Burcu Cabuk | Darren Korber | Takuji Tanaka | Michael Nickerson*
Pulses represent the second most important grain after cereals, with a recorded total production of 77.64 million tonnes in the world, with India, Canada and China representing by far the world’s largest pulse producers1. Pulses are a rich and inexpensive source of proteins, carbohydrates, micronutrients, vitamins, and minerals. These high-energy and nutrient-dense crops have made pulses staple food items in many nations. These benefits that pulses provide are being “rediscovered” as part of a health-conscious diet in North America. Recent studies have shown that consumption of pulses in the diet has a significant positive impact on decreasing the risk of cancer, cardiovascular and coronary heart diseases, and type 2 diabetes. Pulses are also rich in phenolic compounds, a group of phytochemicals found in plant foods that play an important role in protecting cells against oxidative damage2. Several studies have shown that antioxidant properties in pulses may reduce the prevalence of some forms of cancer3-5. Within the food industry, pulses are now being exploited for their bioactive compounds, as functional foods and in pharmaceutical products6-9. Pulses continue to grow in popularity as an important food source, so much so that the Food and Agriculture Organization designated the year 2016 as the International Year of Pulses.
In developing countries where consumption of animal proteins are expensive and the supply is scarce, pulses represent the main source of dietary protein intake10. Moreover, a growing number of studies have demonstrated higher levels of cholesterol and risk for cardiovascular diseases associated with consumption of animal proteins, leading to significantly greater interest in producing pulse-based products for the food industry due to high protein content in developed countries11. However, the low digestibility and low bioavailability of proteins due to protein structures/complex and anti-nutritional factors (ANFs) are significant concerns that potentially might restrict the widespread use of pulses12. The presence and negative impacts of these ANFs (e.g., protease inhibitors, phytic acid, phenolics, lectins, and tannins) have been revealed in many studies13-16. Therefore, methods aimed at complete elimination or reducing these ANFs prior to their safe use in human consumption have been studied widely. These methods can be divided into two categories: physical and biochemical methods12.
Among these methods, fermentation processes are increasingly gaining attention in decreasing the level of ANFs and therefore improving the nutritional quality of pulses. Besides the elimination of ANFs, fermentation of pulse proteins offers various advantages that include improved functional properties, increased vitamin and mineral content, enhanced flavour, production of bioactive compounds and essential amino acids, and increased carbohydrate availability17-19. This review will provide an overview of the impact of fermentation on the nutrition and functionality of pulses.
Factors Influencing Fermentation
Fermentation has been used for thousands of years, however only recently have we sought to better understand fermentation’s potential to modify and improve the nutritive value and in vitro protein digestibility of foods. The process of fermentation not only enhances the nutritive values, but process also improves the appearance and taste of some foods20. However, the effectiveness of the fermentation process depends on several factors, the most significant of which is the type of the microorganism used to ferment the food. There are three main kinds of microorganisms used in the fermentation of legume pulses; lactic acid bacteria (LAB), bacteria of the genus Bacillus, and Aspergillus molds. In various industrial fermentations, LAB have seen widespread use for the development of functional foods and production of enzymes/metabolites. Lactobacillus plantarum, for example, has been seen application for the production of industrial enzymes such as tannase, and fermentation of foods for the reduction of ANFs21. Coda et al.22 showed that fermentation with L. plantarum VTT E-133328 was able to reduce the ANFs and improve the nutritional properties of faba bean flour. This study demonstrated that trypsin inhibitor activity and condensed tannins (by more than 40%) can be significantly reduced by fermentation technology. On the other hand, strains of Aspergillus, especially Aspergillus niger, produce large amounts of highly-active, extracellular fungal phytases23,24. Application of A. niger fermentation for the removal of phytic acid from pulses has been reported25,26. For instance A. niger fermentation was found to remove phytic acid, resulting in a degradation of 57% in soybean26. Similarly, Bacillus spp. are well-known for the production of large amounts of alkaline proteases27,28 and may accordingly be used to increase the biological availability of essential amino acids through the degradation of anti-nutrients. Other factors known to affect the rate and extent of fermentation include: fermentation pathway (homo- versus hetero-fermentative), substrate type, cell density, tolerance to various metabolites, end products or by-products, temperature, pH, fermentation mode (batch, feb-batch, or continuous), solid state versus submerged fermentation, and fermentation time.
