Keywords: bilberries; Vaccinium myrtillus
; black currants; Ribes nigrum
; cell wall; polysaccharides; structure; rhamnogalacturonan II; xyloglucan;juice processing; high pressure processing
Cell wall polysaccharides play an important role during berry processing. Due to their polymeric character they can cause thickening after mashing of the berries and cause problems during extraction and clarification of fruit juices. Therefore, commercial enzyme preparations are used to degrade cell wall polysaccharides during processing. However, the berry cell wall composition of berries has never been studied in detail.
Aim of this thesis was to analyse cell wall composition and structure of bilberries and black currants to use the obtained data to monitor cell wall components during current processing and during high pressure processing.
A general characterisation of cell wall polysaccharides showed a higher pectin content with similar structure in black currants compared to bilberries. The major storage polysaccharides in black currant seeds were mannans and xylans in bilberry seeds. Rhamnogalacturonan II was enzymatically released in its dimeric form from all polymeric pectic populations of black currants and bilberries. Analysis of xyloglucans with different analytical techniques showed major differences between bilberries and black currants: while xyloglucan structure in black currants was simple, it was very complex in bilberries due to the presence of a rare side chain in partly novel building blocks.
During conventional processing using pectolytic enzymes, pectic polysaccharides were degraded to a large extent. Some polymeric pectic elements were extracted into the juice. One third of these polysaccharides were monomeric and dimeric rhamnogalacturonan II, two thirds were modified hairy regions. During high pressure processing pectins become more calcium sensitive and more extractable, which leads to the formation of a strong gel that hinders juice pressing. When high pressure is combined with commercial enzyme preparations, pectins are, however, further degraded than at atmospheric pressure.
The detailed characterisation of cell wall polysaccharides in bilberries and black currants led to the identification of novel structures and showed the complexity of the plant cell wall. Influences of novel processing technologies were now substantiated. A higher viscosity and lower juice yield were assigned to changes in pectin structure and extractability. This opens the possibility to use high pressure together with commercial enzyme mixtures to obtain a synergistic effect.