| Summary: | 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).
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