Elsevier

Food Hydrocolloids

Volume 90, May 2019, Pages 330-340
Food Hydrocolloids

Reducing stickiness in spray dried dairy emulsions

https://doi.org/10.1016/j.foodhyd.2018.12.033Get rights and content

Highlights

  • Stable liquid emulsions with protein content as low as 1% w/w.

  • Powders with 2–8% w/w protein had high free fat values despite good emulsion stability.

  • Increased susceptibility to powder stickiness with decreasing protein content.

  • Lactose replacement with glucose syrup decreased powder susceptibility to stickiness.

  • Fatty acid profile influences surface fat and fat distribution within the powder particles.

Abstract

High-protein intake in early childhood may cause negative health effects in later life. However, low protein powdered follow-on milks are challenging to manufacture due to stickiness caused by increased free fat and lower glass transition points. Skim milk powder and palm oil emulsions, ranging from 0.25 to 5% w/w protein were assessed for their stability and subsequently spray dried to produce powders ranging from 1 to 20% w/w protein. Powders containing 2–8% w/w protein had high free fat levels despite the initial emulsion being stable, suggesting small adjustments in protein concentration can have a significant impact on emulsion stability during drying and atomisation. Reducing the protein content via substitution with lactose reduced the powders glass transition point (Tg) from 65 to 49 °C (powder contained 4%w/w protein, 67%w/w carbohydrate and 28%w/w fat). Substituting lactose with maltodextrin (DE6) at a 45%w/w level of inclusion increased the Tg of the powders from 50 to 113 °C, however, viscosity increased significantly. Interestingly, using glucose syrup (DE39) to substitute lactose at a 15%w/w level of inclusion, increased the powder Tg by 21 °C with no significant change in emulsion viscosity compared to a control with lactose as the sole carbohydrate. XPS analysis showed fat type (milk, palm, sunflower) influenced powder surface fat coverage. The inner fat contained increased levels of saturated fatty acids (palmitic and stearic) compared to the surface and encapsulated fat. In conclusion the Tg of low protein spray dried emulsions can be increased and the surface fat decreased, helping to reduce susceptibility to stickiness.

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.

References (46)

  • K.D. Foster et al.

    The contribution of milk fat towards the caking of dairy powders

    International Dairy Journal

    (2005)
  • N. Fu et al.

    Colloidal transport phenomena of milk components during convective droplet drying

    Colloids and Surfaces B: Biointerfaces

    (2011)
  • C. Gaiani et al.

    Surface composition of dairy powders observed by X-ray photoelectron spectroscopy and effects on their rehydration properties

    Colloids and Surfaces B: Biointerfaces

    (2006)
  • C. Gaiani et al.

    How surface composition of high milk proteins powders is influenced by spray-drying temperature

    Colloids and Surfaces B: Biointerfaces

    (2010)
  • H. Habtegebriel et al.

    Effect of operating parameters on the surface and physico-chemical properties of spray-dried camel milk powders

    Food and Bioproducts Processing

    (2018)
  • S.A. Hogan et al.

    A novel technique for determining glass–rubber transition in dairy powders

    Journal of Food Engineering

    (2010)
  • S.A. Hogan et al.

    Influence of milk proteins on the development of lactose-induced stickiness in dairy powders

    International Dairy Journal

    (2010)
  • T. Huppertz et al.

    Lactose in dairy ingredients: Effect on processing and storage stability 1

    Journal of Dairy Science

    (2016)
  • M. Jayasundera et al.

    Surface modification of spray dried food and emulsion powders with surface-active proteins: A review

    Journal of Food Engineering

    (2009)
  • G.M. Kelly et al.

    Physical characteristics of spray-dried dairy powders containing different vegetable oils

    Journal of Food Engineering

    (2014)
  • G.M. Kelly et al.

    Effect of hydrolyzed whey protein on surface morphology, water sorption, and glass transition temperature of a model infant formula

    Journal of Dairy Science

    (2016)
  • S. Kentish et al.

    Milk skin formation during drying

    Chemical Engineering Science

    (2005)
  • E.H.J. Kim et al.

    Surface characterization of four industrial spray-dried dairy powders in relation to chemical composition, structure and wetting property

    Colloids and Surfaces B: Biointerfaces

    (2002)
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