Detection of fresh palm oil adulteration with recycled cooking oil using fatty acid composition and FTIR spectral analysis

Figures & data

Table 1. Comparison of fatty acid compositions between fresh palm olein (FPO), recycled cooking oil (RCO), and Codex standard (as a percentage of total fatty acids).

Fatty acids	FPO	RCO	Codex Alimentarius^c
C8:0	-	0.03 ± 0.02	N/A
C10:0	-	0.02 ± 0.01	N/A
C11:0	-	0.02 ± 0.00	N/A
C12:0	0.21 ± 0.04^a	0.21 ± 0.02^a	0.1–0.5
C14:0	0.92 ± 0.05^a	1.06 ± 0.04^b	0.5–1.5
C15:0	0.04 ± 0.01^a	0.06 ± 0.00^a	ND^d
C16:0	43.09 ± 4.64^a	40.63 ± 0.24^a	38.0–43.5
C16:1	0.15 ± 0.06^a	0.10 ± 0.01^b	ND-0.6
C17:0	0.10 ± 0.01^a	0.11 ± 0.00^a	ND-0.2
C17:1	0.02 ± 0.01^a	0.08 ± 0.03^a	ND-0.1
C18:0	4.92 ± 0.62^a	4.33 ± 0.17^a	3.5–5.0
C18:1 (trans)	-	0.09 ± 0.01	N/A
C18:1 (cis)	39.85 ± 4.26^a	43.85 ± 0.16^a	39.8–46.0
C18:2	9.91 ± 0.95^a	8.40 ± 0.18^a	10.0–13.5
C18:3	0.20 ± 0.06^a	0.26 ± 0.09^a	ND-0.6
C20:0	0.38 ± 0.04^a	0.25 ± 0.06^a	ND-0.6
C20:1	0.22 ± 0.02^a	0.12 ± 0.03^b	ND-0.4
C20:5	-	0.05 ± 0.03	N/A
Saturated fatty acids (SFAs)	49.67 ± 5.28^a	46.73 ± 0.08^a	45.9–51.3
Unsaturated fatty acid (USFAs)	50.33 ± 5.27^a	53.27 ± 0.08^a	49.8–61.2
Monounsaturated (MUFAs)	40.23 ± 4.30^b	44.57 ± 0.05^a	39.8–47.1
Polyunsaturated (PUFAs)	10.10 ± 0.96^a	8.71 ± 0.12^a	10.0–14.1
Odd-chain fatty acids (OCFAs)	0.16 ± 0.03^b	0.27 ± 0.03^a	ND-0.3

- indicated not detected in present study; ND, non detected; N/A, not available in the reference standard

^cAdapted from the Joint Food and Agriculture Organization of the United Nations/World Health Organization. ^[21]

^dJoint Food and Agriculture Organization of the United Nations/World Health Organization.^[22]

Rows marked with different lower case letters (a-b) show that means are significantly different (p < 0.05) with each other.

Figure 1. The gas chromatograms of the fatty acid composition analyses of (a) fresh palm oil, (b) recycled cooking oil, and (c) adulterated oil.

(continue).

(continue).

Table 2. Comparison of fatty acid compositions between fresh palm olein, recycled cooking oil, and adulterated oil samples with different adulteration levels (expressed as percentage of total fatty acids).

