Psychrotrophic bacteria and their negative effects on milk and dairy products quality

Osobine i svojstva mikrobnih populacija sirovog mlijeka u momentu prerade presudne su za pojavu kvarenja, vrijeme održivosti, organoleptičku kvalitetu i randman mlijeka i mliječnih proizvoda. Hlađenje i vremenski duža pohrana sirovog mlijeka na niskim temperaturama, uobičajena u današnjim uvjetima proizvodnje, pogoduje rastu psihrotrofnih bakterija. Zbog toga, broj tih bakterija u ohlađenom sirovom mlijeku u odnosu na broj mezofilnih aerobnih bakterija značajno je viši od njihovog idealnog omjera od 10 %. Većinu psihrotrofnih bakterija karakterizira sposobnost tvorbe ekstracelularnih i/ili intracelularnih termostabilnih enzima (proteaze, lipaze, fosfolipaze) koji mogu uzrokovati kvarenje mlijeka i mliječnih proizvoda. Određene vrste psihrotrofnih bakterija pokazuju i prirodnu otpornost na antibiotike i/ili mogu stvarati toksine te se istovremeno smatraju i uvjetno patogenim bakterijama. U smislu kvalitete sirovog mlijeka i mliječnih proizvoda psihrotrofne bakterije postale su ozbiljan problem s kojim se suočava današnja mljekarska industrija. Svrha ovog preglednog rada bila je opisati negativan utjecaj koji psihrotrofne bakterije imaju na kvalitetu sirovog mlijeka i gotovih mliječnih proizvoda. Također, u radu su opisane najznačajnije vrste, te značenje kontrole kojom se može umanjiti kontaminacija mlijeka i mliječnih proizvoda psihrotrofnim bakterijama.The characteristics of bacterial populations in raw milk at the time of processing has a significant influence on shelf-life, organoleptic quality, spoilage and yields of raw milk, processed milk as well as on the other dairy products. Unfortunately, cold and extended storage of raw milk, as a common practice in dairy sector today, favour the growth of psychrotrophic bacteria. Therefore, their count in the refrigerated milk is more than the ideal limit of 10 % of the mesophilic count. Psychrotrophic bacteria are generally able to form extracellular or intracellular thermo-resistant enzymes (proteases, lipases, phospolipases) which can contribute to milk and dairy products spoilage. In addition, besides exhibiting spoilage features, some species belonging to the psychrotrops are considered as emerging pathogens that carry innate resistance to antibiotics or produce toxins. In sense of quality, psychrotrophic bacteria have become major problem for today’s dairy industry as leading cause in spoilage of cold-storage milk and dairy products. This review article focuses on the impact of psychrotrops on quality problems associated with raw milk as well as on th final dairy products. Means of controlling the dominant psychrotrophic species responsible for undesirable activities in milk and dairy products were also discussed

Staphylococcus aureus in cheese

Rast bakterije Staphylococcus aureus u siru tijekom proizvodnje i pohrane sira može uzrokovati stvaranje enterotoksina odgovornih za nastanak bolesti ljudi. Međutim, zbog specifičnih osobina te bakterije i složenosti sira kao medija za njihov rast, u praksi je često vrlo teško procijeniti stvarnu sposobnost preživljavanja tih patogenih bakterija u različitim vrstama sira. Osim toga, podatci iz literature o učestalosti kontaminacije i stupnju enterotoksičnosti sojeva S. aureus značajno se razlikuju. Radi objektivnije procjene potencijalnog mikrobiološkog rizika za čovjeka, zbog moguće prisutnosti bakterije S. aureus u siru, ovaj pregledni članak ukratko opisuje osnovne karakteristike bakterije S. aureus i njihov utjecaj na higijensku ispravnost sira. Također, u radu su prikazani vlastiti rezultati istraživanja prisutnosti S. aureus bakterija u tradicionalnim (autohtonim) tvrdim ovčjim sirevima proizvedenim od sirovog mlijeka.Growth of Staphylococcus aureus in cheese during production and storage can lead to production of enterotoxins responsible for human diseases. Due to specificity of those bacteria and complexity of cheese as a grown media, sometimes in the field it is very difficult to estimate an initial risk assessment of the S. aureus surveying in different cheese varieties. Moreover, the literature data on frequency and proportion of enterotoxigenic strains that cause cheese contamination are significantly different. The purpose of the present review is to objectively assess the risk of the potential occurrences of S. aureus in cheese and significance with respect to safety. The basic characteristics of Staphylococcus aureus, their presence in cheese and its potential risk for health are briefly reviewed. The own results of study relating to the presence of S. aureus in traditional (autochthonous) hard sheep cheese made from raw milk are also discussed in this review

Metode za izravnu identifikaciju bakterija mliječne kiseline u mliječnim proizvodima

