Infant milk formula powders: effects of composition, processing and storage variables on physicochemical properties

The demand for safe and nutritious infant foods is continuously growing and infant milk formula (IMF) powders have consequently received considerable research attention. IMF powders are prepared by combining macronutrients (carbohydrate, fat and protein) and micronutrients (minerals, vitamins and others) to simulate the nutritional composition of human milk. In addition to meeting nutritional requirements, IMF powder should have desirable properties such as high solubility and low propensity for caking. The physicochemical stability of IMF can be influenced by product composition, processing variables (heating, homogenisation, evaporation and spray drying) and conditions encounter during transportation and storage (temperature and relative humidity). The stability of emulsion (feed) during processing and powder during transportation and storage is of great importance that strongly influences nutritional value and shelf life. Instability in emulsions and powders leads to quality issues in IMF powders such as lactose crystallisation, caking, free fat formation, Maillard induced browning, change in surface properties and loss of solubility. Solving these issues requires a systematic study to understand the mechanisms that occur during product formulation, processing and storage. This understanding will help to develop powders that are nutritionally superior and have desirable physicochemical characteristics. Therefore, this study investigated the effect of product composition (lactose-to-maltodextrin (L: M) and whey protein-to-casein (WP: CN) ratios), processing variables (spray drier inlet and outlet temperatures) and storage parameters (temperature and relative humidity) on the physicochemical properties of IMF powders. In the first stage of this study, three IMF powders were produced with varying lactose-to-maltodextrin ratios (L: M 100:0, L: M 85:15 and L: M 70:30). All ingredients were added in water and the wet-mix was heated, homogenised, evaporated and finally spray dried to obtain the powders. The results show that the replacement of a portion of lactose with maltodextrin did not affect the droplet size of fat in emulsions; however, it decreased the zeta potential and increased the apparent viscosity after evaporation. Regarding powder properties, these IMF powders had similar moisture, protein, fat, carbohydrate and ash contents reflecting the reproducibility of powder manufacture. Replacement of lactose with maltodextrin increased the glass transition temperature (Tg, by 4 and 8 °C at 15% and 30% replacement level) and decreased the crystallinity and yellowness of the powders. Water activity, particle size, free fat content and solubility of IMF powders were unaffected by various L: M ratios. The surface composition of IMF powders was distinctly different to their bulk composition, and the powder surface was predominantly covered by fat, followed by protein and carbohydrate. The morphology of the powder particles was mostly smooth and spherical with little or no agglomeration. The physicochemical properties of IMF emulsions and their spray dried powders in terms of whey protein-to-casein ratios (WP: CN 60:40, 50:50 and 40:60) were studied. The average particle size of emulsions containing different WP-to-CN ratios did not differ significantly after heating and homogenisation; however, it differed significantly after evaporation. The values of zeta potential for both heated and homogenised and evaporated feed were highest in samples with 60:40 ratio, followed by 50:50 and 40:60. Apparent viscosity of emulsions did not differ significantly after heating and homogenisation; however, it differed significantly after evaporation especially in samples with high whey protein content. In terms of powder properties, average particle size and water activity of powders decreased with decreasing whey protein-to-casein ratio. Tg, crystallinity, surface free fat content and solubility of the powders were similar and unaffected by varying protein ratios. Surface composition and surface morphology of IMF powders were also similar and were not affected by the varying protein ratios. The whey protein in these powders was found to be aggregated due to their high heat sensitivity, whereas casein remained mostly unaffected. The effects of storage conditions on the physicochemical stability of IMF powders containing different lactose-to-maltodextrin ratios (L:M 100:0, 85:15 and 70:30) were studied. The powders were stored at 22 and 40 oC at three relative humidity levels (11%, 23% and 54% RH) for 180 days, and characterised for moisture content, Tg, crystallinity, surface properties and solubility. The results show that IMF powders stored at 22 and 40 oC up to 23RH were mostly stable for the entire storage period. Deteriorative physicochemical changes were noticed in powders stored at 54RH and that these changes occurred faster at higher temperature (40 oC) than at lower temperature (22 oC. Moisture content during storage was identified as the most critical factor that affected the storage stability of IMF powders. Higher moisture uptake at 54RH had a strong plasticisation effect which reduced the Tg and induced other deteriorative effects including lactose crystallisation and caking. Surface fat continued to increase as storage time increased which was accompanied with a proportional decrease in surface protein and surface carbohydrate contents. The surface of the powder particles changed from smooth and spherical to rough, crystalline and agglomerated particles. The influence of spray drying temperatures and storage parameters on the properties of IMF powders were also studied. Six powders were manufactured at six inlet-outlet temperature combinations (180-80, 180-90, 180-100, 200-80, 200-90 and 200-100). These powders were stored at two temperatures (22 and 40°C) and three RH levels (11RH, 23RH and 54RH) for 180 days. The combination of inlet and outlet air temperatures was found to influence the properties of freshly spray-dried powder in terms of moisture content, water activity, particle size and Tg. All of these IMF powders remained free flowing for the entire storage period (180D) when stored at 22 and 40°C up to 23RH. However, significant changes in moisture content and other physicochemical properties occurred at 54RH at both storage temperatures due to which the powders became unstable and caked. Higher moisture uptake at 54RH had a strong plasticisation effect which significantly reduced the Tg below storage temperature. The stored powders showed three distinct characteristics based on the difference between storage temperature (T) and Tg: when (T-Tg)> 5°C, all powders were crystallised and caked. The powders became lumpy without crystallisation when (T-Tg) was -5°C to +5°C. The powders remained free flowing at (T-Tg) < -5°C. Significant changes in powder properties including crystallinity, surface composition, surface morphology, solubility was noticed only when (T-Tg)> 5°C. Overall, an increase of temperature and RH during storage decreased the Tg, and increased the crystallinity, surface fat, surface roughness, aggregation and caking, all of which reduced the solubility. This study provides fundamental insights into the physicochemical properties of IMF emulsions and their spray dried powders which are often modified by the product composition including lactose-to-maltodextrin and whey protein-to-casein ratios. This study also shows that physicochemical properties of IMF powders can be preserved longer time if the lactose is partially replaced by maltodextrin. It also shows that the choice of inlet-outlet temperature combination during spray drying affects the physicochemical properties of freshly dried IMF powders. Similarly, the choice of temperature-RH combination during storage substantially affects the physicochemical properties of IMF powders. Overall, this study provides new insights on how the product composition, processing parameters and storage conditions affect the characteristics of IMF powders and how best to design/control them to produce better quality powders and store them for longer.