2.1. α-Galactosides

The most common oligosaccharides in pulse seeds are α-galactosides or the raffinose family of oligosaccharides (raffinose, stachyose, verbascose and ajugose), raffinose and stachyose being the most ubiquitous sugars. Some authors ^{[3][5][6][9][10][22]} have reviewed the content of α-galactosides in different seeds and varieties, ranging from 0.4 to 16.1% (dry matter—d.m). Singh et al. ^[11] reported that lentils presented the lowest amount of total galactosides (37.5 mg/g), followed by faba beans (52.0 mg/g), beans (60.9 mg/g) and peas (66.3 mg/g). Muzquiz et al. ^[9] reported concentrations of raffinose, ciceritol, stachyose and verbascose in different varieties of Spanish beans, peas, chickpeas, faba beans, soybeans and lupins; raffinose ranged from 1.0 mg/g in Phaseolus vulgaris var. Palmeña to 32.15 mg/g in Lupinus mariae-josephi; stachyose varied from 9.22 mg/g in Vicia faba var. Alameda to 59.08 mg/g in Lupinus albus var Multolupa. Verbascose was not detected in the reported chickpeas, the highest amount being found in Pisum sativum var. Luna (50.25 mg/g). Another α-galactoside is ciceritol, an α-D-digalactoside of pinitol that does not belong to the raffinose family of oligosaccharides. It is not present in all pulses, and its amount ranged from 1.61 mg/g to 29.65 mg/g in pea var. Iceberg and chickpea var. Duraton, respectively.

The α-galactosides are not digested or hydrolyzed by humans. However, they are fermented by colonic bacteria with the production of hydrogen, carbon dioxide, methane and short-chain fatty acids (SCFA), mainly butyric and propionic. While the gases produced are responsible for flatulence, bloating and diarrhea, the SCFA are mainly related to the prebiotic effect associated with the α-galactosides, promoting the growth of beneficial gut microflora (bifidobacterias and lactobacilli) and reducing the enterobacteria population. α-Galactosides have also been shown to help in normalizing bowel function, reducing potentially carcinogenic compounds (such as N-nitroso compounds), and enhancing the immune system and increasing resistance to infection ^{[3][9][11][23]}. In addition, it has also been reported that propionic acid reduces serum cholesterol, helping to reduce the risk of cardiovascular diseases, and butyric acid induces apoptosis and stops the growth and differentiation in colon cancer cells ^[3][9][11]. Despite there not being a recommended dietary intake for α-galactosides, Martinez-Villaluenga et al. ^[23] documented that a dose of 3 g/day of α-galactosides produces an increase in the intestinal bacteroides, bifidobacterias and eubacteria without any flatulence discomfort.

2.2. Myo-Inositol Phosphates

Phytate (IP6) or myo-inositol hexakisphosphate is the main form of phosphorous storage in pulse seeds (up to 75% of total seed phosphorous), and is stored as salts (phytate–mineral complex) or bound with proteins or starch ^[3][9]. According to the literature, the total content of inositol phosphates ranges from 0.2 to 2.34% ^{[3][9][10][11][22]}, and as described above for α-galactosides, the amount of inositol phosphates varies between species and varieties, as well as with the soil phosphorous ^[24]. Sparvoli et al. ^[3] reported that varieties of peas (2.1–7.1 mg/g), chickpeas (2.8–12.6 mg/g), lentils (2.5–12.2 mg/g) and mung beans (1.8–5.8 mg/g) contain relatively lower amounts of IP6 than those of common beans (2.4–28.7 mg/g), faba beans (5.9–15.0 mg/g) and soybeans (4.8–20.1 mg/g). Muzquiz et al. ^[9] reported the total inositol phosphate content in different varieties of some Spanish legumes, the average content being 0.4% in beans, 0.75% in faba beans, 0.6% in chickpeas and beans, 0.7% in lupins and 1.2% in soybean varieties.

