Plant-based meat analogues: using neutrons to peek into the black box

A recently published study uses neutron scattering to shed light on texturisation - the black box for plant-based meat analogues - through in-situ real-time observations of the process. The challenge is increasingly relevant for the food industry as consumer concerns about sustainability, health and animal welfare rise.

Plant-based meat analogues: the challenge of the imitation game

Meat production has a large environmental impact and is inevitably linked to ethical issues and animal welfare concerns. High meat consumption may also increase the risk of several chronic diseases, such as cardiovascular disease and colorectal cancer. For many reasons, people are increasingly turning to vegetarian or flexitarian diets, which is gradually making meat substitutes more mainstream.

Today, plant-based meat analogues represent an important share of the global market for meat substitutes. However, in spite of the significant efforts made to improve the imitation of animal meat, an exact replica of the sensory experience of animal meat is still a complex challenge for the food industry, involving appearance, taste, flavour and texture.

The products that most closely resemble animal meat are characterised by meat-like fibrous structures. These are typically obtained via a process called high-moisture extrusion cooking: plant protein powders and water (optionally mixed with other ingredients) are fed separately into an extruder barrel, where they are mixed at temperatures between 130 and 170 °C. The mixture is then conveyed towards a cooling die where it solidifies. During solidification, a unique fibrous structure is formed, which gives the final product its meat-like appearance and texture.

While such fibrous structures have been well studied both at the millimetre and micrometre scales, they have rarely been studied at the nanoscale. Most importantly, the mechanisms behind the formation and bearing structure of the fibres have not yet been clearly identified and are a subject of discussion and ongoing research. In fact, methods that involve extracting samples cannot provide such information: the equipment needs time to cool down before it can be opened, and during this time the sample structure may change. The main issue, however, is that the entire system is something of a "black box", preventing the use of a whole suite of in-line detection techniques, such as near infrared (NIR) spectroscopy, Raman spectroscopy and nuclear magnetic resonance (NMR) spectroscopy.Neutrons and X-rays are, however, the exception.

Connecting the dots

The study now published in the scientific journal Food Hydrocolloids is part of the PhD project that Tong Guan is working on in the framework of InnovaXN, an EU-funded doctoral training programme in which 40 PhD students tackle a variety of subjects driven by industrial challenges and exploiting the advanced characterisation techniques of the large-scale European facilities. Tong's project is a collaboration between the ILL in Grenoble ETH in Zurich and the industrial partner Planted Foods AG (Switzerland). An additional industrial partner, Three-Tec GmbH, was involved in designing the technical equipment needed for the study.

Tong is connecting the dots to help bring clarity to the picture - or, rather, to shine a light into the black box. Her project brings together the right partners, facilities and expertise to achieve something that has not been done before. While a cell neutron-transparent windows has been previously designed and used to investigate the structure evolution of plant proteins in situ, such studies have not yet been performed inside an extruder or under extrusion-like temperatures. With its ability to probe structures at all scales, including the little studied nanoscale, small-angle neutron scattering (SANS) is particularly useful for this project. In fact, SANS has already been successfully applied to a variety of food systems (such as food-protein gels and protein-sugar systems).

Neutron scattering: a game changer

For this study, the research team designed and performed a complex neutron scattering experiment which allowed them to follow the texturisation of a plant-based meat analogue (made of a soy protein concentrate) in situ and in real time, inside a custom-made extruder. Importantly, the experiment was performed without interfering with the extrusion process.

A custom-designed cooling die equipped with three neutron-transparent windows allowed the structuring process to be observed on the nanoscale by small-angle neutron scattering (SANS) on a length scale from 1.3 to 436 nm. Measurements were performed on SANS-1 at the Paul Scherrer Institut (PSI) in Villigen, Switzerland and on the instrument D22 at the Institut Laue-Langevin (ILL) in Grenoble, France.

Small-angle neutron scattering (SANS) explores the structure of substances on length scales ranging from 1 nanometre to close to 1 micron. In a SANS experiment, a beam of neutrons is directed at a sample. The neutrons are elastically scattered by nuclear interaction with the nuclei in the sample. SANS measures the deviation at small angles (from much less than one degree to several degrees) of the neutron beam due to structures of such sizes in the sample. It is frequently a unique way to obtain direct structural information on disordered systems on these length scales. The high neutron flux and the flexibility of its setup make D22 an instrument that is particularly well suited to real-time experiments and weakly scattering samples.

As always, the results obtained raise new questions and open up specific research paths. One important next step is to study the viscoelastic behaviour of raw materials further under extrusion-like conditions, as they may play a relevant role in all three of the proposed mechanisms. Another is undoubtedly to study the formation of protein nano-aggregates, as they might well be the pivotal feature for structuring. Are they already present in the raw materials or do they develop during the extrusion process? These insights may enable targeted food structure engineering, which holds the potential to design tailored machinery explicitly suited to producing meat-like structures and hence to transform processes and prospects in the food industry.

» Original publication

Source: Institut Laue-Langevin (ILL)