Technical paper
Effect of blanching periods and milling conditions on physical properties of potato powders and applicability to extrusion-based 3D food printing
2022 Volume 28 Issue 3 Pages 207-216
Details
2022 Volume 28 Issue 3 Pages 207-216
A three-dimensional (3D) food printer is an attractive tool for making highly customized foods. The quality of powders and pastes used as ink in printers has a significant impact on print performance. The present study investigated the effect of the powder preparation method on particle size and flowability of the powder. The particle size of potato powders milled by jet mills was smaller than that milled by hammer mills, and the mean diameter depended on blanching conditions prior to drying and milling. The flowability of powders decreased with decreasing particle size, and improvement in flowability was required for the fine powders manufactured by jet mill. The printability of the potato pastes was influenced by blanching conditions and particle size. In addition, manufacturing procedures for powders play an important role in determining the optimum water content of pastes for 3D printed foods.
In recent years, three-dimensional (3D) food printers have attracted considerable attention, and many companies are placing 3D food printers on the market (Tan et al., 2018). Many studies have analyzed printing performance in terms of physicochemical and rheological properties of food materials to improve the printing technology (Pallottino et al., 2016). The 3D food printer enables the creation of complicated and detailed objects, which are difficult to create using other fabrication methods. Thus, 3D food printers have the potential to make highly customized foods in terms of appearance, taste, texture, and nutrition value according to personal preferences and/or health conditions by depositing the desired nutrition components, flavors, and tastes (Nachal et al., 2019; Kumar et al., 2020). In addition, customized foods would be helpful as care foods for the elderly with difficulties swallowing or chewing, and it is an attractive technology for a society like Japan, where aging of the population is particularly advanced.
The 3D food printers employ several food printing methods, such as extrusion-based printing, inkjet printing, and binder jetting (Sun et al., 2015; Singhal et al., 2020). Among these, the extrusion-based printer is the most widely used and has the advantage of having compact size and low maintenance cost (Sun et al., 2015; Sun et al., 2018). The mechanism of the extrusion-based printer is similar to that of extrusion cooking. In extrusion-based printing, food materials are extruded through nozzles. The nozzles or printing stage is digitally controlled by a G-code path that is designed in advance, and the food materials are deposited layer by layer to form 3D objects. The required characteristics of food materials are limited for extrusion-based 3D printers, and paste, gel, or semi-solid food materials can be used for this technique. Researchers have focused on the application of extrusion-based 3D printers using chocolate (Lipton et al., 2010), cookie dough (Lipton et al., 2010; Pulatsu et al., 2021), baking dough (Yang et al., 2019), processed cheese (Le Tohic et al., 2018), rice flour (Thangalakshmi et al., 2021), and potato (Liu et al., 2018; Martinez-Monzo et al., 2019). Printability of food materials using extrusion-based 3D printers is associated with the ability to shape a 3D model by depositing layer after layer of food materials and for it to keep its shape for a certain period after printing (Godoi et al., 2016). Although printer settings, such as nozzle diameter, nozzle movement speed, infill levels, and layer height affect the molding performance (Anitha et al., 2001; Yang et al., 2019) and texture of 3D printed foods (Huang et al., 2019), rheological and physicochemical characteristics are key factors in designing the mold for printing and supportability after printing. The optimization of physicochemical and rheological parameters plays an important role in making highly advanced printed foods.
One way to fabricate 3D food printer ink is by making a powder of food materials followed by pasting. This process has the advantage that powdered foods have a long shelf-life because of their low water activities, are effective in reducing food loss, and the powders can easily be made into pastes. In this case, food materials are treated with drying and milling, and these processes affect powder and paste qualities. An important parameter of powders is their fluidity. Powders with insufficient fluidity cause clogging and are lumpy, which would be a practical problem for binder jetting types of 3D food printers, such as the ones in the research by Holland et al. (2018). Therefore, evaluation of the flowability of food powders is key in designing highly improved ink for 3D food printers. This study focused on the flowability of potato powders. Potato is one of the most important crops after rice, wheat, and corn as far as having high production and a stable supply (Wijesinha-Bettoni and Mouille, 2019), providing high energy from starch and being rich in vitamins and minerals (Al-Mughrabi et al., 2013; Beals, 2019), which makes it an attractive raw material for 3D printing. Several researchers have studied the flowability of potato starch (Crouter and Briens, 2014; Wang et al., 2016); however, the knowledge on flowability of potato powder for application to 3D food printers is insufficient. Potato powder contains not only starch but also protein and dietary fiber (Andre et al., 2007; Wijesinha-Bettoni and Mouille, 2019), which may have different flowability properties. In addition, dehydrated vegetables, such as potato powders, can be adversely affected by enzymes during the manufacturing process, such as through darkening and an increase in off-flavors (Mate et al., 1998; Carillo et al., 2009). Therefore, it is necessary to protect raw vegetables from the action of the enzymes during the production process. Blanching is performed on potatoes and other vegetables prior to drying to denature the enzymes, and blanching conditions potentially affect the physical properties of potato powders. Therefore, potato powder was prepared from raw potatoes with various blanching periods prior to drying and milling under several conditions to investigate the effect of powder preparation method on flowability. In addition, powders from different production conditions may have different rheology when mixed with water to make a paste (Hu et al., 2020), which may affect the print performance; however, this has not been studied in detail. In this study, potato pastes were made using potato powders with different production conditions and the effects on the printability and physical properties of potato pastes were investigated.
