Purification and Characterization of Pinto Bean Protein Using Membrane Technology

Abstract

Pinto beans, an underutilized legume, are abundant in protein content and contain a variety of beneficial phytonutrients. However, the commonly used protein extraction method, alkaline extraction, is associated with several drawbacks. These drawbacks include low extraction yield and purity as well as the production of large amounts of wastewater that can lead to environmental hazards. In this regard, membrane technology has gained considerable recognition as a superior method for extracting proteins. A combined processing scheme was developed, which included alkaline extraction at pH 10.5, ultrafiltration with a concentration factor of 5.5, diafiltration with a diavolume of 4, and isoelectric precipitation at pH 4.5 followed by freeze drying. The specific functional characteristics (nitrogen solubility index, water and oil holding capacity, and emulsifying and foaming properties) of the protein concentrates were assessed and compared with those of a commercially available soybean protein isolate. Based on pinto bean flour containing 23.9% protein, 85.5% of the protein was recovered in the products of this process: precipitated protein concentrate (PPC) with 86.4% protein, acid-soluble protein concentrate (ASP-C) with 56.3% protein, and meal residue with 6.1% protein. The mass yields were 17.3% in PPC, 3.9% in ASP-C, and 54% in the meal residue. The precipitated protein showed higher emulsifying activity, and the acid-soluble protein showed a high nitrogen solubility index (NSI) and oil-holding capacity. Both proteins had comparable foaming properties to commercial soy protein isolate. The project demonstrated the feasibility of protein production from pinto beans and highlighted the proteins' useful food functionality and good potential for commercialization.

Keywords: diafiltration; extraction; membrane processes; pinto bean protein; ultrafiltration.

FIGURE 1

Process flow diagram for protein extraction from pinto bean flour.

FIGURE 2

Effect of pH on protein extractability of pinto bean flour. Data represents the means of three replicates ± SD.

FIGURE 3

The amount of precipitated protein in the protein feed solution at varying pH. (▲) Measured from the protein content of precipitate; (■) Measured from the mass balance of protein in combined supernatant. Data represents the means of three replicates ± SD.

FIGURE 4

Acid soluble protein concentrate (ASP‐C) and precipitated protein concentrate (PPC) recovered from pinto bean glour.

FIGURE 5

Foaming properties, whippability (A), and foam stability (B) of protein products from the pinto bean and soybean protein isolate. Different small letters indicate a significant difference at various time points for each sample (p < 0.05). Different capital letters demonstrate a significant difference among the various protein samples at a consistent time (p < 0.05). ASP‐C, pinto bean acid soluble protein concentrate; PPC, pinto bean precipitated protein concentrate; SPI, commercial soybean protein isolate (Supro 500E).