Microwave drying and NIR spectroscopy for the rapid moisture measurement of yerba mate (Ilex paraguariensis) leaves during storage

Figures & data

Table 1. ANOVA for leaves/sticks. Dependent variable: moisture (g/100 g).

SOURCE	DF	SQ	QM	F-statistic	p-Value
Corrected model	47	303.080^a	6.449	42.94	0.000
Linear coefficient	1	2671.900	2671.900	17791.96	0.000
Method	1	0.919	0.919	6.12	0.015
Time	5	88.005	17.601	117.20	0.000
Temperature (T)	1	21.975	21.975	146.33	0.000
Relative humidity (RH)	1	142.204	142.204	946.93	0.000
Method*Time	5	10.597	2.119	14.11	0.000
Method*Temperature	1	0.085	0.085	0.57	0.454^b
Time*Temperature	5	6.536	1.307	8.70	0.000
Method*RH	1	0.017	0.017	0.11	0.739^b
Time*RH	5	11.367	2.273	15.14	0.000
Temperature*RH	1	3.461	3.461	23.05	0.000
Method*Time*T	5	6.733	1.347	8.97	0.000
Method*Time*RH	5	0.820	0.164	1.09	0.370^b
Method*T*RH	1	0.979	0.979	6.52	0.012
Time*T*RH	5	6.939	1.388	9.24	0.000
Method*Time*T*RH	5	2.443	0.489	3.25	0.009
Error	96	14.417	0.150
Total	143	317.497

Note: ^aR² = 0.955 (adjusted R² = 0.932); ^bnot significant at 95% confidence.

Table 2. ANOVA for leaves. Dependent variable: moisture (g/100 g).

SOURCE	DF	SQ	QM	F-statistic	p-Value
Corrected model	47	412.748^a	8.782	54.38	0.000
Linear coefficient	1	2762.677	2762.677	17106.37	0.000
Method	1	12.566	12.566	77.81	0.000
Time	5	276.001	55.200	341.80	0.000
Temperature (T)	1	25.846	25.846	160.04	0.000
Relative humidity (RH)	1	45.519	45.519	281.85	0.000
Method*Time	5	17.190	3.438	21.29	0.000
Method*Temperature	1	0.002	0.002	0.01	0.912^b
Time*Temperature	5	3.693	0.739	4.57	0.001
Method*RH	1	0.835	0.835	5.17	0.025
Time*RH	5	13.187	2.637	16.33	0.000
Temperature*RH	1	1.874	1.874	11.60	0.001
Method*Time*T	5	0.524	0.105	0.65	0.663^b
Method*Time*RH	5	8.491	1.698	10.51	0.000
Method*T*RH	1	0.420	0.420	2.60	0.110^b
Time*T*RH	5	2.799	0.560	3.47	0.006
Method*Time*T*RH	5	3.801	0.760	4.71	0.001
Error	96	15.504	0.161
Total	143	428.252

Note: ^aR² = 0.964 (= 0.946); ^bnot significant at 95% confidence.

Figure 1. Pearson correlation between moisture content of yerba mate determined by the official method (kiln) and microwave for samples of leaves/sticks (solid line, ▲) and leaves (dotted line, ●).

Table 3. PLS models for moisture content prediction in yerba mate in comparison with the official method and microwave.

Methods and conditions	Pretreat	LVs	Calibration		Cross validation
			R^2	RMSEC	R^2	RMSECV
Official (Kiln) 105°C/± 6 h	First derivative	5	0.9754	0.7942	0.9622	0.9847
Microwave 900 W/4.3 min	MSC + First derivative	5	0.9730	0.7907	0.9584	0.9799

Note: Pretreat = pretreatments; LVs= latent variables; MSC = master scattering correction. obtained values for RMSEC and RMSECV and their respective R² are similar for microwave and kiln, so they are presented with 4 decimal places.

Figure 2. PLS prediction of moisture content in yerba mate (leaves/sticks and leaves) by kiln drying (a) and microwave (b).

Figure 3. Moisture content during storage time of yerba mate leaves/stick (a) in conditions of NCC (18°C/81 RH (■)) and ACC (45°C/31 RH (▲)). Moisture content during accelerated storage in laboratory: (b) yerba mate leaves/sticks and (c) leaves: 32°C/50 % RH (■), 48°C/50 % RH (●), 32°C/10 % RH (▲), 48°C/10 % RH (▼), and 40°C/30 % RH (×).