In general, phytate, myo-inositol hexaphosphate or IP6 has been considered as an antinutrient that interferes with nutrient (mineral, protein and starch) digestibility and bioavailability ^[25]. However, this mechanism of action also produces health benefits. As IP6 binds starch and the calcium necessary for α-amylase activity, the starch digestibility is reduced, improving the glycaemic response of pulses and the management of diabetes type 2 ^[3][22]. Due to its mineral-binding capacity, IP6 has been linked with other beneficial health effects, such as the prevention of kidney stone formation, the prevention of cavities and plaque in teeth and protection from demineralization. It has been also reported that a diet with 1% sodium phytate added can control hypercholesterolaemia and atherosclerosis, and can reduce the risk of colon cancer and improve irritable bowel syndrome ^[26][27]. Further, it has been reported that the lower phosphorylated forms (IP5–IP3) can promote the absorption of minerals and show strong antioxidant and anti-inflammatory activities, inducing apoptosis and normalizing abnormal cell proliferation ^{[3][9][26][22]}. In addition, some authors ^[28][29] have reported that iron absorption can be improved when IP6 is below 10 mg/g protein in one serving dose.

2.3. Protease Inhibitors

Protease inhibitors in legumes belong to the Kunitz and the Bowman–Birk families, and both are capable of inhibiting trypsin and chymotrypsin enzymes. There is a high number of isoforms of both inhibitors that vary with the legume species and variety ^[30]. The trypsin inhibitor content ranges from 5.75 to 15 trypsin inhibitor units (TIU)/mg in peas, from 5 to 10 TIU/mg in faba beans, from 12.60 to 19 TIU/mg in chickpeas, from 3 to 8 TIU/mg in lentils and from 8.57 to 82.70 TIU/mg in soybeans ^[9][19]; while the chymotrypsin inhibitor content varies from 2.19 chymotrypsin inhibitor units (CIU)/mg in Vicia narbonensis to 17.30 CIU/mg in bean var. Riñón, but is not detected in soybean var. Ostrumi ^[9][10]. Protease inhibitors have a negative effect on animal growth due to the inhibition of gut protein digestion. However, in a Western diet, there have not been any reported toxic problems related to the intake of these compounds, mainly because pulses are cooked prior to consumption and protease inhibitors are thermal-labile compounds. Over the past two decades, different studies ^{[3][6][9][26][22][31][32]} have shown that protease inhibitors are effective in preventing or reducing colon, lung, liver, prostate and breast cancer progression. Sánchez-Chino et al. ^[22] reported that some possible mechanisms of action are: (i) the reduction of protein digestibility reduces the availability of essential amino acids for the cancer cells; (ii) the protease inhibitors act as insoluble dietary fiber able to absorb carcinogens (such as free radicals) in the gut; and (iii) the inhibiting of proteases produced by cancer cells. Even though there is not a recommended amount of protease inhibitor consumption, it is important to note that the traditional Japanese diet contains about 420 protease inhibitor units/day; further, it has been reported that the consumption of the purified protease inhibitor at 25–800 CIU per day during a period of 12 weeks exerted a protective effect against cancer development, and doses of up to 2000 CIU/day did not cause health problems in humans ^{[9][10][11][26][31][32][33][34][35]}.