Culture-independent molecular tools have been introduced into food microbiology during the last ten years. Most of them are based on the amplification of a bulk bacterial DNA extracted directly from a sample, the targeting of a selected gene, or a variable region of the selected gene. Many studies have explored indigenous lactic acid bacteria in dairy products by culture-independent molecular approaches. It is well known that indigenous microbiota significantly contribute to the uniqueness of artisanal cheeses. However, there is no molecular method that can provide complete qualitative and quantitative insight into the microbiota associated with a certain ecosystem. Therefore, a combination of molecular approaches should be applied to get a more objective picture of the microbiota. This paper aims to present the most widely used culture-independent molecular tools for identifying lactic acid bacteria in dairy products.Molekularne metode za izravnu identifikaciju koriste se u mikrobiologiji hrane desetak godina. Većinom se temelje na amplifikaciji ukupne bakterijske DNA izolirane neposredno iz uzorka, a cilj je amplifikacije određeni gen ili varijabilna regija toga gena. Takvi su molekularni pristupi primijenjeni u mnogim studijama identifikacije prirodne populacije bakterija mliječne kisline u mliječnim proizvodima. Također je poznato da autohtona mikrobna populacija bitno pridonosi specifičnostima okusa i mirisa tradicionalnih sireva. Međutim, ne postoji molekularna metoda koja omogućava potpun kvalitativan i kvantitativan uvid u mikrobnu populaciju bilo kojega ekosustava. Zato je potrebno primijeniti kombinirane molekularne pristupe da bi se dobio objektivniji rezultat. Svrha je ovoga rada prikazati najčešće primjenjivane molekularne metode za izravnu identifikaciju bakterija mliječne kiseline u mliječnim proizvodima

Determination of total bacterial count in raw milk by flow cytometry

Instrumentalna metoda protočne citometrije prihvaćena je u Hrvatskoj kao rutinska metoda procjenjivanja ukupnog broja bakterija u sirovom mlijeku. U odnosu na referentnu metodu utvrđivanja ukupnog broja bakterija, metoda protočne citometrije značajno se razlikuje u iskazivanju rezultata. Dosadašnja iskustva u primjeni protočne citometrije u mljekarskim laboratorijima - detaljno su opisana. Istaknuti su principi rada, metodološki detalji i čimbenici koji utječu na rezultate analiza. Da bismo spriječili nesukladnosti u tumačenju rezultata ukupnog broja bakterija u mlijeku, objašnjena je transformacija rezultata procijenjenih protočnom citometrijom u rezultate utvrđene referentnom metodom. Transformacija rezultata jedne u drugu metodu, uvjetovana je hrvatskom legislativom.The automatic flow cytometry as routine method for total bacterial count determination of raw ex-farm milk has recently been accepted in Croatia. This method significantly differs from the reference method (Standard Plate Count) mostly in the presentation of the results obtained. Therefore, this paper summarized experiences in the application of flow cytometry in the dairy laboratories practice. The principle and the practice of the method, methodological details and factors influencing the results were described. In order to avoid problems regarding the interpretation of the results, which are general problems of the quantitative microbiology, this article try to explain an appropriate conversion of the results with regards to SPC/ml, as an official method for the bacteriological quality proposal by the national legislation

Microbiota of kefir grains

Bakterije i kvasci, a ponekad i filamentozne plijesni u kefirnim zrnima žive u složenom simbiotskom odnosu koji kefirna zrna čini jedinstvenom mikrobnom zajednicom u prirodi. Složenost i kompleksnost njihove fizičke i mikrobne strukture razlogom su što su kefirna zrna još uvijek mikrobiološki nedovoljno i nepotpuno istražena. U istraživanju mikrobnog sastava kefirnih zrna koriste se različiti mikrobiološki i molekularni pristupi. Razvojem metagenomike, bazirane na identifikaciji bez kultivacije, otvaraju se nove mogućnosti identifikacije do sada još neidentificiranih mikrobnih vrsta sadržanih u kefirnom zrnu. Do sada je identificirano preko 50 vrsta mikroorganizama prisutnih u kefirnom zrnu. U radu su prikazane do danas identificirane mikrobne vrste sadržane u kefirnim zrnima različitog podrijetla. Također, radi tehnološkog i mikrobiološkog značenja koja imaju kefirna zrna sama po sebi, u radu su detaljnije prikazani molekularni eksperimentalni pristupi koji se koriste u istraživanju njihove mikrobne bioraznolikosti.Kefir grains represent the unique microbial community consisting of bacteria, yeasts, and sometimes filamentous moulds creating complex symbiotic community. The complexity of their physical and microbial structures is the reason that the kefir grains are still not unequivocally elucidated. Microbiota of kefir grains has been studied by many microbiological and molecular approaches. The development of metagenomics, based on the identification without cultivation, is opening new possibilities for identification of previously nonisolated and non-identified microbial species from the kefir grains. Considering recent studies, there are over 50 microbial species associated with kefir grains. The aim of this review is to summarise the microbiota composition of kefir grains. Moreover, because of technological and microbiological significance of the kefir grains, the paper provides an insight into the microbiological and molecular methods applied to study microbial biodiversity of kefir grains

