Identification And Control Of Off-flavor Compounds From Red Radish Pigment Product

Color plays a very important role in the acceptability of foods as indicator of food quality,therefore,colorants are essential for food industry.However,the use and safety of synthetic colorants has been questioned during the last decade,leading to a reduction in the number of permitted colorants.The interest in natural colorants has significantly increased as a consequence of both legislative action and consumer awareness to the use of synthetic additives in their foods.In view of the high content of the highly stable anthocyanin in red radish roots,the plant is considered as a potent source of natural pigments.However,the development of off odor during the pigment extracts has limited so far their applications in food and pharmaceutical industries.The main aim of this work was to identify the off-odor active compounds,develop a suitable method to remove or reduce the off-odor during processing without affecting the pigment yield,then propose the off-odor formation mechanism in radish red during processing and finally,study the application of the odorless radish red in various food products.In this paper,the radish red off-odor and aroma active compounds were identified by sensory evaluation,gas chromatography-mass spectrometry/olfactometry and partial least square regression;the effect of different medium pH and preheating time on the anthocyanin yield and off-odor were evaluated;the off-odor formation mechanism was also explained;and then the radish reds were applied in different foods.The main results are shown as follows:First phase of this research reports,the aroma-active compounds of radish red extracts including water extracts?W?,phosphoric acidified water extracts?PAW?,water or phosphoric acidified water preheated extracts?HW or HPW?,Chongqing extracts and Yunnan extracts were identified and characterized via sensory evaluation,gas chromatography-mass spectrometry?GC-MS?and gas chromatography-mass spectrometry with olfactometry?GC-MS/O?.Three sensory attributes,namely radish odor?desirable?and radish off-odor and sulfur-like odor?undesirable?,were selected to characterize the radish red extracts.The correlation between the aroma-active compounds and sensory attributes was studied using partial least square regression?PLSR?.The sensory evaluation results revealed that Chongqing?9.30 and8.80?,water?9.00 and 9.10?,Yunnan?7.80 and 8.00?,and HW?6.90 and 7.20?extracts showed higher radish off-odor and sulfur-like odor scores,while PAW-2.5?7.70?and HPAW-2.5?7.60?extracts showed higher radish odor scores.Sulfur-containing compounds,allyl isothiocyanate,1-butyl isothiocyanate,acetonitrile,oxygen-containing compounds,terpenes,and naphtha-lene were positively and significantly correlated with radish off-odor and sulfur-like odor but negatively and significantly correlated to radish odor.3-?Methylthio?propyl isothiocyanate and pyridine were positively and significantly correlated to radish odor.Additionally,3-buten-1-yl isothiocyanate and 4-?methylthio?-3-butenyl isothiocyanate showed positive but not significant impact on radish odor.Based on their higher volatile concentration,the aroma-active compounds of the four extracts were evaluated using GC-MS/O.26 aroma-active compounds with different odor intensities were identified to be the major odorant compounds for radish red extracts by panelists.Conclusively,the radish red extraction conditions play a great role in its sensory characteristics.In the next phase of research,we developed a suitable method for extracting the off-flavor free red radish root pigments,while conserving high pigment yield.Results showed that phosphoric and citric acidified hexane?pH 2.5?extracts yielded maximum pigment of 208.37mg/100g and 202.89 mg/100g fresh radish,respectively with lower percentage polymeric color and higher color density,chroma and hue angle compared to other extracts at different pH.Tentative anthocyanin identification by UPLC-TQ-MS showed glycosylated anthocyanins substituted at C3 and C5 in phosphoric acidified extracts and only in citric acidified extracts.In addition,analysis of total glucosinolates?GSLs?revealed significant inhibition of glucosinolate degradation in acidified hexane extracts?pH 2.5?compared to the other extracts.The off odor evaluated by GC-MS revealed a significant decrease of isothiocyanates,sulfides and nitrogen containing compounds in acidified hexane?pH 2.5?compared to other extracts.Furthermore,dehydration and rehydration of extracts showed that over 70%of off odor compounds could be removed in two phases?aqueous and hexane?during hexane removal.Results of sensory evaluation confirmed that the red radish root anthocyanin from the phosphoric acidified hexane?pH 2.5?was closer to the color and odor characteristics of the commercial red radish anthocyanin.Therefore,phosphoric acidified hexane method could be a suitable method to extract off odor free pigment from red radish roots.In the next phase of study,we evaluated the effect of sodium chloride?NaCl?concentration in phosphoric acidified medium pH 2.5 on removal of off odor and pigment properties from red radish.Results showed that the total glucosinolate was highly degraded at high NaCl concentration?