the cyclones of a multistage spray dryer can increase the free fat content (Donz, Boiron, & Courthaudon, 2014; Vignolles et al., 2010). Recent research showed that atomisation appears to disrupt the oil water interface leading to the formation of non-emulsified fat which manifests itself as surface fat in the final dried powder particle, causing uncontrolled powder agglomeration (Foerster, Gengenbach, Woo, & Selomulya, 2016).
While the physical process of manufacturing a spray dried emulsion impacts the surface composition the chemical properties of the droplet also influence surface composition. Single droplet drying studies have shown fat starts to accumulate at the surface of a droplet during the initial stages of drying and continues to build up until the middle stage of drying is complete, showing migration of the lipid phase occurs within the droplet during drying (Fu, Woo, & Chen, 2011). Studies in whole milk powders have shown differences to exist in the fatty acid profile of encapsulated fat compared to the free fat, suggesting the chemical properties of the fat influence the migration of the fat within the droplet prior to the completion of drying (Murrieta-Pazos et al., 2012). The migration of fat towards the surface of spray dried emulsions can lead to uncontrolled powder agglomeration causing the powder to self-agglomerated leading to a powder with poor functional properties.
The overall aim of this work was, therefore, to reduce the susceptibility of low protein spray dried emulsions to stickiness caused either by amorphous lactose transitioning to a rubbery state or high levels of surface fat facilitating powder self-agglomeration via fat bridging. The first study objective was to reduce the protein content of the spray dried emulsions without increasing the levels of free fat. As maltodextrins and glucose syrup have higher glass transition temperatures than that of lactose, a second study objective was to investigate the potential of maltodextrins and glucose syrup to partially replace lactose in a low protein spray dried emulsion. The resulting powders were assessed for their physicochemical properties as well as changes in the glass transition point. The final study objective sought to understand the impact of the fatty acid profile of the lipid phase on the migration of fat towards the surface of the atomised droplet prior to powder particle formation.
Section snippets
Materials
Skim milk powder (SMP 36% w/w protein, 0.5% fat w/w, 52.7% carbohydrate w/w, 7.3 %w/w ash, 2.5–3 %w/w moisture) was purchased from Arrabawn Co-op (Nenagh, Co. Tipperary, Ireland). Lactose and milk fat were supplied by Glanbia Ingredients Ireland (Ballyragget, Co. Kilkenny, Ireland). Refined palm oil and sunflower oil were purchased from Trilby Trading (Drogheda, Co. Louth, Ireland). Maltodextrin (DE 6) and glucose syrup (DE 39) were obtained from Roquette (Lestrem, France) and AllinAll
Impact of reducing protein content
To investigate the impact of reducing protein content on the stickiness of spray dried emulsions, the effect of reducing protein content on emulsion stability was first investigated. This is important as destabilisation of the emulsion would lead to a powder with high levels of free fat. Fig. 1a shows the droplet size distribution for emulsions ranging in protein content from 0.25% to 5% w/w. The emulsions containing 0.25% protein were highly unstable displaying a bimodal size distribution with
Conclusion
Emulsions containing 1 and 2% w/w protein were stable to gravitational separation, however, upon atomisation and drying they resulted in powders with significantly higher levels of free fat when compared to emulsions containing ≥2.5% protein. This suggests that the emulsion may undergo some degree of disruption during spray drying and small adjustments in protein level can significantly impact the emulsions stability during drying. This highlights that there is a requirement for a specific
Conflicts of interest
All the authors declare no conflict of interest.
Acknowledgements
This work was supported by the Irish State through funding from the Technology Centres programme- Grant Number TC/2014/0016.
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2022, Journal of Food EngineeringCitation Excerpt :It has been reported that the standard relative amounts of C and O on the surface of lactose were very close, 52.2% and 47.8%, respectively, but for fat and protein, the relative content of C was much higher than O (milk fat: 89.1% C and 10.9% O; casein: 65% C, 19% O and 16% N) (Kim et al., 2002). This means that most of the particle surface was fat, consistent with previous research (O'Donoghue et al., 2019; O'Neill et al., 2019). In addition, after breakage, the relative amount of C and O slightly decreased and increased, respectively (Fig. 4).
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