Test materials Commercially available potatoes (cv. Mayqueen) were obtained from Hokkaido Prefecture, Japan. The potatoes were stored at 20 °C for up to a maximum of 18 h prior to the experiments.
Procedure for making potato powder The potatoes were peeled and diced to approximately 10 mm cubes. Subsequently, the diced potatoes were dipped in water at 25 °C for 10 min and were treated by hot water blanching at approximately 100 °C for 2 min or 4 min. Potatoes without blanching treatment were also prepared. The diced potatoes with or without blanching treatment were dried at 60 °C for 18 h. After drying, the potatoes were stored in plastic bottles with desiccant for use during milling tests.
The milling was conducted using two methods to produce potato powders of different particle sizes. A hammer mill (1018-S-3, Yoshida Seisakusho Co. Ltd., Tokyo, Japan) is an impact grinder widely applied to food materials used for milling. In the hammer mill, material to be processed is transferred to the milling chamber where 12 movable hammers with attached disks are rotated at 8 000 rpm, and the supplied materials are milled by collision with hammers and the lining. The milled materials were passed through stainless steel screens with specified mesh opening size to sample containers. Screens with mesh opening sizes of 0.5, 1.0, and 2.0 mm were used to make potato powders with various particle sizes. These mesh sizes were within the range for stable powder production; below this size, clogging occurred. The feed rate was kept at 6 kg/h. A jet mill (IDS-2, Nippon Pneumatic Mfg. Co. Ltd., Osaka, Japan) was used to make super-fine potato powders. The jet mill is an effective grinding method for obtaining super-fine powders in a dry process. In the installation, compressed air is sprayed to produce airflow with high velocity. The supplied materials were accelerated and collided between materials and the collision plate. The fine particles were transported to a sample container through a classifier. The internal pressure of the nozzle and the distance to the impact plate were kept at 0.45 MPa and 92 mm, respectively. The feed rate of dried potatoes was 6 kg/h.
Measurements of particle size distribution Particle size was determined by a laser diffraction particle size analyzer (LS 13 320, Beckman Coulter Co. Ltd., Brea, CA). The measurement was carried out using a dry powder system module, and the mean particle diameter (VMD: volume mean diameter) was obtained. The refractive index of the sample powder was assumed as 1.60.
Evaluation of flowability Carr's flowability index (CFI) was calculated to evaluate the flowability of potato powders based on aerated density, packed density, compressibility, angle of repose, angle of the spatula, and uniformity using a powder property tester (MT-1001, Seishin Enterprise Co. Ltd., Tokyo, Japan). Samples were collected in 100 mL containers through a sieve with a mesh opening of 250–355 µm. Powder was removed from the top of the container, and the aerated density was calculated. The packed density was then obtained by adding the sample to a container fitted with an attachment to tap it 180 times at a drop height of 18 mm. The compressibility was calculated using the aerated density and packed density. The powder was allowed to fall naturally through a sieve with a mesh opening of 250–355 µm to the center of a circular table with a diameter of 80 mm, and the angle of the cone formed by the deposition was measured as the angle of repose. The sample was deposited on a 22 mm x 105 mm spatula, and after the spatula was gently raised upward, the tilt angle of the sample on the spatula was measured to obtain the spatula angle. The uniformity was obtained from the results of particle size measurements by calculating the ratio of D60 to D10, where D60 and D10 are the diameters of particles that are finer than 60% and 10% of cumulative particle size distributions, respectively. The CFI was calculated by summing the scores for compressibility, angle of repose, spatula angle, and uniformity; the higher the score, the better the flowability.
