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Direct Observation of Bacterial Exopolysaccharides in Dairy Products Using Confocal Scanning Laser Microscopy

Ashraf N. Hassan^*, Joseph F. Frank^* and Karsten B. Qvist

^* Department of Food Science and Technology, Athens, GA 30602, USA
Center for Advanced Food Studies, and Department of Dairy and Food Science, The Royal Veterinary and Agricultural University, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark

Corresponding author:
J. F. Frank; e-mail:
frank{at}flavor.fst.uga.edu.

The objective of this work was to develop a methodology for direct visualization of bacterial exopolysaccharides (EPS) in fully hydrated dairy products. The new method involved staining EPS with wheat germ agglutinin labeled with Alexa fluor 488 or staining with concanavalin A 488. Samples were observed using confocal scanning laser microscopy. Distribution of EPS produced by Lactococcus lactis (CHCC 3367), a combination of Streptococcus thermophilus (CHCC 3534) and Lactobacillus delbrueckii ssp. bulgaricus (CHCC 769) and Lactobacillus delbrueckii ssp. bulgaricus RR in milk was compared in stirred and unstirred fermented milk. The EPS and proteins were observed as distinct entities, with EPS present in the protein network pores. EPS was observed in greater amounts in milk fermented by the ropy L. lactis culture than in milk fermented by the less ropy strain of S. thermophilus. Stirring the fermented milk caused aggregation of EPS into more extended structures. The more ropy the culture, the longer and larger the strands formed during stirring. The method was also applied to Feta cheese made with an EPS-producing strain of S. thermophilus. EPS was observed in the cheese as thick sheets filling pores in the protein network.

Key Words: exopolysaccharide • confocal scanning laser microscopy • yogurt

Abbreviation key: CSLM = confocal scanning laser microscopy, EPS = exopolysaccharides, SEM = scanning electron microscopy

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