Study On The New Technology And Volatile Flavor Compounds Of Kohlrabi

Kohlrabi is one kind of traditional curing food with unique flavor, which is very popular in China. Traditional processing of kohlrabi need sun drying, and several times salt dehydration. Further desalination process is needed because of high slat content after ripening, moreover, the long production cycle made it not appropriate for modern production. While one-time heat production can sharply shorten the processing cycle and reduce process steps. Research about Kohlrabi is mainly concentrated on the desalination process of traditional pickled turnip, however, less research on the volatile compounds. The study on the volatile compounds of kohlrabi not only can speculate the composition of its flavor and prevent the formation of bad odors, but also provide theoretical basis for the optimization of the process and for the mix of flavor.The pre-treatment process of this new technology and the changes of physical and chemical indicators of different products with different process were investigated in this paper, with Kohlrabi as raw materials. Orthogonal experiments were used to optimize the process parameters of this new technology. Besides, SDE-GC/MS was used to analysis the volatile flavor compounds of different Kohlrabi and raw materials. The main conclusions were as follows:1. new process of kohlrabi:kohlrabi→Cleaning→shred→pre-salt cured→mechanical dehydration→cooling→add salt→vacuum-sealed bags→ripening in dark→finished product j insulation at 25℃0.5% pre-salt cured was used in the Kohlrabi dehydration, which can decrease the loss of nutritional content. While hot air dehydration can reduce the germ-carrying of kohlrabi and realize the mechanization of this product.2. optimal parameters for the new process:water content of Kohlrabi 73%-76%, with 6% salt and vacuum 0.091 MPaChanges in physical and chemical characteristics of kohlrabi produced with new technology: (1) changes of protein content: protein has been decreased with the curing time. Degradation rate of protein decrease with the increase of salinity. The protein content of kohlrabi cured with 5% salt droped from 6.084% to 4.47%, and protein content of kohlrabi cured with 11% salt droped from 6.087% to 5.22%. (2) Changes of amino acid nitrogen content: the amino acid nitrogen content of kohlrabi taken on an rising trend in the ripening process, a negative correlation with protein content. (3) Changes of lactic acid content: in the ripening process, lactic acid of kohlrabi also taken on an rising trend. While the produce of lactic acid in low-salt pickled Caisi was faster with high yield. (4)Changes in reducing sugar content: overall in ripening process reducing sugar of kohlrabi taken on deline trend. Reducing sugar of kohlrabi cured with 5% salt reduced when marinated 60 days, while this happened when marinated 90 days in kohlrabi cured with 9% salt. (5) Changes in vitamin C content: vitamin C content has been declining in ripening process. The content of Vc in low salt Kohlrabi is high, relatively large loss in Kohlrabi of high-salt. (6) Changes in nitrite content: the nitrite content of Kohlrabi has a peak and the peak time was delayed with the increase of salt concentration, possibly because of the decomposition effect of high content of lactic acid in low-salt kohlrabi. (7) Color change:in curing process, the color of Kohlrabi gradually deepened, particular faster in low salt kohlrabi. (8) Hardness Change:the hardness of raw Kohlrabi materials decreased after dehydration. In the ripening process, kohlrabi hardness rised in the first month, then declined, and finally maintain at a stable hardness level.3.nutrition of kohlrabi produce with new process:compared with traditional kohlrabi, the nutritional value of this product increased. In products of new process, protein content increased 23%, reducing sugar increased 7.8%, vitamin C content increased 115.8%. While lactic acid and nitrite contents were decreased 272.5% and 33.5%, respectively.4. The market acceptance of new technology and traditional turnip kohlrabi have no significant difference, there is sufficient market acceptance.5. The analysis method of volatile flavor compounds in raw materials and finished product were established in this paper. The methods was:take simultaneous distillation and extraction (SDE) method.100 g sample were accurately weighed and then put into a 2 L round bottom flask, added 1 L of distilled water, then extracted by adding 40 mL of anhydrous ether into the extraction bottle, maintain a slight boiling stage in the round bottom flask, while extraction bottles heated in 40℃water bath pot, circumfluence for 2 hours, anhydrous ether extracts was dryed through anhydrous sodium sulfate at 0℃for 12 hours and then filtered, the filtrate was concentrated to 1 mL with a rotary evaporator under the conditions of ice bath. GC conditions: Quartz capillary column Rtx-wax (30 m X 0.32 mm X 0.25μm), temperature-programmed, Column temperature 40℃, keep 1 min, rose to 120℃with 5℃/min rate, then rose to 160℃with 20℃/min speed, finally, rose to 220℃with 5℃/min rate, keep 3min, carrier gas was high-purity He, column flow 1.02 mL/min, inlet temperature 250℃, split ratio 20:1, interface temperature 250℃. MS conditions:EI electron source, scan range 35-450 m/z. Gallery: NIST05. Under these conditions, it can better complete the separation and analysis of volatile flavor compounds in turnip and raw materials.6.90 kinds of volatile aroma substances were detected from raw materials, kohlrabi of new technology and the traditional turnip by SDE-GC/MS, including 22 esters,16 alcohols,14 aldehydes,8 acids,5 ketones,6-methyl sulfide,3 nitriles,9 hydrocarbon compounds and seven other heterocyclic compounds.The main flavor components in Kohlrabi and materials are:phenethyl isothiocyanate, cyclohexanol, allyl isothiocyanate, phenyl acetonitrile; the major flavor compounds of Kohlrabi of new technology are phenethyl isothiocyanate, allyl isothiocyanate, cyclohexanone, dibutyl phthalate, ethyl linolenate, dimethyl trisulfide, methyl linoleate, ethyl linoleate, croton nitrile and methyl palmitate; the main flavor of traditional turnip is 15 acid, cyclohexanol, phenethyl isothiocyanate, allyl isothiocyanate, dimethyl trisulfide; the main flavor component of desalination kohlrabi are pentadecylic acid, (E)-9-stearic acid, linoleic acid, linoleic acid methyl ester, musk ketone, etc.68,56,28 kind of volatile aroma components were detect in Kohlrabi of new technology, the traditional turnip and desalination kohlrabi, respectively. From the detected volatile aroma components species, the Kohlrabi of new technology has better quality than traditional turnip and desalting kohlrabi. As can be seen from the ion current, desalination resulted in the loss of aroma components of kohlrabi. Combined with the result of the sensory evaluation of kohlrabi, desalination kohlrabi has less ester aroma but with heavy sour taste, thus has less flavor than traditional kohlrabi and new process kohlrabi.