(623e) Alkaline Peanut Protein Extraction Assisted with Microwaves (MAE)

Alkaline peanut protein extraction assisted with microwaves (MAE)

Abstract

Over the last years there has been a growth in the demand for animal-based protein, driven mainly by the increase in the population, leading to the search for new sources of protein. Vegetable proteins are an alternative to animal-based in food applications, because of the availability and renewably of the raw material, wide variety of sources and relatively low environmental impact compared with animal counterparts. Peanut is a leguminous with high protein content (26-29%) and with an excellent amino acid profile, used as food ingredient because of its functional and nutritional properties. Peanut have been grown throughout the world as an oilseed crop for production of edible oil, yielding defatted flour as one of the final products. This defatted flour is a protein-rich (50 to 55%) and non-expensive by-product mainly used as a low-value added ingredient in livestock feed,  providing an excellent opportunity to produce high-protein food ingredients (protein concentrate or isolate) with a better economic and social revenue (Zhao et al., 2012). The development of a peanut protein isolate (PPI) from defatted peanut flour would also provide the food industry with a new high protein food ingredient for product formulation and protein fortification.

One method used for protein extraction from vegetable sources is alkaline solubilization followed by acid precipitation. It is a classical technique and also the most efficient because of the good final protein yield (Kain et al., 2009). On the other hand, when this method is used for protein extraction in a by-product of the oilseeds-industry (as defatted peanut flour), the final protein yield is reduced because of the changes in protein structure during oil extraction, which favor protein aggregation and thus extraction unavailability of the molecules. 

An increase in protein extraction for the development of peanut protein ingredients could certainly contribute to spread their use by the food industry. Therefore, the pursuit of alternative extraction process for deffated peanut flour is desirable.

In the last years the use of novel methods in food industry has arisen, standing out microwave (MAE), because of their convenience from economic and process point of view but also because of their efficient energy-use. MAE is the use of electromagnetic waves, with a frequency from 300 MHz to 300 GHz. The microwave energy generates vibrations between the molecules of the solution, allowing them to be separated. MAE has been used for extraction of specific soluble compounds in fluids (liquids or gases). Only a few papers has been published using solvent extraction assisted with microwaves and only one for protein extraction using this technology in a soybean matrix (Choi et al., 2006).

Being peanut one of the most important oilseed cultivated worldwide and peanut deffated flour a main by -product of peanut industry, the aim of this research was the evaluation of microwave (MAE) effect in alkaline protein extraction from peanut defatted flour in terms of crude yield, protein extraction and purity of the final high-protein product (isolate). 

In order to reach this objective, microwave assisted alkaline extractions were made using deffated peanut flour as raw material (ratio 1:25, solid: distillated water) and a commercial microwave (LG, MS1142GW, Mexico) varying: A) power (145, 290, 435 and 580 watts) and B) processing time (2, 4, 6 and 8 minutes). Treatments were executed over the mix flour/water adjusted with NaOH 50% to pH 9.0 followed by an 1 hour in the incubator (Model RF 1575, VWR, USA, 70 rpm at 50°C) and then by a centrifugation (Model 5804R, Eppendorf, Germany, 10 minutes, 10,000 rpm at 23°C). Supernatants were separated and acidified with HCl (15%) up to a pH of 4.5. Protein and moisture of the previously separated curd (with centrifugation of the acid mix during 10 min, 10,000 rpm, 23°C) were obtained using AOAC official methods (925.10 and 984.13 respectively).

The final curds had a high purity (>95%), with no difference among treatments. The main effect of microwaves was detected in the extraction yield (final protein obtained in the isolate compared to the peanut flour initial protein content), because the higher was the microwave power and treatment time, the higher was the extraction yield. The increase most noticeable compared with the control was observed for 290, 435 and 580 watts, all at 8 minutes (augment of 7.86, 19.51 and 45.6% respectively). Treatment at the lowest power (145 watts) yielded non-significant difference compared with the control. An augmentation of 45% in the protein extraction from deffated peanut flour imply a recovery of more than 50% of the initial protein present in the flour, reducing thus the waste of high quality peanut protein and increasing of course the economic convenience of the process.

As expected, the extraction yield was directly related to crude yield (final isolate solid weight among initial solids in raw material) with a R²= 0.97. The effect of microwave treatment is not related only with an increase in the temperature of the deffated peanut protein dispersion, because the temperature was measured after all the microwave treatments and even when temperature was not higher than control, extraction yield was higher.

One interesting future study derived from these results is the evaluation of the functionality of different isolates obtained with microwaves. The ability of peanut isolates to absorb water or oil as well as their capacity to retain air are some of the highly appreciated characteristics as food ingredients. Also the evaluation of different processing times (higher than 8 minutes) is recommended.

The use of microwaves to assist alkaline protein extraction from deffated peanut flour opens thus the possibility to use a new technology, cost and energy efficient, in the treatment of an oilseed industry by-product and in food ingredient production.