Valorization of functional protein from a plant based food waste: Sour cherry kernel, and its physicochemical characteristics

Sour cherry fruit has been considered as a functional food because of its high content of antioxidant compounds. It has large amounts of water-soluble vitamins including C, B1, B2, B3, B6. In addition, gallotannins, melatonin and other low molecular weight phenolic compounds are also responsible for the high water-soluble antioxidant properties. The anthocyanins, hydroxycinnamic acids and other flavonoids found in sour cherry fruits, such as isorhamnetin rutinoside and quercetin, have been reported to possess various phytotherapeutic activities. According to National Agricultural Statistics Services of United States (NASS), almost 99% of the produced sour cherry fruit (1.4 million tonnes) has been processed into jam, juice, or canned food worldwide. Turkey is among the top four sour cherry producer countries of the world. In Turkey, forty percent of the total annual sour cherry crop has been processed into juice due to consumer preferences. During sour cherry juice processing, the seed is removed since the hard kernel shell is not edible and its existence is prohibited by authorities such as FDA. As far as the production yield of sour cherry fruit and its processes considered, it is observed that high amounts of sour cherry seeds arise as a waste every year generating huge disposal problems. There are recent studies on utilizing the sour cherry seed on areas other than food such as biofuel generation, animal feed production, and activated carbon generation for removal of hazardous compounds from industrial wastewater. However, these options can be utilized for less valuable by products of food industry such as the non-edible hard shells of fruits. In addition, the researches investigating the process effect on sour cherry juices enlightened the potent nutritional value of sour cherry seed and kernel, exclusively and defined sour cherry kernel as a nontoxic low-cost plant material. A seed of sour cherry contains 76.5% hard shell and 23.5 % edible kernel w/w. The chemical composition of the sour cherry kernel as investigated in previous studies is 46.6% total carbohydrates, 29.3% protein, 17% total lipids, 3.9 % moisture, 3.1% ash w/w and 30.25% dietary fiber by weight. Thus, the sour cherry kernel can be evaluated and further utilized in terms of its rich protein, lipid, and dietary fiber content. Moreover, sour cherry kernel has a number of functional properties, some of which have still being investigated. Among the many, dermaprotective, cardioprotective, anti-inflammatory, anti-diabetes, antioxidant, and therapeutic effects can be stated as important ones. The protein fraction of sour cherry representing 29.3% w/w makes it a valuable plant protein. Plant proteins play significant role in human nutrition, especially where average protein intake is less than the essential amount. Because of inadequate supplies of food proteins, there has been a growing interest for plant proteins, as new protein sources, to be used as both xx functional food ingredients and nutritional supplements. In addition to their nutritional value, proteins offer great potential as functional food ingredients providing useful properties when incorporated into foods. In order to utilize a by-product as a protein source it should both present high protein content and quality based on well-balanced essential amino acids, and be free of allergic or toxic substances. Several protein products (flour, concentrates, or isolates), depending on their protein content, can be added to food products in order to improve their functional properties. Sour cherry kernel protein is a nontoxic plant protein rich in conditionally essential amino acids such as glutamic acid, arginine, aspartic acid, and serine. It is a good source of essential amino acids such as lysine, which is limited in most cereals, and phenylalanine, as well. The amino acid composition of sour cherry kernel was evaluated previously as follows: Glutamic acid 27.96 % Arginine 9.30 %, Proline 8.25%, Aspartic acid 7.55 %, Phenylalanine 7.05 %, Glycine 6.52 %, Lysine 5.28 %, Alanine 4.57 %, Serine 4.49 %. Although the scientists have investigated the sour cherry kernel flour and its functional properties coming from protein constituents, to the best of our knowledge, the protein fraction of sour cherry has not been investigated by researchers in detail; it would be beneficial to analyze sour cherry kernel proteins and proceed to its valorization as a plant based food waste. Therefore, the aims of this study were the optimization of protein extraction conditions from sour cherry kernel based on the protein yield at first, and secondly to investigate the physicochemical and functional properties of the protein isolate which were produced in the optimum extraction conditions. Sour cherry kernel flours were defatted prior to extraction, thermal properties of dispersions containing defatted sour cherry kernel flour were evaluated by a differential scanning calorimeter, the protein denaturation transition of sour cherry kernel was observed as an endothermic peak at 80.750C. The overall surface charge of the defatted flour samples was determined by measuring the electrophoretic mobility (UE) of defatted sour cherry kernel flour solutions (0.01%, w/w) at pH 2-11. The isoelectric point of defatted sour cherry kernel flour, the point at which zeta potential is zero, was found to be at around pH 4.2, by simultaneous zeta potential measurement at different solutions from pH 2 to 11. Then, sour cherry kernel protein isolate was extracted by isoelectric precipitation technique using the parameters of pH, solid/solvent ratio, and extraction time with respect to Box-Benhken experimental design of Response Surface Methodology (RSM). In the first set of experiments, the effects of three variables, X1 (pH, 8 to 11), X3 (solid/solvent ratio, 1:30 to 1:10 g:100 mL), X3 (extraction time, 1-3h), at three levels on sour cherry kernel protein extraction were investigated in order to determine the conditions giving highest protein yield by RSM. A total of seventeen experimental designs (i.e. twelve factorial points and five central points) were carried out for three factor experiment. Runs at the central point of design were applied to estimate the possible pure error. Also, the protein yield was used as the response variable corresponding to the combination of the independent variables. The maximum yield (79.64%) was found under the experimental conditions of X1 (pH) = 9.5, X2 (solid/solvent ratio) = 1/ 30 g/mL, and X3 (time)= 3h. Data from extractions were fitted to RSM by means of a reduced cubic model of ANOVA. The response surface model was evaluated by Design-Expert® 8.0.5 to determine a set of experimental conditions for the optimum protein yield. Secondly, the verification experiment was carried out with optimum conditions from software, the predicted (62.28%) and experimental yield (63.76%) xxi were consistent with each other, in that, the experimental result was within the range of 95% confidence level. After extracting proteins at optimum conditions, the desired functional properties including water and oil absorption, emulsifying, foaming properties; as well as digestibility and solubility, are investigated. Sour cherry kernel protein isolate exhibited functional properties that are comparable to other extensively used plant proteins such as soy but it has yellowish color and acrid taste. In particular, with a foaming capacity of 375%, least gelation concentration of 6.0 g/100 g of protein, and oil absoption capacity of 3.56g/g protein; sour cherry kernel protein isolate can be a good alternative to soy protein isolate in fortification of bakery and dairy food products with low amounts so that sensory properties of products are not changed.