Thesis (MSc)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: Developing countries such as South Africa are in dire need of nutritionally adequate dairy food and beverage sources that are ambient stable due to minimal access to refrigeration. One such product is Kefir, a naturally fermented milk beverage that originated in Caucasian China many centuries ago. The microorganisms responsible for fermentation of the milk are held together in a carbohydrate matrix in the form of small grains. These grains are then removed from the beverage prior to consumption, and added to fresh milk for new fermentations. This beverage holds great potential for large scale development due to the self-propagating nature of the grains, the lack of sophisticated equipment and knowledge necessary for production, and the appealing sensory characteristics of this beverage. This study was therefore performed as an initial investigation to determine the optimum fermentation conditions for large-scale grain production and optimal sensory appeal. Kefir grain production was found to be proportional to incubation temperature in the range studied (18°, 22°, 25° and 30°C), with maximum grain biomass increases of 500% for the Kefir incubated at 30°C over the 10 d trial. During fermentation of Kefir grains in milk, lactic acid and other metabolites are produced. Lactic acid results in coagulation of the milk, necessary to provide the characteristic texture and flavour of Kefir, as well as exerting a preservative effect. Lactic acid production was found to be strongly proportional to both incubation temperature and inoculum concentration. The samples containing 2% (w/v) Kefir grain inoculum concentration that were incubated at 25°C for 24 h were found to have optimum lactic acid levels for good quality Kefir (pH of 4.4 - 4.6 and TA of 1.0 - 1.15%). The other metabolites produced during Kefir fermentation are responsible for the specific flavour of Kefir, and include acetaldehyde, diacetyl, ethanol, acetone and 2-butanone. These compounds were studied using headspace gas chromatography over the fermentation period, which yielded good resolution and separation of all these compounds, however, only acetaldehyde, ethanol and acetone were found to be major metabolites in this study, These analytical results were then further compared to sensory results for key identified attributes, as obtained from a trained sensory panel, to enable recommendations for optimum fermentation conditions to be made. The studied attributes included sourness, sweetness, butteriness, creaminess, yoghurt flavour, cowiness, effervescence, yeastiness, smoothness and overall acceptability. It was apparent from this study that correlations between analytical and sensory data could be drawn, and that panellists were particularly accurate in detecting the attribute sourness resulting from the accumulated lactic acid in the Kefir. Overall acceptability also seemed to be intricately linked to the attribute creaminess, hence the regular literature references to full-cream Kefir as optimum for best sensory appeal. From this study, it was evident that Kefir with optimal sensory appeal is obtained with incubation for 18 h at moderate temperatures (22° or 25°C) and grain inoculum concentrations (0.8% w/v). / AFRIKAANSE OPSOMMING: In ontwikkelende lande soos Suid-Afrika, bestaan daar 'n groot behoefte aan voedsame suiwelprodukte wat stabiel is by kamer temperatuur aangesien 'n groot deel van die bevolking beperkte toegang tot verkoelingsfasiliteite het. Een so 'n produk is Kefir, 'n natuurlike gefermenteerde suiwelproduk wat sy oorsprong eeue gelede in China gehad het. Die mikroorganismes wat verantwoordelik is vir die fermentasie, is saamgebind in 'n koolhidraat matriks in die vorm van klein korrels. Hierdie korrels word verwyder uit die drankie voordat dit gedrink word, en word dan weer by vars melk bygevoeg vir 'n verdere fermentasie. Hierdie gefermenteerde produk het baie potensiaal vir massa-produksie, omdat die korrels natuurlik vermeerder, geen gesofistikeerde toerusting of kennis nodig is nie, en die finale produk hoogs aanvaarbare sensoriese eienskappe het. Die doel van die studie was om 'n inleidende ondersoek uit te voer om die optimum fermentasie toestande vir massakweking van korrels en die mees aanvaarbare sensoriese eienskappe te bepaal. Uit hierdie studie is gevind dat Kefirkorrel vermeerdering proporsioneel is tot die verhoging in inkubasie temperatuur in die gebied 18°, 22°, 25° en 30°C, met maksimum biomassa toenames van tot 500% vir Kefir wat vir 10 dae by 30°C geïnkubeer was. Gedurende fermentasie van Kefirkorrels in melk, word melksuur en ander metaboliete gevorm. Melksuur lei tot die verlaging van die pH van die melk, en veroorsaak stolling, wat noodsaaklik is vir die kenmerkende tekstuur en geur van Kefir, maar dien ook as 'n preserveermiddel. Daar is ook gevind dat melksuur produksie 'n direkte verband het met die inkubasie temperatuur en inokulum konsentrasie. Die monsters met Kefirkorrel inokulum konsentrasie van 2% (miv) wat vir 24 h by 25°C geïnkubeer is, het die optimale melksuur konsentrasies vir goeie kwaliteit Kefir bevat (pH van 4.4 - 4.6 en TA van 1.0 - 1.15%). Ander metaboliete wat belangrike geurkomponente van Kefir is, is asetaldehied, diasetiel, etanol, asetoon en 2-butanoon. Hierdie metaboliete is bepaal en geëvalueer met bodamp gaschromatografiese tegnieke gedurende die fermentasie, wat 'n goeie resolusie en skeiding gelewer het. In hierdie studie is slegs asetaldehied, etanol en asetoon as hoof Kefir metaboliete gevind. Die analitiese data is verder vergelyk met die sensoriese data van die hoof sensoriese komponente, soos bepaal deur 'n opgeleide sensoriese paneel, om die mees gunstigde fermentasie parameters te bepaal. Die geëvalueerde eienskappe was suurheid, soetheid, botterigheid, romerigheid, joghurt geur, koeismaak, gas inhoud, gisagtigheid, gladheid en algehele aanvaarbaarheid. Uit hierdie data is gevind dat daar wel 'n sterk korrelasie bestaan tussen die analitiese en sensoriese resultate, en dat paneellede in staat was om die suurheid, as gevolg van die gevormde melksuur, te bepaal. Algehele aanvaarbaarheid is definitief gekoppel aan romerigheid, daarom word volroommelk Kefir verkies bo die wat met afgeroomde melk berei is. Die data uit hierdie studie het ook getoon dat Kefir met optimale sensoriese eienskappe verkry is na 'n inkubasietyd van 18 h by "matige temperature" (22° of 25°C) en 'n Kefirkorrel inokulum van 0.8% (mIv).