Spoilage microorganisms in milk and dairy products

Mikroorganizmi - uzročnici kvarenja svojim prisustvom u mlijeku i mliječnim proizvodima mijenjaju njihova primarna svojstva i osobine. Nastale promjene, koje se događaju prije i nakon tehnološke obrade, uvjetovane su brojem i vrstom mikrobnih uzročnika. Najčešće se te promjene odnose na samo jednu pogrešku okusa, mirisa, arome ili konzistencije mliječnog proizvoda. Međutim, u slučajevima većih mikrobnih kontaminacija, sve te pogreške mogu se dogoditi istovremeno. Osim toga, svaka i najmanja promjena, uvjetovana prisutnošću mikrobnih uzročnika kvarenja, umanjuje prehrambenu vrijednost proizvoda. Usprkos značenju koje ima za ukupnu kvalitetu, kontrola mikroorganizama uzročnika kvarenja za mljekarsku industriju nije obvezna, te ih u proizvodima kontrolira zanemariv broj proizvođača. U radu je opisan neželjeni učinak mikroorganizama uzročnika kvarenja na kvalitetu sirovog, pasteriziranog i steriliziranog mlijeka, fermentiranih mlijeka, vrhnja, maslaca i sira s namjerom da se potakne važnost i značenje njihove kontrole u mljekarskoj industriji.Spoilage microorganisms cause changes of primary characteristics and properties of milk and dairy products. The product defects depends on the specific species and number of microorganisms involved in pre- and post- technological processing. Most often, these changes are related to single undesirable sensory characteristic, smell, flavour or conistency. However, in the case of heavier microbial contamination all these undesirable characteristics can occur simultaneously. Besides, even small changes caused by presence of spoilage microorganisms lead to decreased quality of milk and various dairy products. Despite of the importance for the overall quality, the control of spoilage microorganisms for dairy industry is not obligated and therefore, only a few producers control them. Therefore, the present study describes the undesirable effect of spoilage microorganisms on quality of raw, pasteurized and sterilized milk, fermented milk, butter, sour cream and cheeses with the intention to emphasize the importance and significance of their control in the dairy industry

Viscosity of stirred yoghurt during storage

Svrha rada bila je utvrditi promjene viskoznosti tekućeg jogurta proizvođača A (n = 20) i proizvođača B (n = 20) tijekom pohrane od 21 dana. Udjel masti, suhe tvari, bezmasne suhe tvari, količine mliječne kiseline, pH i viskoznost tekućeg jogurta (A i B) utvrđeni su 0., 7., 14. i 21. dana pohrane na temperaturi od 4 °C. Značajna promjena viskoznosti tekućeg jogurta (A i B) utvrđena je 0. i 21. dana (p < 0,05). Vrijednost viskoznosti tekućeg jogurta A od 0,324 Pa s prvog dana, povećala se na 1,912 Pa s 21. dana pohrane. Istovremeno, vrijednost viskoznosti tekućeg jogurta B povećala se od 0,208 Pa s 1. dan na 0,883 Pa s 21. dana pohrane. Tekući jogurt proizvođača B u odnosu na tekući jogurt proizvođača A imao je stabilniju strukturu gela tijekom pohrane od 21 dana. Stabilnija struktura gela vjerojatno je posljedica značajno veće količine suhe tvari bez masti i veće količine mliječne kiseline (p < 0,05) utvrđene za tekući jogurt proizvođača B.The aim of this paper was to determine the changes in viscosity of stirred yoghurt from producer A (n = 20) and producer B (n = 20) throughout 21 days of storage. The content of fat, dry matter, non-fat dry matter, lactic acid, pH and viscosity of stirred yoghurt were determined on the 0, 7th, 14th and the 21st day of storage of the product at the temperature of 4 °C. Considerable change in viscosity of stirred yoghurt (p < 0,05) throughout the maintenance period was determined on the 0 and the 21st day for both yoghurts. The viscosity of the stirred yoghurt from producer A was increased from 0,324 Pa s on the 0 day to 1,912 Pa s on the 21st day. At the same time the viscosity of the stirred yoghurt from producer B was increased from 0,208 Pa s on the 0 day to 0,883 Pa s on the 21st day. In comparison with producer A, a considerably higher quantity of non-fat dry matter and higher quantities of lactic acid (p<0,05) in the stirred yoghurt from producer B resulted in a more stable gel structure throughout the whole research period