Fatty acids	Adulteration level (FPO:RCO)
	100:0 (FPO)	99:1	98:2	97:3	95:5	90:10	85:15	80:20	70:30	60:40	50:50 (excluded)	0:100 (RCO)
C8:0	-	-	-	0.02 ± 0.01	-	0.02 ± 0.02	0.16 ± 0.23	0.01 ± 0.01	-	0.02 ± 0.00	-	0.03 ± 0.02
C10:0	-	0.01 ± 0.00	-	-	-	-	0.22 ± 0.22	-	-	0.01 ± 0.00	-	0.02 ± 0.01
C11:0	-	0.01 ± 0.01	0.01 ± 0.01	0.01 ± 0.01	0.01 ± 0.00	0.01 ± 0.01	0.10 ± 0.15	0.01 ± 0.01	0.01 ± 0.01	-	-	0.02 ± 0.00
C12:0	0.21 ± 0.04	0.29 ± 0.01	0.23 ± 0.02	0.19 ± 0.03	0.17 ± 0.01	0.17 ± 0.01	0.41 ± 0.27	0.19 ± 0.00	0.18 ± 0.01	0.20 ± 0.01	0.18 ± 0.06	0.21 ± 0.02
C14:0	0.92 ± 0.05	0.98 ± 0.01	0.91 ± 0.01	0.92 ± 0.01	0.90 ± 0.02	0.92 ± 0.01	1.12 ± 0.25	0.93 ± 0.01	1.01 ± 0.01	1.00 ± 0.01	0.75 ± 0.12	1.06 ± 0.04
C15:0	0.04 ± 0.01	0.04 ± 0.00	0.04 ± 0.01	0.03 ± 0.01	0.04 ± 0.00	0.04 ± 0.01	0.13 ± 0.19	0.04 ± 0.00	0.03 ± 0.02	0.03 ± 0.03	0.02 ± 0.03	0.06 ± 0.00
C16:0	43.09 ± 4.64	38.36 ± 0.14	41.16 ± 0.09	37.94 ± 0.22	38.22 ± 0.35	38.97 ± 0.13	36.87 ± 4.07	39.78 ± 0.48	36.69 ± 0.12	39.95 ± 0.23	56.68 ± 0.46	40.63 ± 0.24
C16:1	0.15 ± 0.06	0.19 ± 0.01	0.19 ± 0.04	0.16 ± 0.03	0.20 ± 0.02	0.22 ± 0.03	0.30 ± 0.22	0.19 ± 0.08	0.19 ± 0.08	0.24 ± 0.06	0.09 ± 0.09	0.10 ± 0.01
C17:0	0.10 ± 0.01	0.09 ± 0.01	0.10 ± 0.01	0.08 ± 0.02	0.09 ± 0.01	0.08 ± 0.02	0.15 ± 0.21	0.08 ± 0.01	0.07 ± 0.01	0.09 ± 0.01	0.04 ± 0.07	0.11 ± 0.00
C17:1	0.02 ± 0.01	0.03 ± 0.00	0.08 ± 0.00	0.03 ± 0.01	0.03 ± 0.00	0.07 ± 0.01	0.11 ± 0.15	0.02 ± 0.00	0.01 ± 0.01	0.04 ± 0.01	0.01 ± 0.02	0.08 ± 0.03
C18:0	4.92 ± 0.62	4.28 ± 0.03	4.97 ± 0.06	4.18 ± 0.05	4.21 ± 0.04	4.15 ± 0.05	4.35 ± 0.11	4.22 ± 0.02	5.73 ± 1.55	4.01 ± 0.09	6.05 ± 0.45	4.33 ± 0.17
C18:1 (trans)	-	-	-	0.03 ± 0.01	-	-	-	-	-	-	0.04 ± 0.07	0.09 ± 0.13
C18:1 (cis)	39.85 ± 4.26	44.02 ± 0.08	41.74 ± 0.09	44.67 ± 0.11	44.40 ± 0.24	44.13 ± 0.25	44.30 ± 0.01	43.57 ± 0.39	44.81 ± 1.61	44.20 ± 0.18	29.71 ± 0.92	43.85 ± 0.16
C18:2	9.91 ± 0.95	10.86 ± 0.03	9.79 ± 0.04	10.95 ± 0.07	10.93 ± 0.04	10.74 ± 0.02	10.28 ± 0.46	10.39 ± 0.10	10.64 ± 0.01	9.65 ± 0.15	5.98 ± 0.30	8.40 ± 0.18
C18:3n6	0.02 ± 0.01	0.08 ± 0.09	0.02 ± 0.00	0.02 ± 0.01	0.02 ± 0.01	0.03 ± 0.00	0.42 ± 0.60	0.03 ± 0.01	0.03 ± 0.01	0.02 ± 0.01	0.20 ± 0.35	0.07 ± 0.04
C18:3n3	0.18 ± 0.06	0.18 ± 0.04	0.19 ± 0.09	0.18 ± 0.06	0.17 ± 0.03	0.07 ± 0.01	0.16 ± 0.23	0.05 ± 0.01	0.08 ± 0.03	0.08 ± 0.03	0.03 ± 0.06	0.18 ± 0.05
C20:0	0.38 ± 0.04	0.28 ± 0.01	0.25 ± 0.01	0.25 ± 0.05	0.31 ± 0.02	0.14 ± 0.08	0.54 ± 0.76	0.19 ± 0.01	0.40 ± 0.17	0.10 ± 0.05	0.12 ± 0.21	0.25 ± 0.06
C20:1	0.22 ± 0.02	0.30 ± 0.02	0.30 ± 0.02	0.32 ± 0.02	0.28 ± 0.02	0.16 ± 0.18	0.10 ± 0.14	0.22 ± 0.15	0.11 ± 0.04	0.19 ± 0.12	0.03 ± 0.06	0.12 ± 0.03
C20:5	-	0.01 ± 0.02	0.02 ± 0.03	0.02 ± 0.02	0.01 ± 0.02	0.03 ± 0.05	0.28 ± 0.39	-	-	0.09 ± 0.02	0.05 ± 0.08	0.05 ± 0.03
SFAs	49.67 ± 5.28	44.34 ± 0.09	47.68 ± 0.05	43.62 ± 0.19	43.96 ± 0.32	44.61 ± 0.04	44.05 ± 1.91	45.46 ± 0.43	44.12 ± 1.53	45.49 ± 0.09	63.84 ± 0.59	46.73 ± 0.08
USFA s	50.33 ± 5.27	55.66 ± 0.09	52.32 ± 0.05	56.38 ± 0.19	56.04 ± 0.32	55.39 ± 0.04	55.95 ± 1.91	54.54 ± 0.43	55.88 ± 1.53	54.51 ± 0.09	36.16 ± 0.59	53.27 ± 0.08
PUFAs	10.10 ± 0.96	11.13 ± 0.05	10.01 ± 0.09	11.17 ± 0.09	11.13 ± 0.05	10.87 ± 0.04	11.14 ± 1.69	10.48 ± 0.13	10.75 ± 0.03	9.84 ± 0.13	6.27 ± 0.48	8.71 ± 0.12
MUFAs	40.23 ± 4.30	44.53 ± 0.11	42.31 ± 0.04	45.21 ± 0.10	44.91 ± 0.27	44.52 ± 0.05	44.81 ± 0.22	44.06 ± 0.30	45.13 ± 1.55	44.67 ± 0.17	29.89 ± 0.70	44.56 ± 0.04
OCFAs	0.16 ± 0.03	0.18 ± 0.02	0.24 ± 0.02	0.16 ± 0.04	0.17 ± 0.01	0.14 ± 0.02	0.49 ± 0.70	0.15 ± 0.01	0.12 ± 0.04	0.16 ± 0.04	0.08 ± 0.12	0.27 ± 0.03

