Functional properties of date palm proteins
Date palm (Phoenix dactylifera L.) is considered the oldest fruit tree in the world. It is grown in dry and semidry terrains and has been an important foodstuff for the last 7000 years (El-Sharnouby et al, 2009).There are more than 2000 different cultivars of date palm known all over the world (A-Hooti et al, 1997). Dates have been eaten fresh or used by many parts of the food industry to make products such as jam, bar, date syrup, date pies, alcohol, date chocolate and date confectionery etc. (Sablani et al, 2008).
The development of date palm fruit is divided into four stages of maturity traditionally describe by changes in colour, texture and taste. The first stage, Kimri, is the longest stage of growth and maturity and is characterized by green colour, increase in the size and weight of fruit and rapid accumulation of reducing sugars and total solids. In this stage the fruit contain the highest percentage of moisture, acidity and tannin. The second stage is the Khalal stage. At the Khalal stage, green colour change to yellow or red according to variety, moisture content decreases and sucrose content begins to increase. At the third stage Rutab, loss of moisture is rapid due to weight decrease and conversion of sucrose into invert sugar and the fruit skin becomes brown and softer in texture. In the Tamar stage, the final stage of maturity, the fruit shows a massive decrease in moisture, devolps a wrinkled epicarp and characteristic colour of the variety. The fruit also looses tannins and acidity leading to loss of their astringency (Tafti & Fooladi, 2006).
Date palm is a pitted fruit, which consists of a fleshy preicarp and pit (Besbes et al, 2004). The date seeds comprise approximately 10-15 % of the dates weight (Hussein et al, 1998- Hamada et al, 2002). Currently, the global production of date fruits is about 6.7 million tonnes, of which almost 700.000 tonnes of date palm pits are not currently used for human foods and (FAO, 2004 – El-Sharnouby et al, 2009). In the date industry, date seeds there is a problem of what to do with the waste date pits and these are either discarded or used as fodder for domestic farm animals (Hamada et al, 2002; Besbes et al 2004). Most of the previously studies has been allocated to the date fruit (flesh) particularly about their chemical compositions and nutritional value, and there is limited published work focused on date seeds even though they contain potentially useful quantities of protein and fat/oil (see Table below).
Date seeds are used mainly for cattle feeds, as well as date seeds powder and coffee substitute that have been recently introduced to the market (Rahman et al, 2007). Hamada et al (2002), have studied the sensory characteristic of date seeds from different varieties and found they were light to dark brown in colour, odourless and had a bland taste with slight bitterness. Al-Farsi et al (2007) who studied the compositional and functional characteristics of date by-products, suggest that date seeds can be considered a good source of dietary fibre, total phenolics and an inexpensive source of natural antioxidants and could be used as a functional food ingredient.
Date seed oils have been found to be resistant to thermal treatment over long time scales up to 40h and had high a oxidation stability (Besbes et al., 2005). Furthermore, the carbon of date seeds has a high efficiency as a filter aid to remove oxidation products from fired oils (El-Anany et al, 2008). Al-Turki et al (2004) revealed that date seed could be used as an organic fertilizer and for generation of biogas by mixing with cattle faeces.
Proximate analysis of date fruit and date seeds of three cultivars grown in the UAE at tamer stage (% dry basis)
(1) Data from: AL-Hooti et al. (1997).
(2) Data from: Hamada et al. (2002).
Almana & Mohmoud, (1994) found that bread containing 10% coarsely milled date seed fraction gave a product similar to or better than that containing wheat bran, whereas, the fine milled seed fraction caused a decadence in bread odour, flavour, colour, uniformity and overall acceptability. In another study it was found by Hussein et al (1998) that the addition of date seed to broiler chick diets gave a similar improvement in their body weight to that of a diet containing soybean and/or corn meal.
Dates are richer in protein than the majority other fruits (Ahmed et al, 1995). Date seeds could be an excellent source of functional foods components because they contain a balance of fats, proteins, minerals and carbohydrates. Al-Showiman et al. (1992) analysed date seeds from seven varieties from Saudi Arabia and found that the percentage of protein ranged from 4.79 to 7.50%. They found that the proteins contained sixteen amino acids including the essential amino acids lysine, leucine, isoleucine, threonine, methionine, valine and phenylalanine.I In a similar study by Bouaziz et al. (2008) seventeen amino acids were found in the date seeds of the two varieties studied.
