Role of macronutrients and lactose crystallisation on the physicochemical stability of infant formulations

<p> The formulation of infant formula (IF) powders in terms of macro-ingredients (carbohydrates, protein and fats) and micro-ingredients (vitamins and minerals) is designed to cater to the nutritional needs of infants based on their age or dietary requirements. Stage 1 products are intended for infants aged 0–6 months, stage 2 products for newborns aged 6–12 months and stage 3 products for those of 1-3 years of age. The compositional variation has a profound influence on the physicochemical stability of IF powders when exposed to adverse conditions such as temperature and relative humidity (RH) during storage and transportation. This results in various inter-dependent deteriorative reactions, including glass transitions, lactose crystallisation, Maillard reactions and changes in protein conformation and surface free fat. These changes negatively impact physical and functional powder properties, such as colour and appearance, free flowing, wettability and solubility, thus affecting product acceptability.</p> <p> The presence of amorphous and/or crystalline lactose, hydrolysed or non-hydrolysed protein and the composition of fat are critical in understanding these deteriorative changes in physical and functional characteristics of IF powders. Therefore, the studies presented in this thesis investigated the effect of compositional variation (in terms of lactose and protein) and storage conditions (temperature and RH) on the physicochemical and functional properties of IF powders. In the study, the influence of compositional variation on the physicochemical properties of the three stages of two commercial IF product ranges, containing hydrolysed and intact whey protein were investigated. In terms of composition, lactose and fat content varied between 25-50 and 20-25 % respectively, whereas protein content varied from 9-15%.</p> <p> For the products containing intact whey protein, crystalline lactose was low (<5%), but for the stage 1 and stage 2 IF product containing hydrolysed whey protein, 30% of lactose was found to be in the crystalline form. Generally, during IF production, there should be no crystallisation of lactose in solution, unless purposely applied. The presence of notable amounts of crystalline lactose in samples stage 1 and 2 IF products containing hydrolysed whey protein could have arisen from lactose crystallisation prior to (i.e., pre-crystallisation) or during processing or storage, or due to the addition of crystalline lactose through dry-blending. The SEM results did not show the presence of lactose crystals as separate particles, thus suggesting that the presence of crystalline lactose in these products was the result of lactose pre-crystallisation prior to drying. Stage 1 and 2 of IF products containing hydrolysed whey protein exhibited lower surface free fat content than stage 3, which could possible be due to higher spray drying outlet temperature in the former provided a feed concentration >30 % was used. There were significant (p<0.05) differences in fatty acid composition between bulk fat and surface free fat for all samples. Fatty acid composition analysis showed the presence of ~5–10% of unsaturated fatty acids (USFA; mainly C18:1 and C18:2) in the surface free fat in IF powders containing high levels of crystalline lactose, compared to >40 % USFA in the surface free fat in other samples.</p> <p> All IF powders were stored at two temperatures (25 and 45 °C) and five RH (11, 25, 33, 54 and 65 %) for up to 8 weeks and the relationship between different physicochemical properties like lactose crystallisation, surface free fat, glass transition temperature (Tg) and particle size was investigated. While all powders remained free flowing for eight weeks after storage at RH 11-33 % at both temperatures, caking was observed in powders stored at RH> 54 % after 6 and 4 weeks at 25 and 45 °C, respectively most likely due to the influence of moisture sorption on lactose crystallinity and Tg of the powders. For the products containing hydrolysed whey protein, lactose crystallinity increased linearly with the decrease in Tg (influenced by water plasticisation which assists the rearrangement of lactose molecules in a crystal lattice) and the increase in particle size but showed no correlation with surface free fat. For products containing intact whey protein, lactose crystallinity exhibited an exponential relationship with Tg and surface free fat.</p> <p> For stage 1 and 2 products containing hydrolysed whey protein, surface free fat was <15 % and <4 %, respectively, throughout storage, showing restricted presence of USFA while intact whey containing powders exhibited >30 % surface free fat after storage at 45 °C and was largely composed of USFA. These changes negatively influenced the wettability of all powders. In addition, powders containing intact whey protein were investigated for the formation of 5-hydroxymethyl furfural (HMF), colour and conformational changes in the protein structures during storage. Browning increased linearly with HMF content during storage and the presence of higher amount of reducing carbohydrate (through inclusion of glucose syrup) in stage 3 samples resulted in relatively higher changes in browning during storage at 45 °C and 65 % RH. The changes in ratio of α-helical: β-structures of protein followed a linear relationship with the HMF content and simultaneous changes in solubility were observed which were in turn influenced by the composition of the different powders.</p> <p> In the next phase, the effect of different levels of lactose pre-crystallisation was studied in stage 1 and 2 IF powders containing hydrolysed whey protein and their storage stability was evaluated at 25 and 45 °C and three levels of RH (11%, 33% and 54%). The difference in the amount of lactose and maltodextrin in the composition of powders resulted in five levels of pre-crystallised lactose (PCL) between 0-45 % and 0-31 % in stage 1 and 2 products, respectively. Lactose pre-crystallisation was achieved by shock cooling the reconstituted IF mix from 60 °C to 20 °C followed by slow cooling to 6±2 °C for variable times. Powder properties such as particle size, powder morphology, lactose crystallisation, water activity (aw), Tg, surface free fat and color were studied. The proportion of PCL influenced the particle size, surface free fat and aw in powders at week 0. During storage, stage 1 powders showed a reduced rate of change of aw, lactose crystallisation and glass transition in samples containing 18 % PCL, whereas stage 2 powders showed similar observations in powders containing 7-24 % PCL during storage at RH 54 %. Reduced changes in surface free fat and particle size were observed in stage 1 powders containing 18 % PCL relative to control. In stage 2 powders, both 18 and 24 % PCL resulted in the lowest surface free fat (<2%) under all ageing conditions resulting in six weeks of storage at RH 54 % and 45 °C relative to four weeks in others.</p> <p> Overall, this study provides an understanding of the inter-dependency of different changes in powder properties during storage as well as its influence on their functionality. It is evident that although commercial IF powders show a similar storage profile, a difference in their caking behaviour was observed and quantified. Lactose pre-crystallisation positively influenced the changes in physicochemical properties of IF powders during storage; and maltodextrin worked in synchronisation with PCL to result in better product properties. Hence, lactose precrystallisation may be used as an alternative approach to retain powder characteristics for a relatively longer time under the studied temperature-RH conditions.</p>