Table of Content

28 February 2014, Volume 37 Issue 1

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Biotechnology

Acetaldehyde-Producing Properties of Streptococcus thermophilus and Lactobacillus bulgaricus in Fermented Milk during Post-Ripening

ZHA Mu-su, QIN Yan-ting, WU REN Tu-ya, REN Yan, WANG Xue-ni, LIU Wen-jun, MENG HE Bilige, ZHANG He-ping

2014, 37(1):  1-5.  DOI: 10.15922/j.cnki.jdst.2014.01.001

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The aim of this study was to analyze the acetaldehyde-producing characteristics of Streptococcus thermophilus and Lactobacillus bulgaricus in fermented milk during post-ripening and to investigate the relationship between the expression of key regulatory genes and acetaldehyde synthesis. Methods: Fermented milk was obtained from single strain fermentation of reconstituted whole milk by potent strains of S. thermophilus and L. bulgaricus isolated from traditional fermented milk products. After the end of fermentation (pH 4.5-4.6), the fermented milk was ripened by storage at 4 ℃ for 48 h. During the storage period, the change in acetaldehyde content was determined, and the expression characteristics of the key regulatory genes pdc, pdh, ald, and ldh involved in acetaldehyde synthesis were examined by reverse transcriptionqPCR. Results: The amounts of acetaldehyde produced by 6 S. thermophilus strains were 2.59–14.53 μg/g and 9.17–39.45 μg/g by 6 Lactobacillus bulgaricus strains. The synthesis of acetaldehyde was positively correlated with the expression levels of pdc, ald and pdh but negatively correlated with the expression level of ldh. Conclusion: Different bacterial species and different strains have significantly different capacities to produce acetaldehyde during post-ripening, and one strain produces significantly different amounts of acetaldehyde at different post-ripening times, suggesting strain and temporal specificity of acetaldehyde production. The regulatory genes pdc, ald, and pdh promote the synthesis of acetaldehyde, while the gene ldh has a negative effect on acetaldehyde production.

Acid-Producing Capacity of Lactobacillus delbrueckii subsp.bulgaricus Strains Isolated from Fermented Milk Products from Different Regions of Mongolia and Chemical Composition of the Fermented Milks from the Strains

ZHA Mu-su, QIN Yan-ting, WU REN Tu-ya, REN Yan, WANG Xue-ni, LIU Wen-jun, MENG HE Bilige, ZHANG He-ping

2014, 37(1):  6-10.  DOI: 10.15922/j.cnki.jdst.2014.01.002

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In this study, 12 strains of Lactobacillus delbrueckii subsp. bulgaricus which were isolated from fermented dairy products in different regions of Mongolia were selected to perform fermentation experiments in comparison with one commercial strain. The fermentation time required by each strain was recorded, and the titritable acidity, pH and organic acid content (by reverse phase high-performance liquid chromatography, RP-HPLC) in fermented milks were determined after different periods (0-48 h, 7 days, 14 days and 21 days) of low-temperature storage. Strain IMAU20458 that provided weak post-acidification was selected from the tested strains based on acid-producing capacity and the changes in acid components during post-fermentation storage.

Processing Technology

Effect of Starters Consisting of Different Proportions of Streptococcus thermophilus to Lactobacillus delbrueckii subsp.bulgaricus on the Quality of Fermented Milk

CHEN Shi-xian, GAO Peng-fei, ZHANG Xing-chang, ZHAO Zi-fu, SUN Ting, AN Ying

2014, 37(1):  11-15.  DOI: 10.15922/j.cnki.jdst.2014.01.003

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The impact of mixed starters consisting of Streptococcus thermophilus PZST4 and Lactobacillus bulgaricus PZLB5 in different proportions on the fermentation period of fermented milk and its acidity, texture, syneresis and flavor during storage was investigated. The results showed that viable cell ratios between Streptococcus thermophilus PZST4 and Lactobacillus bulgaricus PZLB5 had a striking effect on the quality of fermented milk, and a ratio of 50000:1 was the optimal combination based on the quality parameters tested.

Sodium Bisulfite Impact on the Color of Sterilized Processed Cheese

FAN Ji-peng, LI Xiao-qin, WU Xin, YU Wen-jing, WU Sen-ming, ZHANG Li-ping

2014, 37(1):  16-18.  DOI: 10.15922/j.cnki.jdst.2014.01.004

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In this study, the impact of adding different amounts of sodium bisulfate on the color change of sterilized processed cheese as indicated by L*, a* and b* values as well as ΔE (color difference) was investigated with the aim of solving the browning problem of processed cheese during sterilization. It was found that addition of 0.2–0.8 g/kg of sodium bisulfate showed good inhibition against the browning of sterilized processed cheese.

