Towards creation of artificial plant tissue through 3D food printing

The global food market is constantly evolving due the growing demand for food and changing consumer preferences. Public interest in the concept of personalized foods with tailor made flavor and dietary properties is increasing steadily. In response to this, digital technologies such as additive manufacturing (AM, also known as 3D printing) are being considered for designing and fabricating food in a more flexible manner. Recently, an increasing number of prototypes of 3D printers and applications have been developed to produce edible objects, thus introducing 3D food printing (3DFP) as a novel technique to produce food.

So far, the development of 3DFP has mainly focused on the formulation of simple food-ink recipes having properties suitable for AM technologies and on the optimization of printing parameters to accurately manufacture simple food objects. The diversity of textural properties of food objects that can be produced by 3DFP remains limited as they are rarely taken into account by structure design or adjusted by using more complex printing materials. Hence, real personalization and innovation of food products through 3DFP are aspects that have been barely considered. An interesting concept would be the 3D printing of cells, or even artificial cells, to yield foods with a cellular tissue-like structure of arbitrary design. In this context, the aim of this PhD research was to develop methods to print plant-source materials, including pectins and plant cells, for innovative and personalized food manufacturing towards the creation of artificial 3D cellular plant tissues.

First, an in-house extrusion-based printer was developed in order to print in 3D customizable water-based porous food simulants using low methoxylated pectin gels as food-ink. A series of pectin gels was successfully 3D printed by changing the formulation parameters including stirring rate and pectin, CaCl2, bovine serum albumin and sugar syrup concentrations. It was shown that food objects with variable microstructure and textural properties can be 3D printed by varying the composition of the food-inks. The pectin concentration was the main determinant of firmness and strength of the printed object. Sugar and pectin concentrations increased viscosity and affected the build quality of the printed object. Bovine serum albumin stabilized and promoted the aeration of the food-ink. From the results, a predictive model was established for formulating the food-ink to obtain 3D printed food simulants with a priori defined microstructure and texture.

Subsequently, we showed that texture control could also be realized by printing the food-ink material into designed structures having particular porosities. Analytical and finite element models were used to predict the texture properties of honeycomb structures with varying cell size. Structures were 3D printed using food-inks composed of three different pectin concentrations and characterized with micro-CT and compression analysis. The Young’s modulus of the printed object, considered as a proxy of texture, depended on both the Young’s modulus of the gelled food-ink as well as the porosity of the printed object. The comparison between the texture properties of printed structures and those predicted by analytical and FE modelling as a function of porosity showed that both predicted and actual texture properties matched the same decreasing relationship with increasing porosity. The results emphasized also the importance of structure correspondence for reliable design of texture properties of printed food structures.

In order to speed up the 3D printing process, a coaxial extrusion printhead was designed for 3D printing of pectin-based food simulants in which the inner flow is the food-ink and the outer a Ca2+ crosslinking solution. A series of cubical shaped objects was 3D printed by changing the printing parameters including the food-ink composition, the layer height, and the rate and CaCl2 concentration of the outer flow. The printed objects did not require any incubation post-treatment because the gelation of the food-inks occurred during the printing. The mechanical properties of the printed objects were highly correlated to their Ca2+ concentration, which can be accurately controlled by adjusting the rate and CaCl2 concentration of the outer flow. A predictive model was established for determining the printing settings to print 3D objects with a priori defined texture. We also compared objects printed by coaxial and simple extrusion methods. The method to induce the gelation of the printed objects clearly affected their properties such as the final concentration of Ca2+, the volume and the failure behavior under compression. These differences suggested that the texture perception of printed pectin objects might differ according the used extrusion method.

Finally, a novel approach was tested involving food inks incorporating plant cells in order to print foods that resemble plant tissues in various ways. A 3D printing method was developed based on the extrusion of bio-inks composed of a low-methoxylated pectin gel and embedded lettuce leaf cells. Bovine serum albumin was added in order to increase the air fraction in the printed gel matrix. Objects containing a high cell density were successfully 3D printed. The viability of the encapsulated plant cells depended on the pectin concentration and varied from 50 to 60 %. Although challenges remain in further development to print 3D artificial plant tissue, the reported results can be considered as a first step of this biomimetic concept highlighting that plant-sourced materials can be suitable ingredients to produce food simulants having designed properties using an extrusion-based printing process.