Preparation and characterization of a protein extract from date palm fruit (Phoenix dactylifera L.) : investigation of nutritional and functional properties

Abstract

The world’s population will reach 9 billion by 2050, increasing the pressure to find alternative protein sources to animal protein. The high global demand for soy protein ingredients has resulted in tropical deforestation which is associated with adverse health impact, agronomic, environmental and climate damage and there is a need to find alternative plant protein sources to soy protein. The Kingdom of Saudi Arabia (KSA) is considered as the second largest producer of date palm fruit among the current date-producing countries, resulting also in significant quantities of dates going to waste. The aims of this study were to develop a process for extraction of protein from date fruit suitable to be implemented in the food industry, to characterise the protein’s electrophoretic profile by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), to identify the proteins by Liquid-Chromatography Coupled Tandem Mass Spectrometry (LC-MS/MS), to study the physicochemical and functional properties of date fruit protein extract (DFPE) and the effect of thermal treatment, to determine the chemical composition, nutritional value, anti-nutritional factors and anti-oxidant properties and to develop an infant cereal with reduced allergenicity. Proximate and mineral analysis, anti-oxidant, amino acid composition and digestibility analysis of DFPE, reported in Chapter 2 showed that the protein extract contains all essential amino acids, is a high source of iron and has excellent anti-oxidant properties matching that of ascorbic acid. The extract had a lower protein digestibility-corrected amino acid score (PDCAAS) value than soy protein isolate (SPI) and contained anti nutritional factors e.g. oxalate, tannin and phytate, yet at low quantities that is assumed not to be of anti-nutritional concern. Physicochemical and functional properties of the extract and the respective effect of thermal treatment are reported in Chapter 3. The concentration of free and total sulphydryl groups (FSH and TSH) were significantly less that for SPI, confirming the results of low cysteine in the amino acid analysis of DFPE. The effect of thermal treatment on the profile of sulphydryl (SH) groups indicates that DFPE is less thermally stable than SPI, whilst considering the fact that DFPE had been subjected to heat treatment during the extraction process. This physicochemical profile was mirrored by the corresponding decrease in functionality including decreases after longer heating times in solubility, foam capacity emulsion stability index and increased water separation in emulsions. Chapter 4 describes the development of a protein extract containing 25.8% protein per dry weight using a new extraction process, which is a 13-fold enrichment compared to the 2.8% in the initial sample. The extraction process resulted in 4.2% DFPE, 57.2% date syrup and 38.6% waste. DFPE contains 50% protein whereas the remainder was in the date syrup (26.7%) and in the waste (24.3%) which could also be lost due to protease activity. The electrophoretic profile was established. LC-MS/MS results indicated the two most abundant proteins to be sorbitol dehydrogenase-like with MW (kDa) 39, an energy-related protein and catalase isozyme 2 with MW (kDa) 57, a disease/defense-related protein. Chapter 5 describes the development of an infant cereal based on date fruit protein as a potential competitor to a commercial infant cereal (CERELAC) sold in the KSA. The date cereal product lacked certain amino acids and calcium and contained low levels of oxalate, phytate and tannin. A date-cereal porridge prepared with camel milk had a proximate and mineral composition matching that of a porridge made with CERELAC prepared with cow’s milk apart from lacking fat, calcium and selenium, resulting in a product with reduced allergenicity to gluten and cow’s milk. DFPE could add to the rising need to find replacement for soy protein because the expansion of soy bean production and consumption is associated with environmental threats, such as deforestation.