Thesis (MSc Food Sc )--Stellenbosch University, 2000. / ENGLISH ABSTRACT: The rapid urbanisation of the African population has led to the establishment of large low-income communities in and around almost every major town and city in South Africa. Several factors prevent these people from producing or obtaining their traditional fermented milk drink, Maas (Amasi), often resulting in the occurrence of malnutrition in low-income urban African communities. A product with the potential to satisfy the demand for a fermented milk product is Kefir. Kefir, a self-carbonated fermented milk, is commonly manufactured by fermenting unpasteurised or pasteurised milk with re-usable Kefir grains. These Kefir grains consist of a combination of mainly lactic acid bacteria and yeasts. Neither Kefir, nor Kefir grains are as yet marketed in South Africa, thus creating an excellent opportunity to launch these products locally. It is often difficult for the low-income communities to obtain high quality unpasteurised or pasteurised milk, resulting in a serious health risk. The inhibitory activity of Kefir towards certain spoilage and pathogenic microorganisms was, therefore, studied. Strains of Escherichia coli, Staphylococcus aureus, Bacillus cereus, Usteria monocytogenes and Clostridium tyrobutyricum were inoculated (10 ³ - 10 ⁴ cfu.ml⁻¹ ) into pasteurised milk together with Kefir grains (18 gram per litre) and incubated at 25°C. Uninoculated milk samples and milk samples inoculated only with test organisms served as controls. Growth of all the test organisms were inhibited substantially (>-99.9%) in Kefir over the 30 h incubation period and substantial reductions in microbial log cycles were observed for many of the organisms. This coincided with a steep decrease in pH (6.57 - 4.06) and increase in titratabie acidity (0.20 - 0.72%). If Kefir is eventually marketed to low-income urban African consumers, it will have to compete with Maas and, therefore, comparative sensory testing of Kefir and Maas was conducted. The differences in the sensory properties of Kefir, 'laboratory' Maas (representing traditional Maas) and commercial Maas (containing thickener, colourants and flavourants) were determined by a trained panel. These characteristics were identified as "yeasty" and "cowy" tastes (p < 0.05), "effervescence" (p < 0.01), as well as "sourness," "creaminess" and "smoothness" (p < 0.001). The effect of different incubation temperatures (25°, 30° and 35°C) on the Kefir sensory properties was studied to simulate the effect of the large temperature variations that would be found in the dwellings of low-income African urbanites. The "sourness" and "creaminess" of the Kefir was found to increase with increase in incubation temperature but no strong off-flavours were found to develop. Sensory preference testing was conducted by consumer panels consisting of panellists of different ages and population groups to indicate whether the specific panels significantly prefer Kefir, commercial Maas or laboratory Maas. It was found that commercial Maas was significantly (p < 0.001) preferred to Kefir by young African urbanites. Adult Africans, who presumably still have traditional taste preferences, however, equally (p > 0.05) preferred Kefir and laboratory Maas, identifying this segment of the African population as the appropriate starting target market for Kefir. Kefir and laboratory Maas were also tested for preference by a wider panel consisting of people (aged between 18 and 25) representing the different population groups in South Africa. Kefir and laboratory Maas were preferred equally (p > 0.05) by all the groups. Several arguments supporting Kefir marketing to the low-income urban African population of South Africa have been identified. These include: Kefir's ease of preparation; the re-usability of Kefir grains and subsequent affordability; good packaging, distribution and storage possibilities; Kefir's acceptability by lactoseintolerant individuals; high nutritional value; the inhibitory activity of Kefir against potential spoilage and pathogenic organisms and subsequent enhanced safety and keeping ability; and Kefir's acceptable refreshing taste. / AFRIKAANSE OPSOMMING: Die