Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem Inter-relationship between lactose crystallization and surface free fat during storage of infant formula Juhi Saxena a , Benu Adhikari a , Robert Brkljaca c , Thom Huppertz b,e , Jayani Chandrapala a, ⁎ , Bogdan Zisu d a School of Science, RMIT University, Bundoora, Melbourne, VIC 3083, Australia b FrieslandCampina, Amersfoort, the Netherlands c School of Science (Applied Chemistry and Environmental Science), RMIT University, Melbourne, VIC 3001, Australia d Spraying Systems, Fluid Air, Melbourne, Australia e Wageningen University and Research, Wageningen, the Netherlands ARTICLE INFO Keywords: Infant formula Lactose crystallization Surface free fat Hydrolyzed whey protein ABSTRACT Inter-relationship between lactose crystallization (LC), the amount and composition of surface free fat (SFF); and their effect on physico-chemical properties of infant formula (IF) containing hydrolyzed and intact (non-hy- drolyzed) whey protein in their composition were investigated at two temperatures (25 and 45 °C) and five RH (11–65%) conditions. Results varied with compositional variation of IF. LC increased exponentially with SFF in non-hydrolyzed IF powders. IF composition influenced LC and caused selective migration of triglycerides, re- sulting in higher proportion of unsaturated fats in SFF of powders with large lactose crystals and vice-versa. Increase in SFF with increased proportion of saturated fats in their composition, resulted in reduced wettability of powders. Overall, IF composition affects LC which influences the amount and type of fat migration to particle surface resulting in varying wettability of IF powders. 1. Introduction Caking is a very common occurrence in infant formula (IF) powders, especially in those that are exposed to high relative humidity (RH) and temperature conditions during transportation and storage. High hu- midity causes the free-flowing powder to form lumps, which then form agglomerates before ultimately transforming into a sticky “cake” (Tham, Wang, Yeoh, & Zhou, 2016). Physical stability of an IF powder is a direct function of the product composition, storage conditions and handling (McCarthy et al., 2013). IF powder composition varies de- pending on the stage of infant development whereby, stage 1 IF is in- tended for infants aged 0–6 months, stage 2 for newborns aged 6–12 months, and stage 3 for those older than one year. In most cases, lactose is the predominant carbohydrate and present in amorphous form, making the powder more susceptible to moisture uptake and caking (Vuataz, 2002). IF powders are especially tailored to suit the needs of infants and whey protein hydrolyzates are sometimes used as the protein source for such formulations (Hernández-Ledesma, García- Nebot, Fernández-Tomé, Amigo, & Recio, 2014). However, their in- corporation into powdered formulae is often associated with processing and shelf life difficulties due to their increased hygroscopicity (Drapala, Auty, Mulvihill, & O'Mahony, 2016). IF powders when transported over long distances, become in- evitably exposed to fluctuating temperatures. When the storage tem- perature exceeds the glass transition temperature, lactose crystallizes and liberates moisture which further cause deteriorative changes in the product by inducing cracks on the particle surface (McCarthy et al., 2013; Vuataz, 2002). This effect in turn allows fat migration from the core to the particle periphery which allows particle bridging and hence contributes to caking. (Tham, Xu, et al., 2017). Limited research has been published showing the effects of fat migration to the surface in milk powders (Tham, Xu, et al., 2017; Kim, Chen, & Pearce, 2005; Kim, Chen, & Pearce, 2009). In order to prevent the deteriorative changes brought about by ageing of IF, it is essential to understand the inter- relationship between the composition, lactose crystallization and fat migration. Therefore, the aim of the present study is to establish the inter-re- lationship between lactose crystallization and fat expulsion to the particle surface as a function of ageing at two temperatures (25 and 45 °C) and five RH (11–65%) conditions of IF powders containing hy- drolyzed whey proteins and non-hydrolyzed whey proteins. In addition, the present study focused on the influence of lactose crystallinity on the https://doi.org/10.1016/j.foodchem.2020.126636 Received 28 October 2019; Received in revised form 25 February 2020; Accepted 17 March 2020 ⁎ Corresponding author. E-mail address: Jayani.Chandrapala@rmit.edu.au (J. Chandrapala). Food Chemistry 322 (2020) 126636 Available online 19 March 2020 0308-8146/ © 2020 Elsevier Ltd. All rights reserved. T