Table of Content

01 September 2018, Volume 41 Issue 5

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Basic Research

Influence of Streptococcus thermophilus on the Postacidification of Fermented Milk Containing Lactobacillus helveticus

ZHANG Chenchen, YU Hefei, ZHANG Zhaojun, GUAN Yuxin, GU Ruixia

2018, 41(5):  1-5.  DOI: 10.15922/j.cnki.jdst.2018.05.001

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To improve the fermentation performance of Lactobacillus helveticus, Streptococcus thermophilus as a potential starter culture for functional fermented milk was utilized together with L. helveticus to produce fermented milk. As results, the mixed culture fermentation shortened the curding time by 3.5 h, increased yogurt viscosity by 1.8 times and improved whey syneresis and texture properties as compared to L. helveticus alone. Interestingly, S. thermophilus weakened the postacidification of fermented milk containing L. helveticus, maintaining a pH value greater than 4.2 for 20 d of storage. To account for this finding, the number of living bacterial cells, lactose consumption and the activity of glutamate decarboxylase and β-galactosidase were measured. It was found that the activity of glutamate decarboxylase and β-galactosidase of the fermented milk containing the two strains was lower, resulting in a lower lactose consumption and consequently weakening yogurt postacidification. This study concluded that co-fermentation with S. thermophilus was an effective means to improve the fermentation performance of Lactobacillus helveticus.

Influence and Mechanism of Co-fermentation on Storage Quality of Yogurt

CHEN Nan, XU Qianda, GAO Haoxiang, HE Qiang, SUN Qun, ZENG Weicai

2018, 41(5):  6-11.  DOI: 10.15922/j.cnki.jdst.2018.05.002

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The effect of co-fermentation on the quality of yogurt during storage and the underlying mechanism were investigated. With this aim, we monitored changes in some physicochemical properties (pH value, titratable acidity, water-holding capacity and texture properties), and we also analyzed the color, flavor compounds and microstructure of yogurt using a color difference meter, headspace solid phase microextraction-gas chromatography-mass spectrometry and scanning electron microscopy, respectively. The results obtained showed that co-fermentation markedly decreased the pH value, increased the acid production and gelation and enhanced the water-holding capacity and texture properties of yogurt. At the same time, co-fermentation endowed the yogurt with better color and flavor as well as a more compact, continuous, stable structure three-dimensional network structure. To sum up, co-fermentation could significantly improve the quality of fresh and stored yogurt.

Stability Evaluation of High-Viscosity Acidified Milk Systems

WU Yun, LIU Hongxia, HU Pengli, QIAN Wentao, GAO Zengli, YANG Yonglong, LI Hongliang, MU Zhishen

2018, 41(5):  12-15.  DOI: 10.15922/j.cnki.jdst.2018.05.003

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Acidified milk is deeply loved by consumers because of its delicate taste and unique flavor. However, acidified milk is unstable with respect to whey syneresis, protein precipitation and flocculation during both processing and storage. The synergistic effect of pectin with other stabilizer can stabilize acidified milk and improve its taste. This study investigated the effect of various gums on the viscosity, water-holding capacity, whey syneresis and sensory evaluation of acidified milk. It turned out that the acidified milk with added pectin, propylene glycol alginate (PGA) and carboxymethylcellulose sodium (CMC-Na) had moderate viscosity and good taste and was stable. The optimal mixing ratio of pectin, PGA and CMC-Na was found to be 13:2:2 for better viscosity and the best stability and 13:2:1 for the highest sensory score. The optimal mixing ratio was determined to be 13:2:2 for a better compromise between various response variables such as taste and stability during actual production.

Analysis & Detection

Quantitative Determination of Tetracycline Residues in Milk, Fermented Milk and Milk Powder by Ultra Performance Liquid Chromatography-Tandem Mass Spectrometry

DUAN Guoxia, ZHANG Lijia, Lü Zhiyong, MO Nan, XIE Ruilong, LIU Lijun, LI Cuizhi, HU Xue

