Browsing by Author "Ndlovu, Witness"
Now showing 1 - 1 of 1
Results Per Page
Sort Options
- ItemOptimisation of the enzymatic hydrolysis of rainbow trout processing by-products to manufacture liquid fertiliser(Stellenbosch : Stellenbosch University, 2017-12) Ndlovu, Witness; Goosen, N. J.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: Fish processing by-products are a potential source of proteins for a variety of applications, amongst them fertilisers. Since these by-products are otherwise dumped in landfills, resulting in environmental contamination, their recovery presents an economic opportunity to derive value from these otherwise discarded materials. There exist two main technologies for fish fertiliser production; fish emulsion and fish protein hydrolysates (FPH). Fish fertilisers contain both macro (nitrogen, phosphorous, potassium, calcium and magnesium) and micro (iron, manganese, zinc, copper, molybdenum, and boron) nutrients, with the ability to provide nutrients to both the plants and the soil micro-organisms, thereby building a healthy soil profile. Depending on the characteristics, fish fertilisers can be applied both as soil additives and as foliar spray. South Africa produced 1800 metric tonnes of rainbow trout in 2008, of which a significant amount ends up as by-products after processing. Such a supply of rainbow trout by-products is substantial to sustain the protein recovery processes at a commercial scale. This work is an investigation into the production of a liquid fertiliser, suitable both as a foliar and soil application, using rainbow trout processing by-products (heads) as raw material. The work was divided into three stages. The first stage involved determining the optimum hydrolysis conditions for two pre-selected proteolytic enzymes; SEBPro XL, an exoprotease and SEBDigest F59P, an endoprotease. To determine optimal conditions, three independent variables namely temperature, pH and E/S ratio were optimised at five levels (−α, −1, 0, +1, and +α) by using response surface methodology, central composite design. The degree of hydrolysis, which is the extent of the hydrolysis process, was the dependent variable. The DH was measured and monitored according to the modified spectrophotometric o-phthaldialdehyde (OPA) method. Optimal hydrolysis conditions for optimum DH were found to be a temperature of 60oC for both enzymes. The optimum pH for SEBPro XL was 6.9 whilst for SEBDigest F59P, the optimum pH was 7.6. Within the employed experimental domain, the optimum temperatures and pH were determined at an enzyme to substrate ratio (E/S) of 0.05% and 0.67% for SEBDigest F59P and SEBPro XL respectively. The E/S ratios were based on the protein content of the by-product fish heads. The second stage involved hydrolysing with the two enzymes at their optimum conditions for three hours. The main purpose of this stage was to maximise the DH using the two enzymes and their combinations. With three combinations of the two enzymes being designed, a total of five enzyme treatments were considered for the hydrolysis experiments. These five enzyme treatments were employed in the hydrolysis process and their fish protein hydrolysates (FPH) were compared by way of DH, amino acid (AA) content, free amino acid (FAA) content, macro and micro mineral content of the fertiliser, as well as the heavy metals. The results showed that using SEBPro XL alone results in higher levels of total AA and a higher proportion of FAA in the FPH compared to all other treatments. Its DH was also higher than that of all other treatments. All enzyme treatments produced FPH of within limits heavy metals as per the requirements of the fertiliser regulations and significant amounts of macro and micro nutrients. In the third and final stage of the investigation, SEBPro XL was further employed at varying E/S ratios to maximise on the DH and shorten the reaction time. The E/S ratios ranged from 1% to 5%. The reactions were run for four hours each. At the end of hydrolysis, the DH was 38% for a 1% ratio, 43.1% for 2% ratio, 50.9% for 3% ratio, 58.2% for 4% ratio and 60.2% for a 5% E/S ratio. Therefore depending on the desired DH, a choice can be made for an enzyme concentration and time of hydrolysis. This work established the optimum temperature and pH for hydrolysing rainbow trout processing by-products heads with two enzymes, SEBPro XL and SEBDigest F59P. It also demonstrated the possibility of producing foliar fertiliser from rainbow trout processing by-products. The fertiliser’s nutrient content lies within the legislative constraints. By manipulation of the reaction conditions, temperature, pH and enzyme concentration, as well as reaction time, its quality may be pre- determined.