Effect of sterilization methods on volatile flavor compounds of instant carrot slices

Abstract: The purpose of this paper was to study the effect of 3 different sterilization methods on the flavor of prepared carrot slices from raw materials during cold storage at 4℃. The samples were pretreated by pasteurization (90℃, 30 min), high hydrostatic pressure sterilization (550 MPa, 25℃, 10 min), pressure-assisted thermal sterilization (550 MPa, 50℃, 10 min) and stored at 4℃ for 60 d. The solide-phase micro-extraction (SPME)/chromatography-mass spectrometry (GC-MS) technology was used to detect and analyze the volatile flavor compounds in fresh carrot and sterilized carrot slices during storage. Results showed that carrots had a complex volatile characteristic. Fresh carrot sample's volatiles mainly consisted of terpenes, esters, aldehydes, alcohols, ketones and alkanes. Among these, terpenes were considered to be the most important volatile compounds, such as α-terpinene, terpinolene, β-caryophyllene, β-pinene and D-limonene, which impart the characteristic aroma of fresh carrots. First, a comparison was made between the untreated carrots, pasteurizated carrots, high hydrostatic pressure treated carrots and pressure-assisted thermal treated carrots. In the second step, the volatile compounds of different processing were compared in shelf life of 60 d. The volatile compounds were various in carrot slices during storage, which were treated by different sterilization processing. Firstly, the amount of terpene compounds in each treatment group was declined during the whole storage, and terpenes' oxidation reactions possibly could be the source of this observation. The level of aldehydes was the highest in pasteurizated carrot slices, but the lowest in high hydrostatic pressure sterilizated carrot slices. Furthermore the content of hexanal, pungent and rancid flavor, was increased significantly during the whole storage. The aldehydes were mainly related to unsaturated fatty acid oxidation. Hence, this suggested that pasteurization might result in a decline in the quality of carrot slices, compared to other treatments. The β-carotene degradation products, α-ionone (violet's flower aromas) and β-ionone (Cedar's wood aromas), were detected in sterizated carrot slices 20 d after storage. The level of ionone in high hydrostatic pressure sterilizated carrots was lower than other treatments, which indicated that high hydrostatic pressure sterilization could keep a higher level of β-carotene as well. In other words, the high hydrostatic pressure sterization could keep the quality of carrot slices better. In addition, the levels of esters and alkanes had less change during the storage. The main alcohol was 4-terpineol, the oxidation product of terpinolene. The content of terpenoids decreased after sterilization treatment. And it was higher in high hydrostatic pressure and pressure-assisted thermal sterilizated carrots compared to pasteurizated carrots in 30 d, which might be due to the thermal processing inducing terpinene oxidation. However, the content of β-caryophyllene decreased obviously in high hydrostatic pressure and pressure-assisted thermal sterilizated carrot slices. Pressure could induce membrane damage and bring enzymes and substrates into contact, which would not or less occur during thermal processing. And high hydrostatic pressure sterilizated carrot slices had a higher level of myrcene and terpinolene at earlier storage compared with others, which resulted in a better light balsam smell and pine fragrance kept. Pressure-assisted thermal sterilizated carrot slices had a higher content of β-pinene and β-caryophyllene, and there was better pine resin and spicy aroma. The quality of high hydrostatic pressure sterilizated and pressure-assisted thermal sterilizated carrot slices was superior to pasteurizated carrot slices. The results can provide a reference for the applications of new sterilization technologies in ready-to-eat products.