Evaluation of applicability of 3D food printer A 3D food printer (FP-2500, Seiki Co. Ltd., Yamagata, Japan) based on a screw-type extruder was used to evaluate applicability for printing. Paste samples were prepared by mixing potato powders and distilled water. The mixing was manually carried out for 5 min. For powders without blanching treatment, the moisture content of the paste was adjusted to 52, 55, and 58% w.b., while the moisture content of the samples with blanching treatment was matched to 70, 73, 75, and 78% w.b. This moisture content was selected based on the results of preliminary tests to explore the moisture ranges of the pastes that can be formed using a 3D printer. A 3D model of a hollow cylinder (the cylinder outer diameter was 2.0 cm and height was 2.0 cm with an inner diameter of 1.0 cm) was created using free software of 3DCAD (FeeCAD). The modeling data were processed using freely available slicer software (Slic3r) and exported to G-code to regulate the motion of the 3D food printer. A nozzle with a 2.0 mm diameter was used for printing, and the distance between the nozzle and the printing stage was set at 1.0 mm. The temperature of potato paste was kept at room temperature (approximately 25 °C). The extrusion multiplier was set at 4.0, and the printing speed was 15.0 mm/s. Printability was evaluated based on appearance and the difference in bottom diameters between the designed model and the printed object.
Analysis of physical properties of potato pastes Potato pastes prepared under the same conditions as those in the print performance tests mentioned above were used to measure the physical properties. The physical properties were analyzed using a creep meter (Rheoner II, Yamaden Co. Ltd., Tokyo, Japan) to determine the hardness according to Sagawa et al. (2008) with some modifications. The potato pastes were filled into a stainless-steel Petri dish with a diameter of 42 mm and a height of 13.5 mm, and compression tests were conducted using a creep meter. For the measurement, an acrylic resin plunger with a diameter of 20 mm and a height of 8 mm was used to compress the sample with a clearance of 5 mm and a compression speed of 1 mm/s. The measurements were taken independently three times, and the average hardness value was obtained.
Particle sizes of potato powders The particle size distributions of the potato powders are shown in Fig. 1. The powders without blanching processes milled by the hammer mill with 1.0 and 2.0 mm mesh opening screens exhibited a large peak and two small peaks corresponding to coarse particles (Fig. 1(a)). Conversely, the peaks of coarse particles were not observed in powders from blanched potatoes (Fig. 1(b) and Fig. 1(c)), and the shapes of particle size distribution curves were close to a log-normal distribution. The blanching treatment by dipping potatoes into boiled water was expected to have partially induced gelatinization of potato starch, and the gelatinization caused different physical properties and milling characteristics of dried potatoes that varied with blanching conditions. Other causes were speculated to be that the proteins and lipids in the potato powders were affected by heat treatment (Hallberg and Lingnert, 1991; do Nascimento and Canteri, 2018), and the milling characteristics were consequently changed. In comparison with the hammer mill, the powders milled by the jet mill were finer, the distribution had a single peak, and the shape was almost close to a log-normal distribution. The VMD of potato powders under different blanching conditions and milling methods is shown in Fig. 2. The VMD of the powders milled by the hammer mill were in the range of 38–143 µm. Moreover, larger sizes of screen mesh openings resulted in coarser powders. The VMD of powders milled by the jet mill was 11–18 µm. The particle size of the hammer milled powder without blanching was 38–90 µm, and those of the powders with blanching for 2 and 4 min were 76–117 µm and 73–143 µm, respectively. Thus, the powders from blanched potatoes had larger particle size than those from unblanched potatoes. The blanching treatment prior to drying and milling procedures affected particle size, and blanching for a longer time made the potato powder particle size larger. Therefore, the blanching conditions in addition to the milling method must be designed to produce particle potato powders with the desired size.
Particle distribution of potato powders. The dried potatoes with or without blanching treatments (hot water dipping for 0 min (a), 2 min (b) and 4 min (c)) prior to drying were milled by the jet mill (1) and the hammer mill. The screen mesh opening sizes of the hammer mill were 0.5 mm (2), 1.0 mm (3), and 2.0 mm (4).
Volume mean diameter of potato powders. The potato powders were produced by jet mill (1) and hammer mill. The screen mesh opening sizes of the hammer mill were 0.5 mm (2), 1.0 mm (3), and 2.0 mm (4).