- Dash indicated not detected in the present study.

Figure 2. Magnitude of change in fatty acid compositions versus interval of adulteration level.

Figure 3. Comparison of the FTIR spectra between fresh palm olein (FPO) with refined palm olein (RPO), corn oil (CO), extra virgin olive oil (EVO), and sunflower oil (SFO) at 4000–650 cm⁻¹.

Table 3. Functional group assignment responsible for the specific characteristic band.

				Intensity*
	Wavelength (cm^−1)	Functional group	Mode of vibration	FPO	RCO
1	3529 ^b	O–H	Stretching	vw	w
2	3474 ^a	–C = O (ester)	Overtone	w	w
3	3442 ^b	O–H	Stretching	ab	vw
4	3006 ^a	= C–H (cis)	Stretching	m	m
5	2953 ^a	–C–H (CH3)	Stretching (asym)	w	w
6	2921 ^a	–C–H (CH2)	Stretching (asym)	vs	vs
7	2852 ^a	–C–H (CH2)	Stretching (sym)	vs	vs
8	2732 ^a	–C = O (ester)	Fermi resonance	w	w
9	2680 ^a	–C = O (ester)	Fermi resonance	w	w
10	1744 ^a	–C = O (ester)	Stretching	s	s
11	1655 ^a	–C = C– (cis)	Stretching	m	w
12	1464 ^a	–C–H (CH2, CH3)	Bending (scissoring)	s	s
13	1418 ^a	= C–H (cis)	Bending (rocking)	m	m
14	1402 ^a	–C–H (CH3)	Bending	w	w
15	1377 ^a	–C–H (CH3)	Bending (sym)	m	m
16	1354 ^c	O–H	Bending (in plane)	vw	vw
17	1316 ^a	–C–H (CH3)	Bending	vw	vw
18	1236 ^a	–C–O, –CH2–	Stretching, bending	w	w
19	1159 ^a	–C–O, –CH2–	Stretching, bending	m	m
20	1116 ^a	–C–O	Stretching	w	w
21	1097 ^a	–C–O	Stretching	w	w
22	1032 ^a	–C–O	Stretching	w	w
23	988 ^b	–HC = CH– (trans,trans)	Bending	ab	vw
24	964(FPO)^a, 968(RCO)^a	–HC = CH– (trans)	Bending out of plane	w	w
25	908(FPO)^a, 904(RCO)^a	–HC = CH– (cis)	Bending out of plane	w	vw
26	889 ^b	> C = CH2	Wagging	vw	vw
27	870 ^b	> C = CH2	Wagging	w	w
28	854 ^c	= CH2	Wagging	vw	vw
29	721 ^a	–(CH2)n–, –HC = CH– (cis)	Bending (rocking)	s	s

* Intensity: vw: very weak; w: weak; m: medium; s: strong; vs: very strong; ab: absent.

Adapted from Guillén and Cabo ^{a [36]}, Larkin^{b [40]}.

Figure 4. Comparison of the FTIR spectra of fresh palm olein (FPO) and recycled cooking oil (RCO) at 4000–650 cm⁻¹.

Figure 5. The FTIR spectral regions where variations between fresh palm olein (FPO) and recycled cooking oil (RCO) were significant (p < 0.05).