Analysis & Detection

Determination of Dicyandiamide in Feed, Milk and Dairy Products by UPLC-MS-MS

HU Xue, LI Cui-zhi, BAI Yan-mei, DUAN Guo-xia, ZHAO Jing, LI Hui-xiu

2014, 37(1):  19-22.  DOI: 10.15922/j.cnki.jdst.2014.01.005

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An ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS-MS) method for simultaneous determination of dicyandiamide in feed, milk and dairy products was developed. The extraction methods for feed, milk and dairy products were studied, and this was followed by selection of an appropriate chromatographic column. Samples were extracted with acetonitrile and the extract was then cleaned up on a C18 solid-phase extraction column, frozen and certrifuged to remove protein impurities, and separated on an Acquity BEH Amide column using a mobile phase composed of acetonitrile containing 0.1% formic acid and 10 mmol/L ammonium formate solution by gradient elution. Detection was performed by positive ion electrospray ionization (ESI) in multiple reaction monitoring (MRM) mode. The proposed method had good linearity in the concentration range of 1–1 000 μg/L with correlation coefficients more than 0.995. The quantification limit for feed was the highest, 40 μg/kg. Recoveries at different spiked concentrations were 80.5%– 108.4%, with RSD less than 7.0%. This method proved to be simple, rapid and suitable for the determination of feed, milk and dairy products.

Determination of Urea in Milk and Milk Powder by High Performance Liquid Chromatography with Fluorescence Detection

SONG Wei, ZHANG Shu, CHEN Xiao-xu, SONG Li, ZHAO Yong-biao, SONG Gui-xue

2014, 37(1):  23-26.  DOI: 10.15922/j.cnki.jdst.2014.01.006

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A method for determining urea in milk and milk powder was developed using high performance liquid chromatography with fluorescence detection (HPLC-FLD). Samples were extracted with a mixture of water and ethanol, the supernatant was derivatized with a solution of xanthydrol in acidic condition. The chromatographic separation was performed on a reversed-phase C18 column using a mobile phase consisting of acetonitrile and sodium acetate solution. The derivatized products were separated and tested using a fluorescence detector at λex 213 nm and λem 308 nm. The method showed a good linear relationship in the range of 0.1–10.0 μg/mL, with R2 > 0.999. The spiked recoveries of urea in formula milk powder and liquid milk samples were in the range of 104%–106% and 83.4%–101% with RSD 2.1% –5.7% and 3.2%–4.9%, respectively. The limits of detection (LODs) in formula milk powder and liquid milk were 30 and 12 mg/kg, respectively. This method proved rapid, sensitive, reliable and reproducible.

Reviews

Nutrition and Safety of Yak Milk as Infant Formula Milk Source

ZHOU Rang, WANG Xiu-ying, LÜ Xiao-hua

2014, 37(1):  27-30.  DOI: 10.15922/j.cnki.jdst.2014.01.007

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China has the largest number and species of yak in the world. For some populations, the biological characteristics of yak milk are significant. It shows higher contents of protein, fat, lactose, minerals and vitamins compared to cow and goat milk. The contents of essential amino acids, essential fatty acids, lactose and minerals are similar to those of human milk. In addition, yak milk has some functional fat acids such as eicosapntemacnioc acid and docosahexaenoic acid, which are absent in cow and goat milk. From the viewpoint of food safety, yak milk specific protein is less likely to cause allergies and more easily digested and absorbed than cow milk. Safety evaluation experiments and population studies of conjugated linoleic acid show no side effects. Yak milk is expected to add new varieties in the field of infant formula milk by its unique nutrition and safety.

Study of Stability of Caprine Casein

SHI Yong-cui, LI Yu, LOU Xin-man, WANG Cun-fang

2014, 37(1):  31-34.  DOI: 10.15922/j.cnki.jdst.2014.01.008

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The stability of caprine casein directly determines the quality of caprine proteins. Understanding the stability of caprine casein is necessary to obtain high-quality goat milk. This article overviews the current status of China’s goat milk industry, and makes a brief description of the properties of caprine casein as well as factors affecting its stability. Moreover, the effect of temperature, pH, inorganic salt ions, food additives and ultrahigh pressure treatment on the stability of caprine casein is reviewed. Controlling the stability of caprine casein is of great significance for the development of goat milk products.