Mass cultivation and activity of kefir grains

Abstract

ENGLISH ABSTRACT: Consuming milk in the fermented form is an age-old tradition amongst the ethnic groups of Southern Africa and sour milk is usually made by leaving unpasteurised milk to ferment without the addition of any microbial starter culture. A recently introduced South African law, however, prevents the selling of unpasteurised milk. When pasteurised milk is used for the production of traditional sour milk (Amasi or Maas), the end-product has a putrid taste and aroma. The number of low-income communities in South Africa is increasing as a result of the high rate of unemployment as well as other factors and the inability to maintain a healthy diet often leads to malnutrition. An important need thus exists among these communities for a fermented milk product that is inexpensive, nutritious, palatable and easy to produce. Kefir grains are the natural starter that is used during kefir production and consist of a complex variety of bacteria and yeasts trapped in kefiran, the water-insoluble polysaccharide that keeps the grain together. The nutritious fermented drink that is produced upon incubating kefir grains in milk is called kefir. This has a characteristic acid, mildly alcoholic and slightly yeasty taste. Kefir grains naturally grow in size during incubation in milk and the grain biomass increases between 60% and 100% over 10 days. This is too slow if the grains are to be produced commercially. Various factors that influence grain biomass increase were thus studied in order to develop a method for mass production of kefir grains. These factors included different incubation temperatures (18°, 22°, 25° and 30°C), enrichment of milk with different combinations of tryptose (2%), yeast extract (2%) and urea (0.5%), different volumes of replacement milk, agitation of the cultivation vessel in a shaking water bath and the use of active versus inactive kefir grains. Based on results obtained, final parameters resulting in optimum kefir grains biomass increase were the use of more than 1 % active kefir grains as starter and cultivation of the grains at 25°C in milk containing added urea (0.5%) and yeast extract (2%), as well as agitating the cultivation vessel and replacing all the fermented milk daily. Kefir grain biomass increases up to 327% over 10 days were obtained using this method. An important concern regarding the mass cultivation method was the influence on the activity of the grains. A quick, accurate and inexpensive method had to be developed to determine the grain activity based on the metabolic processes of the kefir grain micro-organisms. The metabolism of lactose to lactic acid is one of the most important metabolic conversions during the growth of lactic acid bacteria, consequently the measurement of the amount of lactose metabolised and lactic acid produced should give a good indication of the activity of these organisms. A wide variety of analytical reference methods exist for the determination of lactose and lactic acid, but these are mostly time consuming, expensive and require a great deal of technical expertise. One of the main advantages of Fourier transform near infrared (FT-NIR) spectroscopy is that once the calibration has been derived, food constituents can rapidly and easily be determined with little or no sample preparation. The application of FT-NIR spectroscopy for the routine determination of lactose and lactic acid in kefir was thus studied. During calibration, the partial least squares algorithm was applied to the spectral data and acceptable calibration statistics (SEVC, r) were derived for lactose (0.3487 g.100 g⁻¹, 0.81), D+ lactic acid (0.1337 g.100 g⁻¹ 0.82) and L+ lactic acid (0.0797 g.100 g⁻¹, 0.93). The use of FT-NIR spectroscopy for the prediction of lactose and lactic acid in kefir can thus be successfully applied during more extensive activity testing of kefir grains.