toenemende verstedeliking van Swart Suid-Afrikaners het gelei tot die vestiging van groot lae-inkomste gemeenskappe in en om die meeste groot dorpe en stede. Verskeie faktore verhoed dat hierdie gemeenskappe hul tradisionele gefermenteerde melk, naamlik Maas (Amasi), self kan maak of koop. Dit lei dikwels tot wanvoeding onder lae-inkomste stedelike Swart verbruikers. Kefir het die potensiaal om te voorsien in die vraag na 'n gefermenteerde melk produk in lae inkomste stedelike Swart gemeenskappe. Kefir is 'n selfgekarboneerde, gefermenteerde melk wat vervaarding word deur die fermentasie van ongepasteuriseerde of gepasteuriseerde melk met herbruikbare Kefirkorrels. Hierdie Kefirkorrels bestaan uit 'n kombinasie van hoofsaaklik melksuurbakterieë en giste. Kefir en Kefirkorrels word glad nie in Suid-Afrika bemark nie, en bied 'n fantastiese geleentheid om hierdie produkte plaaslik bekend te stel. Dit is dikwels moeilik om hoë kwaliteit ongepasteuriseerde of gepasteuriseerde melk in lae-inkomste gemeenskappe te verkry. Die risiko om siektes deur die verbruik van hierdie melk op te doen, bestaan dus. Om hierdie rede is die inhiberende effek van Kefir teenoor spesifieke bederf- en patogeniese bakterieë bestudeer. Rasse van Escherichia coli, Staphylococcus aureus, Bacillus cereus, Usteria monocytogenes en Clostridium tyrobutyricum is geïnokuleer (10 ³ - 10 ⁴ cfu.ml⁻¹) in gepasteuriseerde melk tesame met Kefirkorrels (18 gram per liter) en geïnkubeer by 25°C. Melkmonsters wat slegs geïnokuleer is met die toetsorganismes het as kontroles gedien. Die groei van al die toetsorganismes is substansieël geïnhibeer (>-99.9%) in Kefir gedurende die 30 h inkubasieperiode. Substansiële afnames in logsiklusgetalle is waargeneem vir baie van die organismes. Dit het gepaard gegaan met 'n skerp afname in pH (6.57 - 4.06) en toename in titreerbare suurheid (0.20 - 0.72%) vir die Kefirmonsters gedurende die 30 h inkubasieperiode. lndien Kefir bemark word aan lae-inkomste stedelike Swart verbruikers sal dit moet kompeteer met Maas. Vergelykende sensoriese toetse is dus uitgevoer. Die verskille in die sensoriese eienskappe van Kefir, 'laboratorium' Maas (verteenwoordigend van tradisionele Maas) en kommersiële Maas (wat verdikker, kleur- en geurmiddels bevat) is bepaal deur 'n opgeleide paneel en geïdentifiseer as die" "gis-" en "koeismake" (p < 0.05), die "gasserigheid" (p < 0.01) asook die "suurheid", "romerigheid" en "gladheid" (p < 0.001) van die monsters. Die effek van verskillende inkubasietemperature (25°, 30° en 35°C) op die sensoriese eienskappe van Kefir is bestudeer om die effek van die groot temperatuurvariasies wat in laeinkomste behuising mag voorkom, te simuleer. Daar is bevind dat die "suurheid" en "romerigheid" van Kefir toeneem met verhoging in inkubasietemperatuur terwyl geen afsmake ontwikkel nie. Sensoriese voorkeurtoetse is deur verbruikerspanele van verskillende ouderdomme en bevolkingsgroepe uitgevoer om te bepaal of die spesifieke panele 'n beduidende voorkeur toon vir Kefir, laboratorium Maas of kommersiële Maas. Daar is bevind dat stedelike Swart jongmense kommersiële Maas beduidend (p < 0.001) bo Kefir verkies. Swart volwassenes met verwagte tradisionele smaakvoorkeure het egter Kefir en laboratorium Maas ewe veel verkies (p > 0.05). Hierdie segment van die Swart bevolking is dus die geskikte teikenmark vir die bekendstelling van Kefir. Voorkeur vir Kefir en laboratorium Maas is ook getoets deur 'n paneel (ouderdom 18 - 25 jaar) wat bestaan uit mense van verskillende bevolkingsgroepe. AI die groepe het Kefir en Maas ewe veel verkies (p > 0.05). Verskeie argumente ten gunste van die bemarking van Kefir aan lae-inkomste stedelike Swart gemeenskappe in Suid-Afrika is geïdentifiseer. Dit behels die volgende: die gerief van Kefirvervaardiging; die herbruikbaarheid van Kefirkorrels en gevolglike bekostigbaarheid; goeie verpakkings-, verspreidings- en opbergingsmoontlikhede; Kefir se aanvaarbaarheid vir laktose-intolerante individue; Kefir se hoë voedingswaarde; die inhiberende aktiwiteit wat Kefir teenoor potensiële bederf- en patogeniese organismes het en die gevolglike verhoging in veiligheid en rakleeftyd van melk; en Kefir se aanvaarbare verfrissende smaak.