Flowability of potato powders The aerated and packed density of potato powders are shown in Fig. 3. The aerated and packed densities increased with increasing particle size. Furthermore, fine particles produced by the jet mill exhibited remarkably lower aerated and packed densities than those milled by the hammer mill. Azizi et al. (2020) reported a reduction in the aerated density of potato flour from boiled potatoes compared with that of potato flour from raw potatoes. In this study, it was difficult to closely examine the effect of the blanching treatment on the aerated density because the particle sizes were affected by whether the raw potatoes were blanched or not. Azizi et al. (2020) suspected that boiling caused the loss of soluble components and resulted in a decrease in density. Additionally, hot water treatment of raw potatoes might affect their subsequent milling properties. The compressibility was calculated using the ratio of aerated density and packed density, as shown in Fig. 4. The compressibility was decreased linearly according to the increase in particle size, and particle size was the main factor affecting the compressibility of potato powders. The compressibility of jet mill powders was 31–34%. These values corresponded to 7–10 points of CFI, which indicates that flowability is “poor” or “very poor”. Therefore, measures, such as granulation, will be required, depending on the 3D food printer system.
Aerated and packed density of potato powders. The blanching prior to drying and milling were carried out at 100 °C for 0 min (circle symbols), 2 min (square symbols) and 4 min (triangle symbols).
Compressibility of potato powders. The blanching prior to drying and milling were carried out at 100 °C for 0 min (circle symbols), 2 min (square symbols) and 4 min (triangle symbols).
The angles of repose and spatula are shown in Fig. 5. The angles of repose and spatula were in the range of 36.2–47.2 degrees. The values were comparable to those reported by Stasiak et al. (2013). The angles increased with a decrease in particle size, which implies a decrease in flowability. Overall, the angles of repose for blanched samples were smaller than those of powders without blanching. One of the possible causes is that powders with blanching processes are considered to include gelatinized starches (Zhang et al., 2020), which affect the adhesive strength between particles and result in the difference between the angle of repose and the angle of spatula. However, further examination is required to clarify the influence of processing procedures on the flowability of powders, including detailed analysis of constituent components such as starches, proteins and lipids with consideration of differences among cultivars and cultivation conditions.
Angles of repose and spatula of potato powders. The blanching treatments prior to drying and milling were carried at 100 °C for 0 min (circle symbols), 2 min (square symbols) and 4 min (triangle symbols).
The uniformity of potato powders ranged from 3.0 to 4.9, and no significant differences among powders differentially fabricated (data not shown). The CFI of potato powders is shown in Fig. 6. The index indicates flowability and is estimated from the aerated density, packed density, compressibility, angle of repose, angle of spatula, and uniformity. A larger CFI indicates better flowability. The CFI decreased with decreasing particle sizes; however, there were no significant differences for particle sizes between 35 m and 143 µm. The powders milled by hammer mill had index values in the range of 68–73 points, which indicates that flowability is “passable–fair”. In this range, powder clogging and lumpiness are limited, and no countermeasures might be required when using the powders for food processing machines, including a certain type of 3D food printer. Conversely, the CFI of powders milled by the jet mill was in the range of 53–60 points. Thus, flowability is evaluated as “poor–passable”, and powder clogging and lumpiness are a concern; therefore, measures such as vibration and agitation are required to prevent any hindrances in manufacturing. As an alternative, the granulation process after milling would be an effective measure to improve powder flowability.
Flowability of potato powders. The blanching treatments prior to drying and milling were carried out at 100 °C for 0 min (circle symbols) 2 min (square symbols) and 4 min (triangle symbols).