Table 4. The performances of the PLS calibration models for quantification of adulteration level in adulterated oil samples.

Frequency region (cm^−1)			Equation		R2
	Spectra	Factor	Calibration	Validation	Calibration	Validation	RMSEC	RMSECV	RMSEP
3602–3398 and 3016–2642 and 1845–650	Normal	4	Y = 0.998x + 0.055	Y = 0.965x + 3.638	0.998	0.962	0.796	1.349	3.218
	1st der.	3	Y = 0.997x + 0.060	Y = 0.784x + 5.926	0.997	0.931	0.833	5.228	4.366
3016–2642 and 1845–650	Normal	3	Y = 0.996x + 0.101	Y = 0.999x + 2.233	0.996	0.978	1.078	1.845	2.467
	1st der.	4	Y = 0.999x + 0.021	Y = 0.822x + 5.262	0.999	0.947	0.492	4.525	3.810
3602–3398	Normal	2	Y = 0.929x + 1.647	Y = 0.982x + 7.039	0.929	0.780	4.349	10.966	7.775
	1st der.	1	Y = 0.631x + 8.509	Y = 0.083x + 23.912	0.631	< 0.001	9.887	17.807	17.288
3016–2642	Normal	3	Y = 0.982x + 0.409	Y = 0.812x + 9.116	0.982	0.797	2.167	2.770	7.475
	1st der.	2	Y = 0.983x + 0.398	Y = 0.592x + 8.194	0.983	0.790	2.140	6.568	7.883
1845–650	Normal	3	Y = 0.996x + 0.084	Y = 0.969x + 3.443	0.996	0.965	0.981	1.329	3.095
	1st der.	4	Y = 0.999x + 0.238	Y = 0.846x + 5.550	0.999	0.938	0.523	4.392	4.144

Figure 6. The optimized PLS calibration model for the relationship between actual versus FTIR-predicted adulteration level using FTIR normal spectral at the combined spectral region of 3602–3398 cm⁻¹, 3016–2642 cm⁻¹, and 1845–650 cm⁻¹ for (a) calibration set (CA) and (b) validation set (VA).

Figure 7. DA score plot for (a) the classification between fresh palm olein and adulterated oil samples together with (b) the classification of different packet oils into the group of fresh palm olein.

Table 5. FTIR-predicted adulteration level and fatty acid compositions of different packet oils.

Packet oil (PO) (% of total fatty acid)						Codex Alimentarius^a
	PO1	PO2	PO3	PO4	PO5
FTIR-predicted	20.55	13.05	8.58	7.76	9.67	N/A
Fatty acids
C10:0	0.01	0.01	0.01	-	0.01	N/A
C11:0	0.01	0.02	-	-	-	N/A
C12:0	0.20	0.27	0.25	0.18	0.22	0.1–0.5
C14:0	1.00	1.00	0.98	1.07	0.97	0.5–1.5
C15:0	0.05	0.04	0.04	-	0.04	ND^b
C16:0	39.86	40.52	40.19	40.16	40.82	38.0–43.5
C16:1	0.19	0.23	0.20	0.20	0.21	ND-0.6
C17:0	0.09	0.11	0.10	-	0.10	ND-0.2
C17:1	0.03	0.03	0.04	-	0.08	ND-0.1
C18:0	4.19	4.40	4.30	4.60	4.44	3.5–5.0
C18:1	43.46	42.45	42.87	42.89	42.90	39.8–46.0
C18:2	10.19	10.08	10.26	10.20	9.52	10.0–13.5
C18:3	0.19	0.16	0.18	0.17	0.18	ND-0.6
C20:0	0.33	0.42	0.35	0.33	0.33	ND-0.6
C20:1	0.20	0.22	0.22	0.20	0.19	ND-0.4
C20:5	-	0.01	-	-	-	N/A
C22:2	-	0.02	-	-	-	N/A
Saturated fatty acids	45.74	46.79	46.22	46.34	46.93	45.9–51.3
Unsaturated fatty acids	54.28	53.20	53.77	53.66	53.08	49.8–61.2
Monounsaturated	43.90	42.93	43.33	43.29	43.38	39.8–47.1
Polyunsaturated	10.38	10.27	10.44	10.37	9.70	10.0–14.1
Odd-chain fatty acids	0.17	0.18	0.18	-	0.22	ND-0.3

- Dash indicated not detected in the present study; ND, not detected; N/A, not available in the reference standard.

^aAdapted from the Joint Food and Agriculture Organization of the United Nations/World Health Organization.^[21]

^bJoint Food and Agriculture Organization of the United Nations/World Health Organization.^[22]

Figure 8. Visual observation of the packet oils purchased from the local market after 3 months of storage at a cool and dark place. The white box indicates precipitation happened in these oil samples after 3 months of storage.

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