| Yogurt is a unique flavored dairy product formed by milk or reconstituted milk after pasteurization,which is produced by fermenting probiotics to produce lactic acid and coagulating casein.The quality of yogurt is influenced by three main factors,namely raw materials,strains and fermentation process parameters.At present,most of the yoghurt products on the market contain fruit and vegetable juice ingredients.However,yogurt products prepared with potatoes as raw materials have not yet been released.In addition,when using cholesterol-lowering functional probiotics as a starter,it is necessary to further clarify its cholesterol-lowering mechanism to better achieve industrial applications.In this paper,a new type of functional yogurt was developed by using the probiotics of the laboratory's independent intellectual property as a starter and using fresh potatoes as raw materials.At the same time,its fermentation process is optimized and a more natural and safe method is used to extend the shelf life.To study thecholesterol-lowering mechanismof bifidobacteria,the cholesterol-lowering effect of several strains of Bifidobacteria was analyzed by assimilation,precipitation and adsorption using o-phthalaldehyde colorimetric method.The removal rate of cholesterol in BZ11 was 24.34±0.59%within 48 h,and BZ25,BB12 and BL1 were 10.48±1.19%,15.10±1.43%and 19.78±1.12%,respectively.It was found that bacterial assimilation is the main way to reduce cholesterol in several strains of tested bacteria,while precipitation and cell adsorption are relatively low.The assimilated cholesterol contents of BZ11,BB12,BL1 and BZ25 were 22.00%,12.54%,15.41%and 9.32%,respectively.During the growth phase to the decline period,the cholesterol-lowering mechanism of bifidobacteria is different.Assimilation and adsorption are dominant in the growth and decay phases.The stabilization period mainly includes two ways of assimilation and adsorption.It has also been found that growth cells lower cholesterol by assimilation and co-precipitation,and that resting cells pass through assimilation and adsorption,while dead cells reduce cholesterol by adsorption.The potato yoghurt fermentation process was optimized in this study.Through the sensory evaluation,the content of diacetyl and acetaldehyde as indicators,the essential oils suitable for yogurt products were screened to extend the shelf life.The main content includes single factor and response surface to optimize the process.The bacteriostatic properties of essential oils were studied by double dilution method,and the MIC and MBC were determined by the paper agar method.The most suitable fermentation strains are BZ11,BL1,MT-4,Streptococcus thermophilus.The potato saccharification ratio is 1:3(w/v).The amount of milk powder added is 12%and the inoculation amount is 4%.The proportion of bacteria is BZ11:BL1:MT-4:Streptococcus thermophilus=1:1:1:2.The fermentation temperature is 37? and fermentation time is 12.5 h.The amount of white sugar added was 1.5%.Perilla leaf essential oil has good ability to inhibit fungi,and the addition of essential oil in yogurt is 40?l/100 ml.The shelf life of potato yoghurt added with essential oil at 4? can reach 30d,which is based on the combination of physical and chemical and sensory.The flavor characteristics of yogurt and the physical and chemical effects of cholesterol and cholesterol-lowering effects of potato yoghurt were studied by headspace solid phase microextraction.The results showed that the main flavor substances of potato yoghurt were terpenes,acids,aromatics and ketones.And after the microbial fermentation,the free amino acid level rises.Compared to commercially available yoghurt,potato yoghurt has lower sugar content and higher protein content.It is worth noting that potato yoghurt has a high content of free amino acids before and after fermentation;In addition,the number of probiotics in potato yoghurt is(6.3±0.21)×10~8 CFU/mL,which is 20 times that of commercial yogurt.The amount of bifidobacteriainpotatoyoghurtreached(3.6±0.16)x10~8CFU/mL.Cholesterol-lowering in vitro showed that the cholesterol-lowering rate of mixed-fermented yogurt reached 20.43%,which was 1.86%-15.57%higher than that of single-fermented yogurt.The effects of different storage temperatures(5?,15?,25?)on the pH,titration acidity,lactic acid bacteria and sensory of potato yogurt were studied.With the increase of storage time,the pH,sensory and probiotic live bacteria of yoghurt showed a downward trend,while the titration acidity showed an upward trend.After the yoghurt was stored at 5?,the pH was lower than 4.0 and the number of probiotic bacteria was 9.77×10~6 CFU/mL.The Pearson correlation coefficient of each indicator was compared,and the shelf life prediction model was established by using pH and sensory as indicators.By fitting the regression equation to the first-order kinetics,found pH and sensory prediction models.It has been verified that the pH and sensory shelf life prediction models have good reliability at low temperatures(?15?)and have small errors with their actual values.In terms of the final shelf life,when the potato yoghurt product is stored at?10?,its pH and shelf life show a good correlation.The shelf life of potato yoghurt at 5? and 10? was 29 d and 15 d,respectively,with predicted values of 29 d and 18 d,respectively.This indicates that the model is highly reliable in the low temperature range. |
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