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Antimicrobial Effect of Lactococcus lactis subsp. lactis LDTM 6804 on Psychrotrophic Spoilage and Pathogenic Bacteria in Dairy Products
유제품 내에서 유산균 Lactococcus lactis subsp. lactis LDTM6804의 내냉성 부패 및 병원성 미생물 억제 효과

DC Field Value Language
dc.contributor.advisor허철성-
dc.contributor.author허유경-
dc.date.accessioned2018-05-29T03:44:23Z-
dc.date.available2021-04-13T02:34:41Z-
dc.date.issued2018-02-
dc.identifier.other000000150907-
dc.identifier.urihttps://hdl.handle.net/10371/141673-
dc.description학위논문 (석사)-- 서울대학교 대학원 : 국제농업기술대학원 국제농업기술학과, 2018. 2. 허철성.-
dc.description.abstractIn dairy industry, microbiological spoilage related to the quality and safety of milk and milk products is still an important issue for consumers. The growing demand for safer dairy products has led to consider the use of biopreservatives, mainly lactic acid bacteria (LAB). LAB exert inhibitory effects on the growth of undesirable microorganisms or pathogenic bacteria producing a variety of antimirobial compounds including organic acids, hydrogen peroxide and bacteriocins. Therefore, the purpose of this study was to evaluate a Lactococcus strain isolated from raw milk as a potential bioprotective culture using the dairy product models.
Lactococcus lactis subsp. lactis LDTM6804 (L. lactis LDTM6804), due to the highest antimicrobial activities against psychrotrophic spoilage and pathogenic bacteria, was selected among 306 isolates from raw milk. Furthermore, L. lactis LDTM6804 was characterized and its whole genome sequencing was performed. Gene cluster involved in nisin Z biosynthesis was identified in chromosome of L. lactis LDTM6804. Protective effects of L. lactis LDTM6804 was examined with 10 % skim milk and cheese model. When L. lactis LDTM6804 (6 log CFU/ml) was added to skim milk, viable cell numbers in Pseudomonas fluorescens, Burkholderia cepacia and Listeria monocytogenes were decreased by 9.29, 8.84, 4.93 log units, respectively after 14 d of storage at 15 ℃ compared to skim milk with reference strain L. lactis IL1403. Cheeses were manufactured and infected by L. monocytogenes, Staphylococcus aureus and Escherichia coli, respectively at approximately 5 log CFU/g. In addition, L. lactis LDTM6804 at 1 % was inoculated to cheese as an adjunct to the starter culture. After 25 d of storage, L. lactis LDTM6804 reduced the viable cell numbers of L.monocytogenes, S. aureus and E. coli by 0.42, 0.57 and 0.74 log units, respectively compared to control cheese without L. lactis LDTM6804. During fermentation of 10 % skim milk and ripening process of cheese, nisin activity throughout storage was assessed using quantitative real time PCR (qRT-PCR). An electronic nose apparatus was employed to investigate changes of flavour compounds by L. lactis LDTM6804. Our results demonstrate that L. lactis LDTM6804 exerts strong protective effects against psychrotrophic spoilage and pathogenic bacteria without organoleptic quality changes in dairy products. Further investigations on the optimum level of addition of the protective culture and inhibitory effect of fermentates or purified nisin of L.lactis LDTM6804 were required.
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dc.description.tableofcontentsChapter 1. Introduction 1
Chapter 2. Review of Literature 3
2.1. Microorganisms in dairy products 3
2.1.1. Lactic Acid Bacteria (LAB) in dairy products 4
2.1.2. Psychrotrophic spoilage and pathogenic organisms in dairy products 5
2.1.3. Impact of psychrotrophic spoilage and pathogenic bacteria on quality and safety of dairy products. 6
2.2. Lactococcus lactis subsp. lactis 8
2.2.1. Lactococcus lactis subsp. lactis 8
2.2.2. Antimicrobial activity of Lactococcus lactis subsp. lactis. 8
2.2.3. Lactococcus lactis subsp. lactis in dairy industry. 9
2.3. Protective culture. 10
2.3.1. Definition and classification of protective culture. 11
2.3.2. Industrial value of protective culture 12
2.3.3. Lactococcus lactis subsp. lactis as biopreservatives. 15
Chapter 3. Materials and Methods 16
3.1 Selection of LAB with antimicrobial activity. 16
3.1.1. Raw milk sampling 16
3.1.2. Isolation of LAB 16
3.1.3. Antimicrobial activity 18
3.1.3.1. Paper disc diffusion assay 20
3.1.3.2. Agar well diffusion assay 20
3.1.3.3. Colony based top agar assay 21
3.1.4. 16S rRNA sequence analysis 21
3.2. Characterization and identification of the selected strain 22
3.2.1. API test 22
3.2.2. Growth curve 22
3.2.3. Safety assessment 23
3.2.4. Whole genome sequencing and its analysis 24
3.3. Validation of the selected strain in skim milk 25
3.3.1. Inoculation of milk 25
3.3.2. Bacterial enumeration 26
3.3.3. pH measurement 26
3.3.4. Quantitative real time PCR(qRT-PCR) 27
3.4. Application of the selected strain in cheese 29
3.4.1. Manufacture of cheese 29
3.4.2. Bacterial enumeration 32
3.4.3. Electronic nose analysis 33
3.5. Statistical analysis. 34
Chapter 4. Results 35
4.1. Selection of LAB with antimicrobial activity 35
4.1.1. Isolation of LAB 35
4.1.2. Antimicrobial activity 37
4.2. Characterization and identification of the selected strain 39
4.2.1. Biochemical property 39
4.2.2. Growth curve 39
4.2.3. Safety assessment 39
4.2.4. Genomic analysis 43
4.3. Validation of the selected strain in skim milk 47
4.3.1. Inhibition of pathogens 47
4.3.2. Expression of nisin encoding genes 52
4.4. Application of the selected strain in cheese 53
4.4.1. Inhibition of pathogens 53
4.4.2. Nisin related gene expression analysis 57
4.4.3. Flavor compounds analysis 58
4.4.3.1. Flavor discrimination 58
4.4.3.2. Flavor compounds analysis 59
Chapter 5. Discussion 61
References 67
Abstract in Korean 75
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dc.formatapplication/pdf-
dc.format.extent1738716 bytes-
dc.format.mediumapplication/pdf-
dc.language.isoen-
dc.publisher서울대학교 대학원-
dc.subjectProtective culture-
dc.subjectBiopreservation-
dc.subjectNisin-
dc.subjectDairy product-
dc.subjectLactic acid bacteria-
dc.subjectLactococcus lactis-
dc.subject.ddc631-
dc.titleAntimicrobial Effect of Lactococcus lactis subsp. lactis LDTM 6804 on Psychrotrophic Spoilage and Pathogenic Bacteria in Dairy Products-
dc.title.alternative유제품 내에서 유산균 Lactococcus lactis subsp. lactis LDTM6804의 내냉성 부패 및 병원성 미생물 억제 효과-
dc.typeThesis-
dc.description.degreeMaster-
dc.contributor.affiliation국제농업기술대학원 국제농업기술학과-
dc.date.awarded2018-02-
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Graduate School of International Agricultural Technology (국제농업기술대학원)Dept. of International Agricultural Technology (국제농업기술학과)Theses (Master's Degree_국제농업기술학과)
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