Hamada et al (2002) have investigated that solubility of date seed protein using various solvents. They found that different fractions of the protein were soluble indifferent aqueous solvents. The solubuility of date seed protein in in acetic acid, ethanol, salt solution and 0.1 M NaoH was found to be about 6, 20, 22 and 53% of the total protein respectively. Thus large sections of date seed protein cannot be solubilised by these solvents. However, the proteins extracted would have important biological value, relative tooo egg proteins as a reference standard of proteins, considering their wealth in essential amino acids (Bouziz et al, 2008).
Albumins and globulins are two of the major classes of proteins, albumins are soluble in water, whilst globulins are soluble in salt solutions, Ahmed et al (1995, Smith et al, 1983) reported that most date flesh proteins were water soluble albumins and the prominent protein bands of date flash have a molecular weights of 32 KDa and 72KDa measured using method of sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE), whereas, Bouaziz et al(. 2008) found three prominent protein bands in date seed at 32, 60 and 70KDa.
Objectives of this Study
The objectives of the current study were to convert waste seed into useful protein powder and characterize the protein of date seed and date flesh using a variety of techniques.
Program of study:
(1) Extensive search of literature review.
(2) Study and investigate isolation, concentration and purification of proteins from date seed.
In this study we will attempt to recover a high proportion of the soluble protein from date seed. Previous studies by Hamada et al. suggest that at least 53% of the total protein can be recovered using simple aqueous extraction techniques. We will try to improve on this by using methodologies that are standard in the protein industry, and are commonly used for protein powders derived from other seed products (e.g. soya bean protein). We will investigate different purification methodologies in an attempt to prepare protein powders of a high protein content (80% or above). This will include methodologies such as dialysis and/or ultrafiltration to remove non-protein material from the extracted powder.
(3) Investigate the characterize date seed protein using gel electrophoresis (SDS-PAGE) and Maldi-ToF mass spectrometry.
As far as we are aware the proteome (detailed protein composition) of the date seed, (nor for that matter the date flesh) has been determined and reported. We have access to a state of the art Maldi-ToF (Matrix-assisted laser desorption/ionization time of flight) mass spectrometer that will allow rapid identification of isolated proteins from date seed and flesh.
(4) Study and investigate functional properties of seed protein:
If date seed protein is to be useful as a protein product in manufactured foods we need to confirm that it has acceptable functional properties. We will test the following functional properties of purified date seed and date flesh protein:
- Emulsifying ability.
- Emulsion stability.
- Foaming ability.
- Foam stability.
- Gelation properties.
- Water holding capacity.
(5) Study of compared date seed protein with other food proteins like milk, soybean and egg.
To confirm the usefulness of date seed protein as a functional ingredient we will compare date seed protein functional properties with those of proteins commonly used as food ingredients, i.e. milk, soybean and egg proteins.
(6) Investigate and test functionality of date seed protein in model food systems:
To further confirm the usefulness of date see protein as a functional ingredient we will test the functional properties in a range of food applications. These will include
- As a milk protein replacer and supplement in beverage
(7) Explanation of results based on literature review publication.
(8) Aggregation of PhD thesis.
2. Materials and methods:-
Date palm fruit (Phoenix dactylifera L.)(DPF) at Tamr stage (complete maturity) of the commercially available Tunisian date palm Deglet Nour variety were purchased from the local supermarket in Edinburgh, UK, The seeds of 40 Kg of (DPF) were manually isolated, soaked in water and washed to remove any adhering date flesh, and then air dried for a week, then, they were further dried overnight at - 40ºC. Date seeds were milled in cross beater grinder to pass 1-2 mm screens and then preserved at - 40ºC until analysis. Date flesh was chopped into small slices approximately 8-10mm using a kitchen knife, mixed well then put it in the small polyethylene bags, and preserved at- 40ºC until analysis.
2.2. Preparation of defatted seed powder:
(DPS) powder extraction with chloroform for 16h using a Soxhlet apparatus, the defatted powder was air dried at room temperature, then kept in plastic containers at the refrigerator prior to use.
2.3. Physical analysis:
The weight of one hundred fruits and seeds was measured, and then the average of single fruit and seed was calculated and recorded.
Chemicals and solvents that used in this study obtained from sigma-Aldrich ltd, UK
2.5. Analytical Methods:
Proximate analysis of date palm fruit and date palm seeds were carried out according to association of official analytical chemists (AOAC, 1995), all analytical determinations were performed at triplicate.
2.5.1. Dry matter:
2.5.2. Ash content:
2.5.3. Fat content:
2.5.4. Protein content:
2.5.5. Crude fibre content:
2.5.6. Total carbohydrates:
2.6. Extraction of protein:
2.6.1. Seed proteins:
All the extraction procedures were carried out with the pH value
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