# Rheology-Based Classification of Foods for the Elderly by Machine Learning Analysis
## 3 Results and Discussion  
Figure 1 shows the experimental setup for measuring the hardness of the gel samples prepared with different levels of mung bean starch. As can be seen in Figure 1a, the UDF test allowed a probe to extrude into a food sample that was squeezed out between the probe and the inside wall of the container. Figure 1b illustrates a puncture test where a rod-shaped probe penetrated into the food sample. In a puncture test, there is a general rule called \"semi-infinite geometry\" [26], according to which the sample needs to be larger in diameter than the probe used, since shear force and compressive force are simultaneously involved. Since the size of the probe in the puncture test is relatively small, this test has the advantage of being universally applied to a variety of foods regardless of their shape and dimension [27]. In the cutting-shear (Figure 1c) and compression (Figure 1d), the sample was cut into two pieces using a blade and compressed with a probe larger in diameter than the sample, respectively. As shown in Figure 1, the highest values of hardness were observed in the UDF test, followed by the compression, cutting-shear, and puncture tests. This order seemed to be related to the probe area in contact with the gel samples. In addition, for all the measurements, the hardness values increased with increasing levels of starch in a highly linear way (R\({}^{2}\) > 0.94). This result was favorably compared with that reported by [PERSON], [PERSON], [PERSON], and [PERSON] [27], which showed a highly positive correlation between the tests with rod and cylinder probes for measuring the rheological properties of cooked instant rice. Figure 2a exhibits the relationships between the UDF and each texture method, showing fairly linear correlations. Specifically, the highest coefficient of determination (R\({}^{2}\) = 0.9885) was observed between the UDF and puncture tests. As a fundamental rheological term, stress indicates the ratio of force to area [28]. As shown in Figure 2b, the relationships were investigated in terms of stress (N/m\({}^{2}\)) instead of force (N). The results obtained from the cutting-shear test were excluded in Figure 2b due to the unavailability of the contacting area of the cutting probe used. As can be seen in Figure 2b, the puncture test appeared to be highly inter-correlated with the UDF test. Thus, out of the three rheological methods, the puncture test was selected and further investigated as an alternative method to the UDF test. Figure 1: Experimental setup of rheological measurement of starch gels and their hardness values (Universal design food (UDF) test (**a**), puncture test (**b**), cutting-shear test (**c**), and compression test (**d**)). Figure 2: Correlations between UDF and other texture-measuring methods for starch gels (hardness (**a**) and stress (**b**)). A total of 59 commercial food samples were selected and then subjected to both UDF and puncture tests. Figure 3 presents the stress values of the food samples measured by the UDF and puncture tests. A fairly linear correlation (y = 0.9982x, R\({}^{2}\) = 0.9772) was observed between the two tests for the samples with the stress values lower than that of the starch gels tested in this study (8 \(\times\) 10\({}^{5}\) N/m\({}^{2}\)). On the other hand, when the stress values became higher, the relationship between the UDF and puncture tests was well-fitted to the quadratic polynomial model (y = \(-\)2 \(\times\) 10\({}^{-4}\)x\({}^{2}\) + 1.0836x, R\({}^{2}\) = 0.9706). Several previous studies have classified foods for the elderly based on rheological measurements. The Japan Care Food Conference [9] and Korean Industrial Standards [29] reported that the upper stress limit of the foods for the elderly was 500,000 N/m\({}^{2}\). In addition, [PERSON] et al. [30] compared instrumental and sensory measurements of foods for the elderly, suggesting the stress range lower than 500,000 N/m\({}^{2}\).