Applicability of 3D food printers The appearance of potato paste formed by the 3D food printer is shown in Table 1. Based on results of preliminary tests, we determined the moisture range of potato paste that could be used with the printer. The potato pastes produced without blanching were formed in the range of 52–58% w.b. moisture contents. Conversely, suitable moisture content for blanched potato paste was in the range of 70–78% w.b. Thus, the suitable moisture range for 3D printing was significantly lower for potato paste made without blanching compared to that of the potato paste from blanched samples. The blanching process caused the potato starch to be partially gelatinized (Zhang et al., 2020), which might be one of the reasons why the powder became more viscous when mixed with water. Even under the same moisture conditions, the possibility of molding differed depending on particle size. For example, when potato paste with blanching at 100 °C for 4 min was adjusted to 70% moisture content, extrusion was possible with small particle size powders produced by jet, but not with coarse particles produced by hammer mills. Therefore, moisture content and particle size need to be considered when finding suitable paste conditions for 3D food printers. The model was designed with a diameter of 2 cm, but the actual diameters of the molded products ranged from 1.9 to 3.3 cm (data not shown) and tended to be larger than the design in most conditions. The reason for this result was presumed to be that the potato paste was used as ink for the printer, and the weight of the molded product caused the base to enlarge. Some foods, such as chocolate, cookie dough, and rice starch, can be formed without additives (Pérez et al., 2019; Theagarajan et al., 2020), and potato powder can also be formed under suitable moisture conditions. However, it is expected that using an effective gelling agent, such as agar, will allow the fabrication of modeling objects similar to design objects. In a previous study, Liu et al. (2018) reported that adding potato starch to mashed potatoes allowed the printed structure to maintain a smooth and precise shape. Feng et al. (2018) reported that using potato starch with pea protein improved the quality of the printed structures. Further studies are required on the applicability of various gelling agents, considering their effect on the texture of the final product. Another reason for the insufficient molding accuracy is that the nozzle might have pressed against the material from the upper direction during layering. Severini et al. (2016) reported that when the layering height exceeds an optimum value, the diameter of the formed object increases. Thus, printer settings, such as nozzle diameter, moving speed, extrusion speed, layering height, and the distance between the nozzle and the stage have a significant impact on the forming performance (Wang et al., 2017; Yang et al., 2018). Optimizing the printer settings depending on the materials is an effective measure for improving printability.
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a: Hammer mill A, B and C indicate the mill with screen mesh opening sizes of 0.5 mm, 1.0 mm and 2.0 mm, respectively. The volume mean diameter of each test condition is shown in Fig. 2.
Effect of blanching conditions and particle sizes on hardness of potato pastes The hardness of pastes made from potato powder under different blanching and milling conditions with various moisture contents is shown in Fig. 7. When the moisture content of the paste made from the unblanched powder was adjusted to 52–58% w.b., the hardness ranged from 2.5 to 18.9 kPa, and the hardness decreased as the moisture content increased. For the blanched powders, the maximum hardness was 51.8 kPa, which was higher than that of the paste without blanching, even though the moisture content of the paste increased to 70–78% w.b. The starches, proteins, and lipids affected by the blanching process may have altered the hardness of the paste (Carillo et al., 2009; Zhang et al., 2020). However, the contribution of each of these factors is unknown and further investigation is needed. For the blanched potato paste, the hardness of the paste differed based on the particle size even for the same moisture conditions, and the paste tended to be less firm with smaller particle size powders ground by the jet mill and the hammer mill (mesh of 0.5 mm). Potato powders with smaller particle sizes are expected to have a higher degree of starch damage. The degree of starch damage had a strong influence on the gelatinization viscosity, and the difference in gelatinization viscosity was assumed to cause of the difference in paste hardness. The texture of 3D printed food is an important factor that affects quality when consumption is the intended use. When designing 3D printed food products that meet consumer preferences, it is necessary to consider the moisture content and the conditions of powder preparation, including blanching conditions and target particle size. In particular, the relationship between the conditions for preparation of the powder and the texture of the food after heating need to be studied, assuming that the food will be cooked.
Hardness of potato pastes at various water contents. The blanching prior to drying and milling were carried out at 100 °C for 0 min (a), 2 min (b) and 4 min (c). The potato powders were produced by the jet mill (rhombus symbols) and the hammer mill. The screen mesh opening sizes of the hammer mill were 0.5 mm (circle symbols), 1.0 mm (triangle symbols), and 2.0 mm (square symbols).
Three-dimensional food printers are attractive tools that enable the manufacturing of complex structures that are difficult to produce by other methods. The quality of printer ink made of food powder/pastes is an important factor influencing the precision of 3D food printer outputs. The present study examined the flowability of potato powders and the viscosity of potato pastes manufactured using different procedures to obtain knowledge for designing high-quality inks for 3D food printers. The flowability of potato powders depended on particle size, and smaller particle sizes reduced the flowability. Therefore, some countermeasures, such as granulation, are required for fine powders in order to improve the operationality. The printability of potato pastes made by mixing powders and water was affected by blanching conditions and particle sizes. Accordingly, optimization of fabrication procedures is recommended to make high-quality inks for 3D printing.
Acknowledgements This work was supported by Cabinet Office, Government of Japan, Moonshot Research and Development Program for Agriculture, Forestry and Fisheries (funding agency: Bio-oriented Technology Research Advancement Institution), Grant Number JPJ009237.
Conflict of interest There are no conflicts of interest to declare.