Modification of Protein Functionality by Fermentation
The functional properties of proteins are a function of their physicochemical characteristics. Hydrolysis of proteins during fermentation can therefore be a powerful tool to alter protein functionality through the modification of a protein’s surface chemical properties as well as physical size6. During fermentation, the number of ionizable and hydrophobic groups on the protein surface may increase as the molecule unravels, resulting in a change in physical or chemical interactions with other proteins and substances29. These interactions yield both positive and negative effects as protein functionalities change, depending on the protein and the degree of hydrolysis. For instance, the water-holding capacity of proteins (in flour or protein isolates) can be improved via fermentation since the exposure of buried hydrophilic amino acids can lead to an increase in the affinity for water30. Xiao et al.31 observed a significant increase in water holding capacity by fermentation with C. militaris SN in chickpea flour. Moreover, results revealed that fat absorption capacity and emulsifying properties of chickpea flours were also improved by fermentation. Fermentation of pulses has also been shown to improve protein solubility through the increase in the number of ionizable amino and carboxyl groups, along with formation of smaller peptide fragments33,34. This is supported by Amadou et al.33 who observed a significant increase of nearly 10% in protein solubility during fermentation due to the hydrolytic actions of L. plantarum Lp6. Limón et al.34 also observed the similar trend in protein solubility in fermented kidney beans. Other studies have demonstrated that fermentative hydrolysis of proteins resulted in improved emulsification properties due to production of low molecular weight peptides which can easily migrate to the oil-water interface, resulting in a smaller, and more stable, emulsion35. However, since fermentation can also lead to the exposure of hydrophobic regions, proteins in a solution may aggregate with other partially hydrolyzed proteins, resulting in poorer solubility and emulsifying capacity36. Accordingly, empirical testing is often necessary to benchmark the effects of a particular fermentation process (using a defined organism and set of conditions) for a particular pulse seed, flour or fraction.
Production of Bioactive Peptides and Antioxidants by Fermentation
Bioactive peptides are protein fragments that can elicit a positive effect on human health. These bioactive peptides, derived from proteins, are able to influence basic body systems (e.g., cardiovascular, nervous, gastrointestinal and immunological) and show multi-functional characters. Besides functional, physical and chemical aspects of protein modification by fermentation, formation of bioactive peptides and increasing antioxidant levels in pulses as a result of proteolysis during fermentation has been reported37,38. For instance, bioactive peptides released from fermented soybean products have been shown to possess anti-hypertensive, anti-tumor, and anti-diabetic properties39,40. Another study conducted by Torino et al.41 to produce water-soluble peptide fractions with antioxidant and antihypertensive properties. As a result, an increasing amount of total phenolic compounds and antioxidant capacity were observed throughout the fermentation process with L. plantarum and B. subtilis in liquid (LSF) and solid state fermentation (SSF), respectively. Again by L. plantarum, production of Angiotensin I-converting enzyme (ACE) inhibitory peptides were produced after fermentation of mung bean42.
With growing consumer interest in functional foods and increasing demands from the food industry for alternative functional ingredients, it is increasingly desirable to obtain modified plant proteins with improved physical, chemical and biological functional properties. Fermentation is one of the oldest and most simple biotechnological processes whereby one may modify the properties of proteins in foods. This technology has been “rediscovered” in recent years as a controlled and reproducible means for enhancing the solubility, emulsifying and foaming characteristics, or bioactive peptide liberation, of pulse proteins. In turn, this enables us to improve the nutritional and functional quality of foods by achieving desirable changes in the composition and availability of nutrients. Thus, fermentation of pulses presents the opportunity to add functionality to the fermented product beyond that of simple nutrition. In the near future, food products, as well as diets, will both be tailored via fermentation to meet the specific and varied health needs of our society’s population.
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Burcu Cabuk, Darren Korber, Takuji Tanaka, Michael Nickerson*
Department of Food and Bioproduct Sciences, University of Saskatchewan
(*Corresponding author email: Michael.Nickerson@usask.ca)