STRESS RELAXATION IN COOKED POTATO TUBERS EXPRESSED BY IMPROVED RATE CONTROLLED MODEL

Figures & data

Figure 1. Typical f-functions determined directly for both the varieties after 5 per cent pre-strain. The best model approximations of the data by the sigmoid source f-function are also plotted in the figure (SFC: 4.33% – ‘Nicola’, 7.58% – ‘Panda’).

Table 1. Mean Values of Model Parameters (Sigmoid Model), LSD – Least Significant Difference

Variety	Strain Level (%)	Strain Rate (mm s^−1)	σ0t (MPa)	SFC (%)	K (Pa s^−1)	C 2σ ot (−)	C 1σ0t ^n (−)	N (−)	σ it /σ0t (−)	σ it (MPa)
Nicola	5	0.167	0.0232	7.52	2.54	2.5	8.27	2.28	0.169	0.0061
Nicola	10	0.167	0.0858	7.13	16.13	2.5	6.62	2.41	0.106	0.0092
Nicola	15	0.167	0.1299	2.58	42.73	2.5	6.15	2.66	0.139	0.0180
Nicola	15	0.833	0.1493	12.49	28.83	2.5	9.05	2.01	0.118	0.0191
Panda	5	0.167	0.0236	6.22	2.14	2.5	7.53	3.42	0.188	0.0046
Panda	10	0.167	0.1043	5.01	31.52	2.5	6.35	3.79	0.228	0.0244
Panda	15	0.167	0.1662	6.34	36.90	2.5	7.29	2.87	0.145	0.0244
Panda	15	0.833	0.1878	6.09	77.49	2.5	7.58	3.39	0.185	0.0366
Anova and Fisher's		Variety	*					*	*	*
LSD protected test (p = 0.05)		Strain Level	*	*	*		*		*	*
		Strain Rate	*				*
		LSD	0.0170	2.01	9.43	–	0.475	0.35	0.027	0.0286

Figure 2. The model sigmoid source f-functions calculated using the mean values of the model parameters (Table 1). Symbols at variety denote test details (the pre-strain level: 05 (5%), 10 (10%), 15 (15%), the pre-strain rate S (Slow −0.167 mm s⁻¹), Q (Quick −0.833 mm s⁻¹). a. f-functions for 5% and 10% pre-strain levels, b. f-functions for 5% and 10% pre-strain levels.

Figure 3. Parameter K′ = K/σ_0t plotted against the pre-stress σ_0t . Symbols S and Q at variety title denote pre-strain rates 0.167 mm s⁻¹ and 0.833 mm s⁻¹, respectively.

Figure 4. Parameter C ₁′ = C ₁σ_0t ⁿ plotted against pre-stress. The experimental points were approximated by polynomial of the second order.

Figure 5. Parameter N(MV ± SD) plotted against the pre-stress value.

Figure 6. Activation volume calculated using Eq. 5 for stresses 0.5σ_0t , 0.6σ_0t , 0.7σ_0t , 0.8σ_0t , 0.9σ_0t , and σ_0t is plotted against stress level for all combinations of the test parameters (two varieties, different pre-strain levels, T = 293 K). For symbols see Figure 2.

Figure 7. Ratio of the activation volumes for cooked potato tubers and for raw potato tissue 6 calculated from the model parameters as determined at the same test parameters using Eq. 5. The calculated ratio is plotted against the relative stress (ratio of the actual stress to the pre-stress value). The values obtained at 5% pre-strain were approximated by power formula.

Table 2. Parameters of Power Relation Between Ratio of Activation Volumes (for Cooked and Raw Potato Tubers) and Relative Stress AVR = a.RS^m (see Figure 7)

	a (−)		m (−)
Test Characteristics	Regression (Figure 7)	Eq. 6	Regression (Figure 7)	Eq. 6
Nicola 05	1.70	1.57	−3.04	−3.54
Nicola 10	0.91	0.81	−4.13	−4.97
Nicola 15S	1.07	1.00	−3.01	−3.36
Nicola 15Q	0.82	0.80	−3.53	−3.53
Panda 05	1.03	1.02	−0.48	−0.50
Panda 10	0.89	0.81	−2.43	−3.05
Panda 15S	0.84	0.77	−3.17	−3.71
Panda 15Q	0.78	0.78	−2.45	−2.36

Figure 8. Ratio of the f (σ) values of ‘Panda’ and ‘Nicola’, as obtained at the same stress, plotted against the relative stress in ‘Nicola’. The data obtained at 10% and 15% pre-strains are approximated by exponential equations.