Babies born very preterm or very small have high nutritional needs to support their growth and development. Whilst breastmilk is the best nutrition for them, on its own it may not be sufficient for optimal growth and development. Products based on cows’ milk (human milk fortifiers) can be added to breastmilk to increase nutrient content and these are used widely around the world. However, there are some concerns that fortifiers may affect infants’ tolerance of milk feeds and even contribute to serious complications, such as bowel obstruction from milk curds. Hospital practices around breastmilk fortification, including time of storage before use, vary widely. As an active biological fluid, the physical and chemical properties of fortified breastmilk change over time and this may contribute to curdling. We will determine the impact of these changes over time in order to improve the quality of nutrition for preterm babies and minimise adverse effects.