06 June 2019
Interest in cell-cultured meat has been steadily gaining momentum since the first laboratory-grown burger was unveiled in 2013 by scientists from Maastricht University in the Netherlands.
In general, the process for producing cell-cultured meat requires a few ‘satellite’ cells, which can be obtained from a muscle sample taken from a living animal. These cells can then be transferred to a bioreactor containing a scaffold upon which the cells can attach and grow in a nutrient-rich (and preferably animal-free) medium. The scaffold provides structural support for the cells and promotes muscle fusion, creating ‘strips’ of muscle fibres. The fibres can be mechanically stretched to increase size and protein content and the resulting tissue is harvested and processed into a boneless meat product.
A significant amount of research relating to cell-cultured meat has focused on the formation of processed meat (such as hamburgers and meatballs) and formulation of an economical manufacturing process. In order to effectively scale up production, a suitable scaffold (which is cost effective and preferably edible) to support the growing cells is required. To date, progress in manufacturing such products is advancing beyond all expectations, and slaughter-free meat could be in our shopping trollies sooner than expected. For example, JUST Inc (with laboratories in San Francisco) aims to sell its first cell-cultured meat products this year, whilst Mosa Meat and US-based Memphis Meats aim to have their products on supermarket shelves by 2021.
This focus on producing processed cell-cultured meat is due, at least in part, to the complexity of unprocessed meat, which includes bone, blood vessels and connective tissues, making this structure difficult to replicate. An appropriate scaffold that aids cell alignment will be required to develop unprocessed cell-cultured meat. A significant amount of research and ‘out of the box’ thinking will be needed before we can expect to enjoy a cell-cultured steak or rack of lamb.
In the race to provide a suitable solution to this problem, varied approaches have been taken by companies and research groups within this sector and some are seeking patent protection for new inventions. Some examples of scaffolds being trialled for forming either processed or unprocessed meat are discussed below.
Researchers from Pennsylvania State University and the University of Alabama have collaborated to produce a method of forming edible starch fibre mats using a cost effective wet-electrospinning technique. Starch has the benefit of being one of the least expensive natural fibres.
The wet-electrospinning technique typically comprises a polymer solution, a syringe with a metal needle, a coagulation bath and a grounded collector. The polymers required to form the scaffold are first dissolved in a solution, which is placed inside the syringe and subsequently forced through the needle at a constant flow rate. At the same time a high voltage is applied to the solution. The electric charge draws and stretches the jet of the polymer solution as it is directed to a coagulation bath containing ethanol and water, which precipitates the polymer fibres from solution. The resulting polymer fibres are collected on a rotating drum submerged in the coagulation bath. The thin starch fibres provide a high surface area scaffold upon which cells can adhere and grow to form a structured meat product.
The electrical field that forms between the nozzle and a rotating collection drum draws the starch into long threads.
In a study recently published in Food Hydrocolloids, the researchers built an inexpensive electrospinning device partially using the children’s toy, Lego® to form the rotating drum collector (shown in Figure 1).
The research found that the formation of suitable starch fibres was dependent on the speed of the rotating drum and the amount of ethanol in the electrospinning bath used to collect the fibres.
Figure 1 – ‘Aligned wet-electrospun starch fibre mats’
Israeli company, Aleph Farms, announced that it had produced the first prototype of a cell-cultured steak in December 2018, with thin strips of steak costing around $50 to produce. The company admits that the size and flavour of the steak requires some further research before the product is ready for commercialisation. It has collaborated with the Technion – Israel Institute of Technology, Haifa, to develop the manufacturing method, which includes a bio-engineering platform and innovative approaches to an animal-free growth medium to nourish the cells.
Aleph Farms is reported to use a combination of six technologies, which provide a more economical manufacturing method.
These techniques include innovative approaches relating to an animal-free growth medium to nourish the cells, and bioreactors (the tanks in which the tissue grows). Little information on the specifics of these technologies seems to be publically available.
However, a recent patent application by Technion Research & Development Foundation Ltd published in January 2019 (WO 2019/016795), discloses a method of forming cell-cultured meat on a porous scaffold. The method comprises the steps of incubating a three-dimensional porous scaffold formed from a textured protein, such as a soy protein, and a plurality of cell types including myoblasts (muscle cells) and at least one extra cellular matrix (ECM)- secreting cell type.
The ECM-secreting cells may be adipocytes (fat cells), fibroblasts (which produce the structural framework of animal tissues), progenitor cells (satellite cells), or endothelial cells (cells which line the interior surface of vessels). Once the porous scaffold has been incubated with the different cells, they are allowed to expand on the scaffold. Muscle cell fibres are then formed through the fusion of myoblasts into multi-nucleated fibres (myotubes).
A research group at the University of Bath in the Department of Chemical Engineering has come up with an alternative method to formulating a suitable scaffold for cell-cultured meat. Dr Marianne Ellis, working with a multidisciplinary team with expertise in biochemical engineering, biology and biomaterials, has focused on scaling-up the manufacturing process of cultured meat.
The researchers used stem cells extracted from an animal, which are fed a mixture of glucose, vitamins, minerals and amino acids. The cells are then transferred to a bioreactor containing grass as a scaffold, on which they can attach and grow. To date, the team has used rodent cells to test the effectiveness of this scaffold, as they are cheaper and easier to use compared to stem cells extracted from cows or pigs, for example.
At the present time, it is unclear whether a grass scaffold would be suitable for producing the complex structure of unprocessed meat. This would require a system containing multiple cell types growing in an organised manner and a structure that will need a replicated blood vessel network. A more simplistic and near-term goal is to produce a muscle protein ingredient based on muscle cells alone.
A method that uses decellularised spinach leaves (i.e. spinach leaves in which the cellular material has been removed) to produce a scaffold for tissue engineering has been developed by Glenn R. Gaudette, a professor of Biomedical Engineering at Worcester Polytechnic Institute in Massachusetts.
A patent detailing the decellularisation method (WO 2017/160862) was published in September 2017. This work is based on the similarities in the vascular structure of plant and animal tissues (Figure 2).
Figure 2 – Crossing kingdoms: using decellularised plants as perfusable tissue engineering scaffolds
The method first decellularises spinach leaves by applying a solution containing 10% sodium dodecyl sulphate (SDS) in deionised water for five days. After this, a clearing solution (0.1% TritonX100, 10% sodium chlorite in deionised water) was applied to the leaves for two days. The resultant leaves were colourless and translucent, forming an acellular scaffold consisting of extracellular matrix (ECM), preserving an intact vascular network (Figure 3).
Gaudette, is collaborating with Dr Marianne Ellis of the University of Bath to assess the potential of this technique to develop muscle cells from the stem cells of a cow. If successful, spinach leaves could provide a low cost and edible scaffold for producing more complex meat structures.
Figure 3 – A cellular scaffold consisting of extracellular matrix
Cell-cultured meat has the potential to reduce the amount of land, water and antibiotics required for traditional farming practices. This field is moving rapidly with some processed cultured meat products already poised to enter the marketplace. Unprocessed meat products represent a more elusive target, but research is underway to develop scaffolds that allow the formation of the characteristic texture of a T-bone steak.
This article was first published in The Journal of the Institute of Food Science & Technology in May 2019.
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