AFRIKAANSE OPSOMMING: Gefermenteerde melk speel al vir dekades 'n belangrike rol in die tradisies en eetkulture van die etniese volke in Suidelike Afrika. Dikmelk word gewoonlik gemaak deur eenvoudig melk eenkant telaat staan om dik te word sonder die byvoeging van enige suurselkultuur. 'n Nuwe Suid-Afrikaanse wet is egter onlangs in werking gestel wat die verkoop van ongepasteuriseerde melk verhoed. Indien gepasteuriseerd melk vir die maak van tradisionele dikmelk (Amasi of Maas) gebruik word, lewer dit 'n eindproduk wat nie dieselfde kwaliteit en aroma het as dikmelk vanaf ongepasteuriseerde melk nie. As gevolg van die hoe werkloosheidsyfer en ander faktore in Suid-Afrika, is die aantal lae-inkomste gemeenskappe besig om toe te neem. Die onvermoe om 'n gebalanseerde dieet te handhaaf lei dan ook dikwels tot wanvoeding. 'n hierdie gemeenskappe bestaan daar dus 'n groot behoefte aan 'n gefermenteerde melkproduk wat goedkoop, voedsaam, smaaklik en maklik is om te maak. Kefirkorrels is die natuurlike suursel wat tydens kefirproduksie gebruik word en bestaan uit 'n komplekse verskeidenheid van bakteriee en giste wat saamgebind word deur kefiraan, 'n polisakkaried wat onoplosbaar is in water. Die voedsame gefermenteerde drank wat geproduseer word tydens inkubasie van kefirkorrels in melk, word kefir genoem en het 'n karakteristieke suur, effens alkoholiese en ietwat gisserige smaak. Kefirkorrels vermeerder natuurlik tydens inkubasie in melk en korrel biomassa neem tussen 60% en 100% toe oor 'n tydperk van 10 dae. Hierdie toename is egter te stadig vir kommersiele produksie van die korrels. Verskeie faktore wat 'n invloed op biomassa toename het, is dus bestudeer om 'n metode te vind vir die massaproduksie van kefir korrels. Faktore wat bestudeer is, het verskillende inkubasie temperature (18°, 22°, 25° en 30°C), verryking van die melk met verskillende kombinasies van triptose (2%), gisekstrak (2%) en ureum (0.5%) en verskillende volumes vervangingsmelk ingesluit, asook skudding van die inkubasiehouers in In skud waterbad en die gebruik van aktiewe teenoor onaktiewe korrels. Die finale parameters wat gelei het tot optimum massaproduksie van kefirkorrels, was die gebruik van meer as 1 % aktiewe korrels as suursel, inkubasie van die korrels by 25°C in melk waarby 0.5% ureum en 2% gisekstrak bygevoeg is, skudding van die inkubasiehouers en daaglikse vervanging van al die gefermenteerde melk. 'n Toename van 327% in kefirkorrel biomassa is verkry na 10 dae met gebruik van hierdie metode. 'n Belangrike kwessie aangaande die massakwekingsmetode was die invloed op die aktiwiteit van die korrels. 'n Vinnige, akkurate en goedkoop metode was nodig om die aktiwiteit van die korrels te meet op grond van die metaboliese prosesse van die kefirkorrel mikrobrganismes. Die metabolisme van laktose na melksuur is een van die belangrikste metaboliese omsettings tydens die groei van melksuurbakteriee en die bepaling van die hoeveelheid laktose verbruik en melksuur geproduseer behoort daarom 'n goeie aanduiding van die aktiwiteit van hierdie organismes te wees. 'n Verskeidenheid analitiese verwysingsmetodes bestaan vir die bepaling van laktose en melksuur, maar is meestal tydrowend, duur en vereis 'n hoe graad van tegniese vaardigheid. Een van die belangrikste voordele van Fourier transformasie naby infrarooi (FT-NIR) spektroskopie, na voltooiing van die kalibrasie, is die vinnige en eenvoudige bepaling van voedselbestanddele met minimale of geen monster voorbereiding. Die toepassing van FT-NIR spektroskopie tydens die roetine bepaling van laktose en melksuur in kefir is gevolglik bestudeer. PLS regressie is op die spektra toegepas tydens kalibrasie en aanvaarbare kalibrasie statistieke (SECV, r) is verkry vir laktose (0.3487 g.1 00 g⁻¹, 0.81-), D+ melksuur (0.1337 g.1 00 g⁻¹) en L + melksuur (0.0797 g.1 00 g⁻¹. 0.93). Die gebruik van FT-NIR spektroskopie kan dus suksesvol toegepas word vir die voorspelling van laktose en melksuur in kefir tydens meer volledige aktiwiteitstoetsing van kefirkorrels.