By revisiting previous neutron data, ISIS beamline scientist Gregory Smith has found a better way to model the structure of protein aggregates in milk. Casein proteins in milk self-assemble into nanoparticles called micelles, which are described in our surfactant explainer article. These casein micelles are the structure-forming species in milk.
By modifying the interactions between these casein micelles, milk can be turned into dairy products such as cheese, yoghurt, or ice cream. Therefore, understanding these interactions is of great importance for food scientists and food industries.
Gregory found that the modeling of ultra-small-angle neutron scattering (USANS) and spin-echo small-angle neutron scattering (SESANS) data in the literature could be improved if the casein micelles in the milk are treated as "sticky" spheres with some attraction, rather than as dilute and noninteracting spheres. His study showing this is now available in the journal Food Hydrocolloids.
"A few years ago, I measured the X-ray scattering of milk samples at the Advanced Photon Source in the USA with colleagues from the University of Copenhagen," explains Gregory. “After a lot of work, and many attempts to model the data in different ways, I realised that I needed to think about the casein micelles as sticky to make the model and measurements agree. This has never really been done with scattering data from milk before. I then wondered if this was specific to X-ray data or if it was general to all scattering data."
By comparing series of data collected using two different neutron scattering techniques and from different facilities, he found that the sticky sphere model gave better agreement between the model predictions and measurements for the samples measured. The two techniques are regularly used at ISIS: small-angle neutron scattering (SANS) measurements can be performed on several instruments, and spin-echo small-angle neutron scattering (SESANS) measurements can be performed on the dedicated setup on Larmor.
"Neutron scattering is a great way to study the structure of soft materials, like food, in great detail, but you have to use the right model to get the right answer. It turned out that neutron scattering data is also improved if you think of the casein micelles as sticky spheres. This study will hopefully motivate experiments that revisit casein micelles with neutron scattering," adds Gregory.
Small-angle scattering is a useful tool for generally studying structure on the nanometre scale. Neutron scattering can provide information about the distribution of polymers and proteins, for instance, by exchanging water for heavy water (deuterated water, D2O). Dried milk powder dissolved in heavy water can be used to make up deuterated milk for these experiments. This is why neutron scattering has been used in the past for studying dairy products, primarily the internal structure of the casein micelle in native milk and during processing.
A previous experiment at ISIS used SANS to study casein under different conditions.
The full paper can be viewed at DOI: 10.1016/j.foodhyd.2020.106511