produce bacteriocins and the fundamental knowl- edge gained on the biochemical and genetic char- acteristics of these molecules in the last decade has increased their potential application as biopreser- vatives in the food and beverage industries [46, 50].
Internationally, rapid progress has been made in the last 10 years in the development of tools for the genetic modification of LAB. The major target of the LAB strain development programme of the IWBT is to select for strains that are better adapt- ed as starter cultures for MLF and to better under- stand the physiology, diversity and performance of strains under winemaking conditions. Advances have been made in the development of transfor- mation systems involving the development of inte- gration and amplification vectors, selection criteria and food-grade heterologous expression systems. The genome sequences of several LAB have be- come available contributing to the study of genes and operons in these important bacteria. Research efforts at the IWBT are focused on (i) specific en- zymes that are involved in the production of wine aroma compounds, (ii) bacteriocins that can be used as alternative to chemical preservatives, (iii) investigating the role of LAB in bitterness, (iv) pro- duction of biogenic amines, and (v) compounds causing off-flavours in wine. The research findings will be useful in the identification of target genes for the future selection of improved starter cultures or the direct genetic improvement of LAB strains used in MLF.
The first aim involves the screening of natural wine LAB strains, as well as commercial starter cul- tures for enzymes important in the production of wine aroma compounds. The enzymes focused on include _-glucosidases, proteases, esterases, glu- canases, citrate lyases and enzymes involved in the production of off-flavours such as volatile sulphur and phenols [51, 52].Thus it is important to know if this aroma enhancing potential is realised under MLF conditions and to what extent MLF influences wine flavour composition . Genetic screening was employed to assess the presence of bacteri- ocin-encoding genes. PCR revealed the presence of the plantaricin encoding genes plnA, plnEF, plnJ, plnK in Lb. plantarum strains. Four putative bacte- riocin-encoding genes in the genome of O. oeni were identified and sequenced . It is also known that certain LAB have the potential to affect the wholesomeness of wine by producing biogenic amines [53, 54]. Biogenic amine production is strain related due to the possession of the specific biogenic amine decarboxylase gene, which is influ- enced by winemaking parameters .Therefore, a major thrust was to investigate to assess gene dis- tribution and homology amongst strains.
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Another contribution made by rare strains of LAB is the ability to cause bitterness in wine. Cer- tain LAB possess the ability to degrade glycerol, leading to the formation of acrolein, an intermedi- ate that is able to react with phenolic groups of an- thocyanins, forming a bitter complex . The key enzyme involved is glycerol dehydratase (gdh) that fortunately is not widespread in nature. From 240 natural LAB isolates only 26 contained the gene and they belonged to Lb. plantarum, Lb. pentosus, Lb. hilgardii, Lb. paracasei, Lb. brevis and a Pedio- coccus spp.To our knowledge, this is the first report of the presence of the GD gene in Lb. plantarum, Lb. pentosus and Lb. paracasei . The main aims of the LAB research programme is thus oriented towards developing tools to investigate suitable LAB strains for commercial use in the South African wine industry.
Since 2000 the IWBT has systematically acquired analytical instrumentation and developed re- sources to put the institute in the position to main- tain the forefront cutting-edge analytical capacity needed in the biotechnological environment. To- day, the IWBT has an analytical facility that pro- vides full support for metabolomic and wine ana- lytical approaches, based on HPLC, capillary elec- trophoresis (CE), GC coupled to flame ionisation detection (GC-FID), GC-MS and a range of spec- troscopic instruments.The laboratory is also devel- oping several high-throughput analytical tools.
The volatile composition of South African wines was determined with GC-FID and a combination of GC-FID and Fourier transform infrared (FTIR) spectra have been modelled by chemometric tech- niques to discriminate between the major wine cul- tivars. HPLC is used for quantification of sugars, ethanol, organic acids as well as for phenolic com- pounds in wine.
Fourier transform infrared (FTIR) spectrome- ters designed specifically for applications in the grape and wine industries are recent additions to the arsenal required to handle the demands on modern wine chemistry.The IWBT was the first re- search institution to venture into this technology by acquiring a GrapeScan FTIR spectrometer in 2002. Since then, the IWBT has played an important role in introducing the technology and its potential ben- efits to the South African wine industry, in assisting with the implementation thereof and in facilitating training of users by both local and international ex- perts.This methodology is now used with great suc- cess for routine wine and grape analysis, and it also