<500 mM?compared with control,leading to higher off-odor development.Additionally,correlation analysis results revealed that the glucosinolate degradation was positively and significantly correlated to isothiocyanate,while negative and significant correlation was observed with dimethyl di-,trisulfide,cedrol,triacetin,6-methyl-5-hepten-2-one.Moreover,Results of pigment properties showed that the total monomeric and color properties of extracted anthocyanins were highly degraded at high NaCl concentration?<500mM?compared with control.However,the tentative anthocyanin identification by UPLC-TQ-MS showed 12 glycosylated anthocyanins substituted at C3 and C5 in tested anthocyanin extracts.In conclusion,higher NaCl concentration?<500 mM?could not be useful for red radish off-odor removal and pigment extraction.In the next chapter of this study,the effect of combined pre-heating time?2,5,and 10 min?and extraction medium pH?2.5,3.5,and 4.5?on pigment yield,and removal of off-odor in red radish anthocyanins were evaluated.The tentative anthocyanin identification showed 12pelargonidin anthocyanins and only 9 were identified in samples pre-heated for 10 min.Sample pre-heated for 10 min and extracted at pH 4.5 showed lower pigment yield?98.02 mg/100g,FW?,lighter color and higher percentage polymeric color compared with that pre-heated for 2min and extracted at pH 2.5.However,prolonged pre-heating time?10 min?significantly reduced myrosinase enzyme activity by 73%.Additionally,results showed that pre-heating time and extraction medium pH have negative and significant effect on the pigment yield,while,they showed positive and negative significant effect on the total glucosinolate degradation.Furthermore,the sensory evaluation results revealed no significant difference in radish odor for samples pre-heated for 2 and 5 min followed by anthocyanin extraction in pH 2.5 medium.It might therefore be concluded that the combination of moderated pre-heating treatment time??5min?and lower extraction medium pH could be effective to reduce the off-odor characteristics and maintaining high yield and stable radish red.In the next chapter of this research,we studied the formation mechanism of off-odor in radish red during processing.Firstly,we studied the optimum parameters of purified myrosinase enzyme,and effect of different inhibitors to myrosinase enzyme activity.Secondly,we evaluated and compared the off-odor development through autolysis at different pH,temperature and time.The off-odor was evaluated using gas chromatography mass spectrophotometer?GC-MS?.Finally,we proposed the formation mechanism of red radish anthocyanin off-odor.Results showed that the optimum pH and temperature for myrosinase activity was 6.5 and 37^oC,respectively.Sodium chloride,citric and ascorbic acids showed great myrosinase inhibition of myrosinase enzyme,while magnesium sulfate showed poor enzymatic inactivation compared to the other metal ions.The myrosinase maximum velocity was 2.715U/g,and Michaelis-Menten constant?Km?for sinigrin was 3.75x10^-5M.The off-odor formation mechanism revealed that isothiocyanates formation was related to myrosinase enzymatic reaction.However,sulfur containing compounds might be formed from hydrolysis of S-methyl-L-cysteine sulfoxide by cysteine sulfoxide lyase which is pH dependent reaction.Methanethiol might be considered as the major precursor of different major sulfur containing compounds.In conclusion,to reduce or remove off-odor developed during radish red processing,controlling extraction pH and temperature is the most critical pointThe last part of our research was related to the investigation of the application of odorless radish red in different food products and evaluate the effects of temperature,ultraviolet light,light,pH and metal ions on their stability in various foods.The results showed that odorless radish reds were suitable for acidic foods and ascorbic acid showed potential to reduce the oxidation rate of odorless radish red.However,odorless radish red was significantly affected by highly concentrated salt,alcohol,high temperature,light and ultraviolet radiation.