Impact of Mechanical and Microstructural Properties of Potato Puree-Food Additive Complexes on Extrusion-Based 3D Printing

This paper studies the applicability of extrusion-based 3D printing for constructing novel shapes from potato puree and the effects of four additives (agar, alginate, lecithin, and glycerol) added separately at three concentrations (0.5, 1, 1.5%) on the internal strength, mechanical properties, microstructure, and color of potato puree. The printability of the potato puree and the mixtures was assayed by examining the consistency of the extrusions and the stability and accuracy of the printed patterns. The results indicate that better printing was achieved at a nozzle height of 0.5 cm and a nozzle diameter of 4 mm, with concentrations of alginate and agar between 0.5–1.5% and 0.5–1%, respectively, providing the best printability and end product stability, which was attributed to their respective high mechanical characteristics and specific mechanical energy (SME) values. Scanning electron microscopy (SEM) revealed that more convolutions were induced in the potato puree upon the addition of agar or alginate, which increased the puree stability. 3D printing did not significantly affect the surface color parameters of the final product. This study showed that the 3D printing process is a critical factor for initializing the production of customized healthy products.


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There is a growing demand for the development of customized food for specialized dietary needs, such as 27 products for athletes for recovery after training or products for expectant mothers that vary nutrient component  Three-dimensional food printing is an innovative technique that is of great potential interest and is continuously 37 under debate for both consumers and food scientists due to its broad array of uses (Severini, Derossi, Ricci,

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Caporizzi, & Fiore, 2018). The application of 3D food printing could be summarized as the ability to provide 39 customized food to certain groups of people (de Roos, 2013) and to automatically generate a specific code to 40 adjust composition, density or structure to the preferences and needs of the user. Moreover, 3D printing has 41 demonstrated some interesting applications for industry by enhancing efficiency through the consolidation of 42 multiple steps or even entire food production processes (Bak, 2003;Sun et al., 2015). For instance, the PepsiCo 43 company decided to incorporate 3D printing in the manufacturing of its potato chips to save money and create 44 healthier food after suffering serious problems in the sales of sugary drinks and fatty snacks (Simon, 2015).

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On the other hand, important factors should be taken into consideration when extrusion printing. Maintaining 53 compatibility between specific printing parameters and the corresponding printed substance is crucial to ensure 54 high feasibility for 3D printing. The essential process parameters that can be modulated are the printing speed, 55 the distance between the nozzle and the printing bed and the nozzle size; these are critical criteria that influence 56 the final resolution of the constructed shape (Hao et al., 2010;Zhuo, 2015;Derossi, Caporizzi, Azzollini, & 57 Severini, 2018). Additionally, monitoring the properties and composition of the food material itself (ingredient 58 rheology, electrical conductivity, density, textural quality, and physiochemical and microstructural properties) is 59 imperative and aids in predicting the behavior of a particular food during 3D printing and in assembling a 60 complex shape with many layers that is stable enough to maintain its profile for a long time post-deposition (

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Potato purees, now considered part of the nutritious ready-to-eat food market, could be combined with 64 hydrocolloids that interact with potato starches in an attempt to improve the overall product quality and 65 facilitate processing (Shi & BeMiller, 2002). Therefore, scrutinizing the effects that certain food additives have 66 on the starch structure and textural characteristics is important, because these effects affect the functionality of 67 the whole food product.

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The objectives of this study were to study the effects of food additives (agar, lecithin, glycerol, and alginate) 69 and their concentrations on the mechanical and microstructural properties of potato puree, to evaluate the 70 feasibility of the substances for 3D printing, to characterize the printing process parameters, such as the distance 71 between the nozzle and the printing bed and the nozzle size and to investigate the effects of the printing process 72 on the superficial color of the final products.

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Commercial potato powder and whole milk were purchased from the local supermarket. Agar-agar, soy bean 77 lecithin, sodium alginate and glycerol (food-grade) were procured from Sigma-Aldrich Co. The potato puree 78 samples were prepared according to the following procedure: 450 mL of milk and 50 mL of water were first 79 heated to 40°C, and then, 115 g of commercial potato powder was added. The mixture was then homogenized 80 using an electrical hand blender (Braun, Germany). The same procedure was followed for preparing the puree 81 samples with the different additives at concentrations of 0.5, 1.0 and 1.5% (Shi & BeMiller, 2002). Additives 82 were added at quantities corresponding to the desired concentrations to the warmed solution (milk and water) 83 prior to the incorporation of the potato powder. However, for the agar samples, the solutions were boiled to 84 100°C, and the dehydrated potato was then added. All prepared puree samples were placed in an incubator and 85 held at a temperature of 20ºC preceding any measurements.

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To optimize the 3D printing process, the effects of additives (agar, alginate, glycerol and lecithin), applied 89 speed (1, 2 and 4 mm•s -1 ) and extruder hole diameter (3 and 5 mm) on the extrusion process were studied using 90 a TA.XT Plus Texture Analyzer (Stable MicroSystems, Godalwig. UK) device with a 50-kg cell load.