2018, 41(5):  16-19.  DOI: 10.15922/j.cnki.jdst.2018.05.004

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An ultra performance liquid chromatography-tandem mass spectrometry method for the simultaneous quantitative determination of four tetracycline residues in dairy products, including milk, yogurt and milk powder, has been established. In this method, the pretreatment and instrument operating conditions were optimized. Tetracycline residues were extracted into EDTA-Mcllvaine buffer and then cleaned up by an HLB solid-phase extraction cartridge. After the residues were eluted with a mixture of methanol and acetonitrile (1:9, V/V), the eluate was blown to dryness under nitrogen. The resulting residue was then redissolved. The analysis was carried out using positive ion electrospray ionization under the multiple reaction monitoring mode. The limits of quantification were 10 μg/kg for tetracycline, oxytetracycline, chlortetracycline and doxycycline in milk powder; the limits of quantification were 2 μg/kg in milk and yogurt. The recoveries of the tetracyclines fortified at levels from 2 to 100 μg/kg ranged from 63.1% to 100.6% with relative standard deviation of 1.8%–10.7%, and the matrix effects were 100.5%–122.1%. In conclusion, this method is accurate, rapid and highly sensitive and exhibits good repeatability and wide linearity. Thus it is suitable for the determination of tetracycline antibiotic residues in milk, yogurt and milk powder.

Determination of Food Colorants in Fermented Milk by High Performance Liquid Chromatography

Lü Zhiyong, YUE Hong, CHEN Jing, LIU Chunxia, WU Lunwei, LIU Lijun

2018, 41(5):  20-23.  DOI: 10.15922/j.cnki.jdst.2018.05.005

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A method for the determination of eight colorants, new red, amaranth, allura red, ponceau, tartrazime, sunset yellow, indigotine and brilliant blue in fermented milk by high performance liquid chromatography was developed. The analytes were extracted from samples with weakly alkaline water at 7.7–7.9 and cleaned up on a polyamide solid phase extraction cartridge. The analysis was performed on a C18 column with gradient elution using 0.02 mol/L sodium acetate solution-methanol as mobile phase. The results showed that the recoveries of seven colorants other than indigotine at spiked concentration levels of 0.5–5.0 mg/1 000 g in fermented milk were higher than 90%, while the recovery of indigotine was around 80%. The limit of detection of the method was 0.5 mg/kg. The repeatability relative standard deviation was less than 5%, indicating good precision. With the advantages of accurate results, simplicity and good repeatability, this method is suitable for the analysis of colorants in fermented milk.

Comparison of Soleris Real-Time Optoelectronic System with Aerobic Plate Count Method for Detection of Total Bacterial Count in Raw Milk

CHEN Meilian, ZHANG Hui, WANG Shiying

2018, 41(5):  24-27.  DOI: 10.15922/j.cnki.jdst.2018.05.006

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A method using a Soleris real-time optoelectronic microbial detection system was developed for the rapid determination of the total bacterial count in raw milk and compared with the conventional aerobic plate count method (APC) in this paper. The standard curve equation of the Soleris method was established as follows: y = ?0.651 5x + 8.263 9, where y is total bacterial count (lg (CFU/mL)) and x is detection time (h) (R2 = ? 0.939 6) and it was verified to have good correlation. Repeatability tests showed relative standard deviation (RSD) values less than 5%. No statistically significant difference was observed comparing the two methods (P > 0.05). Under the experimental conditions in this study, it took 0.9–11.8 h to analyze one sample using the Soleris method, which was greatly shortened compared with that (48 h) using APC. Furthermore, the Soleris method was simple and rapid and allowed simultaneous detection of 128 samples. Accordingly, it could be considered as an efficient and stable method for the rapid determination of the total bacterial count in raw milk.

Reviews

Recent Progress in the Application of Leuconostoc

CUI Liang, YUE Yuanyuan, WU Rina

2018, 41(5):  28-34.  DOI: 10.15922/j.cnki.jdst.2018.05.007

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Leuconostoc spp. is a genus of gram-positive, facultatively anaerobic, heterofermentative lactic acid bacteria (LAB), and it is the dominant microflora in many kinds of traditional fermented products. Therefore Leuconostoc spp. can be used in food, chemical, medicinal and other fields. This article reviews the current status of research on the separation, screening and characteristics of Leuconostoc. Moreover, recent progress in the genomic, proteomic and metabolic study of Leuconostoc as well as its application is summarized. The aim is to provide a theoretical basis for better application of Leuconostoc in industrial production and our life.

Recent Progress in High Cell Density Fermentation of Lactobacilus acidophilus

XU Yu

2018, 41(5):  35-38.  DOI: 10.15922/j.cnki.jdst.2018.05.008

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Lactobacilus acidophilus is a kind of probiotics with great commercial value. The high cell density fermentation of Lactobacilus acidophilus has important application value for its industrial production. In this paper, we review the nutritional requirements of Lactobacilus acidophilus, its strategies for adapting to a freezing environment and its high cell density fermentation, aiming to provide theoretical reference for the production of Lactobacilus acidophilus.