Study On Flavor Components Of Purple Sweet Potato Yogurt And The Stability Of Anthocyanins

Purple sweet potato yogurt was made by fermenting fresh milk added withpurple sweet potato powder. As a new kind of yogurt, purple sweet potato yogurtnot merely has nourishment of normal yogurt, but also has healthy function ofpurple sweet potato in scavenging free radicals and delaying aging. The purplesweet potato yogurt is a healthy food worth spreading and it has good marketprospects.The flavor is one of important organoleptic quality targets. It influencesconsumer preferences. Constituents of volatile from samples were analyzed. Thesamples treated by headspace solid-phase micro-extraction (HS-SPME). Thevolatile composition was analyzed by gas chromatography-mass spectrometry(GC-MS).The volatile flavor components of six samples were detected and analyzed.The six samples were separately purple sweet potato powder solution, two kindsof fermentable purple sweet potato powder solution, fresh milk, normal yoghurt,and purple sweet potato yoghurt. The flavor components detected includedaldehydes, ketones, acids, esters, alcohols, sulfur compounds, heteroaromaticcompounds, terpene compounds, alkanes. Firstly,there were43,36,45,57,36,93kinds of volatile flavor components detected in the six samples separately.Secondly, acetaldehyde, an important flavor component of yoghurt, detected onlyin purple sweet potato yoghurt, but not in normal yoghurt. Thirdly, the ketonessuch as diacetyl, acetoin were detected in purple sweet potato yoghurt and theypromoted purple sweet potato yogurt formed good flavor. Fourthly, varieties ofalcohols and alkanes were detected, but they didn’t contribute much to the flavor.Fifthly, purple sweet potato solution could be fermented with lactobacillus andactivated lactobacillus had stronger fermentative capability. The exsistence ofacetoin, NVP, and methyl ketone made the fermentable purple sweet potatosolution had frankincense smell. Sixthly, there were much terpenes, long-chainfatty acids and long-chain aliphatic acid esters in purple sweet potato yogurtbecause of the addition of purple sweet potato powder. Purple sweet potato is rich of anthocyanins. The stability of anthocyanins ispoor. Temperature, pH, lighting, metal ions have great negative impact on it. Thefermentation of yogurt was a complex process. Although low-acid and coldstorage were good to its stability, anthocyanins in purple sweet potato yoghurtstill might be damaged.The extraction conditions of anthocyanins in purple sweet potato yoghurtwere optimized by response surface methodology. The optimal parameters wereas follows: the extractant was0.4%citric acid, the extraction temperature was56℃, the liquid to solid ratio was1:26, the extraction time was32min. Theextraction rate of anthocyanins in purple sweet potato was643μg/g. Addedpurple sweet potato powder to deionized water, fresh milk, and normal yoghurt.The proportion of added powder was the same to purple sweet potato yogurt.Anthocyanins were extracted from the three samples in optimal extractionparameters. The extraction rates were941μg/g，982μg/g,1031μg/grespectively. The extraction rate of normal yoghurt was highest. The result waslikely related to its low pH, the protective effect of protein and saccharides. Itindicated that anthocyanins in purple sweet potato yoghurt could be protected to acertain extent during the fermentation period. By comparison of extraction rates,the rest of anthocyanins in purple sweet potato yogurt was62.4%. It wasspeculated that anthocyanins mainly lost in pasteurization process. The stabilityof anthocyanins in purple sweet potato yogurt is acceptable.