Application of an Electronic Nose and HS-SPME/GC-MS to Determine Volatile Organic Compounds in Fresh Mexican Cheese

Abstract

Electronic devices have been used to describe chemical compounds in the food industry. However, there are different models and manufacturers of these devices; thus, there has been little consistency in the type of compounds and methods used for identification. This work aimed to determine the applicability of electronic nose (e-nose) Cyroanose 320 to describe the differentiation of volatile organic compounds (VOCs) in fresh Mexican cheese (F-MC) formulated with milk from two different dairy cattle breeds. The VOCs were described using a device manufactured by Sensigent and Solid-Phase Micro-extraction (SPME) coupled to GC-MS as a complementary method. The multivariate principal components analysis (PCA) and the partial least squares discriminant analysis (PLS-DA) were used to describe the relationships of VOCs to electronic nose data, sensory data, and response levels. In addition, variable importance in projection (VIP) was performed to characterize the e-nose signals to the VOCs. The e-nose distinguishes F-MC prepared with milk from two dairy breeds. Sensor number 31 correlated with carboxylic acids most in F-MC from Jersey milk. The HS-SPME/GC-MS identified eighteen VOCs in F-MC made with Holstein milk, while only eleven VOCs were identified for F-MC made with Jersey milk. The more significant peaks in both chromatogram analyses were Propanoic acid, 2-methyl-, 1-(1,1-dimethylethyl)-2-methyl-1,3-propanediyl ester in cheese made from Holstein milk and Propanoic acid, 2-methyl-, 3-hydroxy-2,4,4-trimethylpentyl ester in Jersey milk cheese. Both compounds are considered essential carboxylic acids in the dairy industry. Thus, sensor 31 in the electronic nose Cyranose 320 increased its response by essential carboxylic acids identified by HS-SPME/GC-MS as a complementary method. The e-nose Cyranose 320 is potentially helpful for evaluating fresh Mexican cheese authentication independent of cows' milk samples from different breeds.

Keywords: GC-MS; VOCs; e-nose; fresh cheese; sensors.

Figure 1

Diagram of the analysis two types of F-MC. (A) Incubation of F-MC sample for 15 min at 60 °C; (B) Absorption of VOCs in a Tedlar bag; (C) Data processing and pattern recognition. After each sample read, the e-nose required a purge using nitrogen gas (D).

Figure 2

HS-SPME/GC-MS F-MC sampling procedure scheme. (A) External view for the steps of the extraction procedure, where the fiber is exposed to the VOCs. (B) SPME extraction and thermal desorption in GC injector. (C) Analysis and identification of the VOCs.

Figure 3

Sensorgram of the sensors’ responses to F-MC made with milk from Holstein cows (CTF).

Figure 4

Sensorgram of the sensors’ responses to F-MC made with milk from Jersey cows (SMF).

Figure 5

Principal component analysis (PCA) of VOCs with e-nose Cyranose 320 sensor response to two types of F-MC manufactured with milk from different dairy breeds. (Red indicates F-MC made with milk from Holstein cows, green represents F-MC made with milk from Jersey cows).

Figure 6

(A) Partial least squares discriminant analysis (PLS-DA) score plot in 2D graphs using the concentrations of all VOCs grouped by dairy breed. (B) Variable Importance in Projection (VIP) plots derived from the e-nose signals for the VOCs by two types of F-MC manufactured with milk from different dairy breeds (Holstein vs. Jersey).

Figure 7

Heatmap of stable signals of e-nose sensors for six repetitions for each sample of F-MC produced with milk from two different dairy breeds (Holstein vs. Jersey). The blue frame indicates sensors with a negative correlation to the grade; the red frame indicates sensors with a positive correlation to the grade.

Figure 8

Total ion chromatogram of the VOCs and chemical composition in F-MC made with milk from Holstein cows. Chemical composition of CTF by HS-SPME/GC-MS with retention time^(rt): 1. 1-Benzazirene-1-carboxylic acid, 2,2,5a-trimethyl-1a-[3-oxo-1-butenyl] perhydro-, methyl ester; 2. Silicic acid, diethyl bis(trimethylsilyl) ester; 3. 1H-Trindene, 2,3,4,5,6,7,8,9-octahydro-1,1,4,4,9,9-hexamethyl-; 4. 4-Trimethylsilyl-9,9-dimethyl-9-silafluorene; 5. Cyclotrisiloxane, hexamethyl-; 6. Cyclopentasiloxane, decamethyl-; 7. Methyl-[4-[2,6-dimethyl-3-[methylthio]-1,2,4-triazin-5(2H)-ylidene]-2-butenylidene]methylhydrazinecarbodithioate; 8. Cyclohexasiloxane, dodecamethyl-; 9. Propanoic acid, 2-methyl-, 2,2-dimethyl-1-(2-hydroxy-1-methylethyl)propyl ester; 10. Hexane, 3-methyl-; 11. Silicic acid, diethyl bis(trimethylsilyl) ester; 12. Heptasiloxane, 1,1,3,3,5,5,7,7,9,9,11,11,13,13-tetradecamethyl-; 13. Cycloheptasiloxane, tetradecamethyl-; 14. Cyclodecasiloxane, eicosamethyl-; 15. Propanoic acid, 2-methyl-, 1-(1,1-dimethylethyl)-2-methyl-1,3-propanediyl ester; 16. Benzoic acid, 3,4-dichloro-, methyl ester; 17. 1H-Indole, 2-methyl-3-phenyl-; 18. 2,5-Cyclohexadien-1-one, 2,5-dimethyl-4-[(2,4,5-trimethylphenyl)imino]-.

Figure 9

Total ion chromatogram of the VOCs and chemical composition in F-MC made with milk from Jersey cows. Chemical composition of SMF by HS-SPME/GC-MS with retention time(rt): 1. Cyclotrisiloxane, hexamethyl-; 2. Cyclotetrasiloxane, octamethyl-; 3. 2-Methyl-7-phenylindole; 4. Cyclopentasiloxane, decamethyl-; 5. Cyclohexasiloxane, dodecamethyl-; 6. 2-Hexen-4-ol, 5-methyl-; 7. Cycloheptasiloxane, tetradecamethyl-; 8. Propanoic acid, 2-methyl-, 3-hydroxy-2,4,4-trimethylpentyl ester; 9. Nonahexacontanoic acid; 10. 1,4-Dioxaspiro[4,5]decane-7-butanoic acid, 6-methyl-, 2-(methylsulfonyloxy)ethyl ester; 11. 1-Monolinoleoylglycerol trimethylsilyl ether.