The project aims to increase the value of various agro-food processing wastes through fermentation. The processing of fruit and vegetables for making juices and wines leads to large amounts of vegetable residues (pomace), which are landfilled or fed to animals. Biowaste, another type of residue is the biodegradable fraction of municipal household waste and is often collected separately.
The high crude fibre content of vegetable pomace suggests its utilization as a dietary fibre bread improver. An enrichment of different products with crude fibre compounds can raise the dietary fibre uptake of the population. Processing of the vegetable residues to a dietary fibre food additive can be done by lactic acid fermentation, leading to a transformation of low molecular materials and to a microbial stabilization.
Potato waste can be used as a substrate for the low-cost microbial production of enzymes like alpha-amylase, widely used in the food industry and the textile industry. After enzyme extraction, a pomace remains that can be used for the production of technical-grade lactic acid.
Grape waste can be used for the production of wine-pip-oil, colourings and grape flavour. All of the three processes can be performed in a much more efficient way if enzymatic treatment steps are involved. The remaining pomace can be processed to lactic acid. Bioresidue products with too low an overall quality cannot be upgraded to food grade products and will be used for low-cost production of lactic acid for use as a floc-forming supplement in wastewater treatment. Indeed, the loading capacity of a biological aerobic or anaerobic wastewater treatment system is essentially determined by the amount of active biomass retained in the reactor, which can be positively influenced upon by lactic acid. An open system fermentation is to be developed in which bioresidue products are used as substrates for low-cost lactic acid production. The effect of this feed-grade lactic acid on sludge settling properties in a number of different wastewaters and in the different reactor types used for wastewater purification is to be studied.
An emphasis is placed on further optimisation of the amylase production by cloning and expression of the gene in a more suitable host (B.subtilis).
At present large amounts of vegetable wastes (pomace) are generated during juice extraction from fruit and vegetables. These are often dumped in landfills or fed to animals. The organic fraction of municipal waste (here termed biowaste) may have a similar composition to pomace and may also be upgraded in this way. The project considers a number of different wastes (carrot, potato, grape) and a number of products (dietary fibre, enzymes, waste water processing additive, colours and flavours). The main conversion process used for upgrading under investigation is bacterial fermentation using either lactobacilli or bacilli. ACTIVITIES
Conversion of vegetable residues to a dietary fibre food additive was investigated using lactic acid fermentation. The fermentation process should transform low molecular materials leading to a fibre product which is biologically stable. The possibility of using potato waste as a substrate for the production of microbial amylase and technical grade lactic acid was established. Grape waste has been investigated as a source of wine pip oil, colorings and grape flavour. It has been shown that these processes can be enhanced by incorporating enzymatic treatment steps. It has also been shown that low quality substrates (wastes) can be used for low-cost production of lactic acid for use as a floc-forming supplement in wastewater treatment.
Good lactic acid fermentation of carrot pomace was observed, using a lactobacillus isolated from sour dough at ambient temperatures in stainless steel vessels on the production site. Usually the fermented pomace can be kept for a week without deterioration. However, if the carrots have been previously stored and the temperature is low (around 15°C), the activity of the lactic acid bacteria is sub-optimal and mould growth can occur. Following spray drying and milling, the fibre has been used in baking trials, with mixed results on bread volume as well as crumb and crust structure. Addition of water gave some improvement.
In studies of grape pomace fermentation there was found to be a progressive, temperature inactivation of both yeasts and lactic acid bacteria, leading to the selection of a number of resistant strains. Most yeasts have been identified and a number of bacteria have been isolated but not yet identified.
Work on enzyme assisted extraction of pip oil and colour were found to be of less value than expected since this is an alternative methods of extraction and preparation of existing products which have markets which are themselves limited and to a large extent satisfied.
The use of carrot pomace in water treatment focused on active and dense granular sludge in upflow anaerobic sludge blanket (UASB) reactors, which depends on the rapidly acidifying properties (RACOD) of the waste water and the surface tension of the reactor liquid. Several experiments were carried out with lab scale UASB reactors in order to develop a kind of granular growth supplement (GGS) which enhances the sludge granulation and the granular sludge bed stability. Best results were obtained with a GGS containing RACOD in the form of a sucrose/starch mixture and linear alkylbenzene sulphonate (LABS). By adding 20% of RACOD on top of the original influent COD and adjusting the reactor liquid surface tension, granular growth and sludge bed stability could be enhanced significantly within 40 days. When carrot pulp/pomace was applied as an alternative RACOD source under various conditions. The best results were obtained when fresh carrot pulp was added to the lab scale UASB reactors in an integrated recycle contact chamber. This was tried out on a real industrial waste water and was shown to be successful.
Enzyme production on potato waste has yielded two a-amylases from B. stearothermophilus: one is cell bound, the other one is extracellular. These have different characteristics with a relatively low pH optimum which is an advantage in the first step of the starch liquefaction process: the formation of maltodextrins a temperatures of 110°C and at pH 6 7. Addition of extra medium compounds is not necessary; pure potato waste is sufficient for the fermentation. A number of structural features that could be responsible for increased thermostability of the enzymes were identified by X ray diffraction analysis. Five or six salt bridges, new hydrophobic interactions and new hydrogen bonds, absent from the thermolabile CGTase structures, were identified.
Funding SchemeCSC - Cost-sharing contracts
6700 AA Wageningen