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The specific mechanical energy (SME) was measured as an indicator of the energy efficiency and ease of flow

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Varying the extruder hole diameter at a constant extrusion speed showed that decreasing the diameter of the 141 extruder hole caused an increase in the SME of the samples due to higher acquired friction during extrusion,

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which necessitates an increase in the applied force (Table 1). Thus, a larger hole diameter (5 mm) facilitated 143 extrusion with proper ordering of the layers. Moreover, significant differences were seen in the SME exerted at 144 various extrusion speeds at a fixed extruder hole diameter; the SME and the extrusion speed were found to be 145 inversely proportional, where the highest value for the SME was recorded at the lowest speed and gradually 146 decreased significantly as the speed increased (Table 1) Table 2 shows the comparison between the potato puree and the potato purees with concentrations of 1% of 152 different additives (alginate, agar, glycerol or lecithin) at an extruder hole diameter of 3 mm and an extrusion 153 speed of 2 mm.s -1 . The SME value for the potato puree decreased significantly when 1% glycerol or lecithin 154 was added (Table 2). This decrease could be attributed to the ability of glycerol and lecithin to retain moisture 155 via the destabilization of the internal microstructure of the starch granules, therefore softening the material in 156 accordance with Dankar et al., (2018) and Guerrero et al., (2012). Conversely, the addition of 1% alginate or 157 agar in potato puree significantly increased the SME compared with the potato puree alone (Table 2). This

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Furthermore, greater stability in the shape of the extruded layers occurred when alginate or agar was added to 166 the potato puree, since the layers obtained were more consistent and able maintain their shape for a long time additives showed no significant differences when the concentrations were changed from 0,5, to 1 and to 1,5%, 178 whereas significant differences were detected in the mechanical properties of the agar measured at the different 179 concentrations (Table 3).

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However, the only significant difference in the consistency and cohesiveness between the agar and alginate samples 191 was obtained at the concentration of 1%, which was marked by a higher consistency (Fig. 1). This behavior is 192 attributable to the conveyed network structure that occurs between polysaccharide chains and the large-sized long 193 additive molecules (agar or alginate) within the matrix and to the enhancement of the particle-particle surface

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Similarly, the folding formed upon the addition of 0.5% alginate could be attributed to the formation of 225 alginate-cation-polysaccharide complexes (Truong et al., 1995

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Using a 2 mm nozzle, printing with the potato puree and the potato purees with additives produced poor-264 quality products in which the layers did not overlay with one another properly, and the shape was not well-265 maintained, leading to a poor product mainly because the thin filament size that was extruded was not large

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Another consideration for the printing process is the type of substrate to be printed. Of the mixtures 274 prepared, the potato purees with the agar or alginate at the different concentrations tested were able to be 275 printed in stable structures with many built-up layers that held their shape for a long time without collapsing 276 ( Fig. 3a and 3b, puree with 0,5% alginate and potato puree alone, respectively). This result could be directly

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The potato puree and the purees with glycerol or lecithin showed different behavior, in which printing a 285 multiple-layered 3D structure started well with a smooth flow of potato paste (Fig. 3d). Nevertheless, when the 286 structure reached its final stage, the many layers that were printed collapsed into each other (Fig. 3e)

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Generally, increasing the concentration of the additives in the potato puree did not cause any significant 309 differences in the luminosity or hue angle. However, increasing the concentration of lecithin to 1.5% produced 310 a significant difference in the Chroma of the potato puree by further decreasing the degree of saturation (Fig.   311   4). This result could be attributed to the lecithin (at 1.5%) producing increased modifications of the internal 312 starch granule interactions, yielding a dull surface appearance (less saturated).

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On the other hand, Le Tohic et al. (2018) found that the printing process affected the surface color of printed 314 cheeses, inducing a small decrease in the luminosity in contrast to our work, where the 3D printing process had 315 no significant effect on any of the color parameters studied for all the puree samples. Thus, the 3D printing process was proven to not influence the surface color of printed potato purees, which satisfies some consumers 317 and companies.

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Additionally, several attempts have been made to design soft and tasty products to satisfy the desires of the 319 elderly and those facing swallowing and mastication problems, enhancing their appetites with safe, novel and 320 nutritious foods (Aguilera & Park, 2016). Table 4  Alginate (from 0.5% to 1.5%) and agar (0.5 and 1%) were the additives that provided more stability for printed 341 products with corresponding increases in specific mechanical energy (SME).

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The mechanical characteristics of firmness, consistency and cohesiveness showed significant differences 343 (p<0.05) after the addition of agar or alginate to potato purees, and the effect was greater at higher 344 concentrations. Nevertheless, when not mixed with potato puree, only agar had a significant difference in 345 mechanical characteristics among the additives.

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The SEM figures demonstrate the different microstructural characteristics within the potato puree samples, 347 wherein lecithin produced a cotton-like structure, alginate produced more folding, glycerol induced a more 348 continuous network-like structure due to its ability to disrupt the inter-and intra-network interactions between 349 the polysaccharide chains, and agar induced more folding and convolutions, which complements the textural 350 value results.

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The best extrusion conditions for the 3D-printed potato purees were achieved with a nozzle size of 4 mm and a 352 critical nozzle height of 0.5 cm using a printing substrate of potato puree mixed with alginate (0.5 to 1.5%) or 353 agar (0.5 and 1%) to provide the finest resolution of stable end-products with many built-up layers.

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The optimal mechanical characteristic values for obtaining good quality 3D printed potato purees with 355 additives fall within the following ranges: a firmness between 0.94 and 2.10 kg, a consistency between 11.6