Storage stability in a milk based UHT-beverage : effect of pH, carrageenan and storage time

Abstract: The different characteristics of many food products, such as aroma and texture, changes during storage time and become undesirable. The reason is instabilities within the product which become prominent as the product ages. These changes in characteristics are quality losses caused by changes in the product’s intrinsic and extrinsic environment. The stability in UHT-milk and beverages similar to it have been studied in the past decades and a certain focus have been on age gelation which is a common defect in these type of products. Research concerning instabilities in milk-based UHT-beverages containing carrageenan are few. The main objective of the present study was to study the stability of an UHT-milk beverage fortified with protein and containing carrageenan. The specific aim was to study structural changes occurring during twelve weeks of storage and the following five products were analysed: T1 (low pH; low carrageenan), T2 (high pH; low carrageenan), T3 (low pH; high carrageenan), T4 (high pH; high carrageenan) and a reference (center point). The products consequently had two factors varied at three levels; high and low pH and high and low concentration of carrageenan; and a center point with values between high and low. The microstructures of the samples were studied using bright field microscopy, confocal laser scanning microscopy and transmission electron microscopy. In addition the particle size and particle concentration were measured using a NanoSight and the results were further analysed using factorial design and Student’s t-test. The pH was analysed by an ANOVA and Student’s t-test and visual sensory analysis was carried out to detect structural changes in the products during storage. The results from bright field microscopy and visual sensory analysis confirmed that there were structural differences among samples which appeared after different times of storage. The instabilities were observed in the following order starting with the most unstable sample first: T4, Ref, T2, T3 and T1. The results from the pH-measurements showed that the pH increased significantly in T1 and decreased significantly in T4 and the reference between week 0 and week 12. The results from the particle size measurements showed that the particle size increased significantly in samples T3, T4 and reference and decreased significantly in T1 between week 0 and week 8. In the beginning of storage the size was affected by the amount of carrageenan in the sample, and after eight weeks of storage the size was affected by an interaction between pH and carrageenan. The results from the measurements of particle concentration showed that the number of particles significantly decreased between week 0 and week 8 in all samples. The amount of carrageenan, pH level and time affected the number of particles in the samples. The complexity of the products made it hard to conclude what was causing the observed instabilities, resulting only in hypothetical suggestions. The conclusion from the study was however that pH was of primary importance for the stability of this product, where a higher pH seemed to result in less stable products. The effect of carrageenan in this product seemed to be only of secondary importance for product stability.