Mechanical properties and microstructure of apple peels during storage

Figures & data

Figure 1. Tensile specimen (left) and tensile test of apple peel (right).

Figure 2. Shear test of fresh peel.

Figure 3. Stress–strain curve of the tensile tests of fresh peel.

Table 1. Curve fitting coefficients of fresh peels under tensile loading.

Breed	Sample number	Sunlit longitudinal			Shadow longitudinal			Sunlight transverse			Shadow transverse
Danxia	1	89.35	–74.29	23.51	10.35	–26.95	16.40	–105.6	1.93	15.14	–274.3	–6.58	16.34
	2	–11.62	39.07	20.25	–111.6	0.09	15.49	11.71	–24.42	14.38	59.48	–52.67	18.83
	3	–279.3	5.89	20.65	–12.38	–22.47	15.16	104.98	–66.5	18.72	102.16	–69.53	20.43
	4	–558.4	75	16.79	40.15	–64.42	23.22	26.20	–43.99	17.36	44.15	–40.99	15.38
	5	–89.59	–15.35	17.92	–47.70	–19.34	17.26	–686.8	87.78	15.86	47.44	–48.65	18.07
	6	19.60	–54.06	20.76	–5.48	–40.26	16.96	–127.4	–10.62	18.37	–894.1	149.04	10.48
Starkrimson	1	202.36	–111.5	26.83	181.38	–101	24.55	–17.30	–35.36	20.80	184	–116.5	28.48
	2	–671.8	109.12	15.54	–228.6	13.34	19.04	–16.89	–33.14	18.77	–82.99	19.73	19.73
	3	–70.20	–10.53	18.01	–261.8	–14.00	24.37	91.63	–65.55	20.44	52.19	–53.58	18.73
	4	–344.9	47.05	18.12	–271	–4.36	23.07	0.42	–61.41	23.66	–140.4	–19.44	19.51
	5	–570.4	111.27	11.89	–23	–58.99	24.67	–285.4	24.49	17.96	–36.39	–29.48	18.53
	6	–351.5	5.10	23.48	7.16	–42.77	19.32	34.02	–75.67	24.77	–185.2	–8.11	19.80

Table 2. Experimental value and fitted error of fresh peels under tensile loading.

Parts	Fracture strain	Tensile strength (σmax)			Elastic modulus (Emax)
		Experimental value (MPa)	Fitted value (MPa)	Fitting error	Experimental value (MPa)	Fitted value (MPa)	Fitting error
Sunlit longitudinal
Danxia	0.18 ± 0.03a	2.28 ± 0.18b	2.27 ± 0.17	0.0027	21.12 ± 1.39b	20.54 ± 1.78	0.0443
Starkrimson	0.19 ± 0.03a	2.64 ± 0.14a	2.58 ± 0.14	0.0052	24.46 ± 2.95a	22.82 ± 2.62	0.0656
Shadow longitudinal
Danxia	0.20 ± 0.02a	2.19 ± 0.29a	2.18 ± 0.28	0.0027	18.78 ± 1.74b	18.27 ± 1.55	0.0510
Starkrimson	0.19 ± 0.02a	2.45 ± 0.13a	2.41 ± 0.10	0.0064	24.09 ± 2.23a	22.81 ± 2.36	0.0537
Sunlight transverse
Danxia	0.22 ± 0.06a	2.20 ± 0.15b	2.19 ± 0.16	0.0042	17.92 ± 2.62b	17.42 ± 2.96	0.0401
Starkrimson	0.21 ± 0.05a	2.42 ± 0.18a	2.41 ± 0.19	0.0067	21.46 ± 2.24a	20.92 ± 2.35	0.0475
Shadow transverse
Danxia	0.23 ± 0.07a	2.09 ± 0.36a	2.09 ± 0.39	0.0040	17.41 ± 2.70a	17.21 ± 2.54	0.0563
Starkrimson	0.21 ± 0.03a	2.26 ± 0.12a	2.24 ± 0.14	0.0071	21.25 ± 3.57a	20.66 ± 3.72	0.0448

Fitting error of tensile strength indicated the relative error of tensile strength of peel between fitted values and experimental values. Fitting error of elastic modulus indicated the relative error of highest elastic modulus of peel between fitted values and experimental values.

Figure 4. The polynomial fitting curves of Danxia and Starkrimson tensile stress–strain.

Figure 5. Force–deformation curve of shear test of fresh peel.

Figure 6. Fracture sample of fresh peel.

Table 3. Means and standard errors of shear strength of apple peels.

Breed	Sunlit shear strength			Shadow shear strength
	Mean ± SD (MPa)	Coefficient variation	Duncan group	Mean ± SD (MPa)	Coefficient variation	Duncan group
Danxia	3.38 ± 0.39	0.12	b	3.00 ± 0.45	0.15	b
Starkrimson	4.37 ± 1.23	0.28	a	4.03 ± 0.72	0.18	A

Figure 7. Changes of mechanical parameters during storage of Danxia and Starkrimson peels.

Figure 8. Cross-section microstructure of fresh peels observed using SEM: A: Danxia sunlit side; B: Danxia shadow side; C: Starkrimson sunlit side; D: Starkrimson shadow side.

Table 4. The parameters of microstructure of peel.

	Danxia	Starkrimson	Danxia	Starkrimson
Parameters	Range (min–max)		Mean ± standard deviation		p-Value
Thickness of cuticle layer (μm)	(8.18–34.33)	(10.37–18.52)	12.29 ± 2.77	12.86 ± 2.05	0.0001
Length of epidermal cell (μm)	(11.21–27.61)	(12.53–24.00)	17.72 ± 5.48	16.30 ± 3.68	0.2899
Width of epidermal cells (μm)	(4.46–15.33)	(5.98–12.38)	8.41 ± 3.05	8.48 ± 1.59	0.8768
Aspect ratio of epidermal cells	(1.04–4.67)	(1.49–2.43)	2.11 ± 0.61	1.93 ± 1.18	0.0001
Epidermal cells space (μm)	(3.35–17.29)	(5.16–10.89)	7.70 ± 3.00	7.42 ± 1.74	0.1723

Figure 9. The fracture morphology of fresh peel measured using SEM.

Figure 10. The surface microstructure of peels during storage: A: Danxia sunlit side; B: Danxia shadow side; C: Starkrimson sunlit side; D: Starkrimson shadow side.