2.4. Phenolic Compounds

Pulses are rich in phenolic compounds and include different subcategories such as tannins, flavonoids, isoflavones, phenolic acids (such as caffeic, ferulic, sinapic and p-coumaric acids) and anthocyanins. Many of these compounds are located in the seed coat and are responsible for seed color, and are related to the taste and flavor of seeds. In general, the darkest legume varieties tend to have higher amounts of phenolic compounds than the light seeds/varieties ^{[3][11][36][37]}. Even though a great variability in the phenolic content can be found due to the different methods used in their extraction and quantification (spectrophotometrically or by HPLC (high-performance chromatography)), in general, a high variability can be found among legumes and varieties ^{[5][38][37][39][40][41][42]}. For example, among beans, there can be found values from 0.3 mg/g of phenolic compounds in white varieties to 12.6 mg/g in black varieties; peas show values from 0.6 to 2.7 mg/g and chickpeas from 0.6 to 2.7 mg/g ^[19]. Pedrosa et al. ^[5] reported that the raw Curruquilla bean, a cream-colored variety, shows a higher content of total phenols and anthocyanins (2.70 mg/g and 40.10 µg/g) than the Almonga bean (2.38 mg/g and 38.47 µg/g), a white variety. Some authors ^[38][41] reviewed the phenolic content of various legume seeds and documented contents from 11.2 to 48.3 mg/g in dry beans, from 117.8 to 157.6 mg/g in different faba bean genotypes, from 4.9 to 68 mg/g in lentils and from 0.98 to 183 mg/g in chickpeas. Condensed tannins are associated with an astringent taste, as well as with some anti-nutritional effects due to their ability to bind and precipitate proteins, reducing their digestibility. However, from a health point of view, different studies found in the literature report phenolic compounds as bioactive molecules with antioxidant, antimicrobial, anticarcinogenic, immunomodulating, cardio-protective, anti-hypertensive and anti-inflammatory properties, lowering the risk of colon cancer and osteoporosis ^{[6][10][26][37][41]}. Most of these actions are linked to the antioxidant and antiradical activities of the different phenolic compounds. In general, antioxidant activity is positively correlated with total phenolic content ^[41]. Cardador-Martinez et al. ^[43] reported that white beans possess low antioxidative activity in comparison to black, brown and red bean varieties. As documented in some reviews ^[11][41], different authors related this high antioxidant activity to pro-anthocyanidins or condensed tannins, as well as to flavonoid content, and concluded that pulses can be a useful natural source of antioxidants. Zhang et al. ^[44] reported that lentil phenols also inhibit glucosidase and lipase, and would therefore contribute to controlling blood glucose levels and obesity in humans. In spite of these health benefits, there is not in fact a recommended daily intake of phenolic compounds, mainly due to the differences in their total intake for the overall population; however, some reports recommend a minimum daily dose of 300 mg of total phenolic compounds to benefit from their health properties ^[45].

3. Effect of processing (autoclaving or extrusion) on the bioactive compounds of pulses

It is well known that pulses are an inexpensive and sustainable source of nutrients and they represent functional foods that are rich in different bioactive compounds, providing many health benefits when consumed regularly in a balanced diet. Pulses are processed prior to their consumption; therefore, even though a pulse seed contains a high amount of bioactive compounds, it does not mean that the processed pulse has a high content of the bioactive components because their processing modifies their content. The extent of these modifications depends on the seed, the variety, the food matrix, the formulations, and the technological processing. Galactosides were reduced up to 77%, 68% and 82% in autoclaved, extrusion/cooked and cold-extruded plus cooking samples, respectively. Although some paper reported an increase in some of the extrusion/cooking processed samples. Myoinositol phosphates were more affected by autoclaving and extrusion/cooking, reaching reduction between 2-100% and 1-99%, respectively. Cooked pasta reduced their content up to 15%. All the processed reviewed reduced drastically the protease inhibitors content (80-100%), therefore it can be concluded that these samples would show an improved protein digestibility. The majority of the revised papers reported a reduction of the phenolic compounds up to 80%, 99% and 51% for autoclaved, extrusion/cooking and col-extruded/cooked products. One serving of autoclaved legumes, extruded/cooked and cold-extruded/cooked products can supply on average 3.2, 1.8 and 1.3 g of galactosides, and even though there is no a recommended daily intake, according to the literature it would be enough to obtain prebiotic benefits on gut. Considering one serving of each product and depending on the legume seed and the processing conditions some of the processed legumes could supply less than 10 mg/g protein and thus improving the mineral absorption. The low remaining trypsin inhibitor activity (<1 to 1.5 TIU/mg) in the processed legumes included in this review would have a positive effect in human health. One serving of majority of the revised product can supply more than the minimum daily dose (300 mg) of total phenolic compounds to obtain health benefits. Therefore, from the research documented in this review, it can be concluded that processed pulses and pulse-based foods can supply not only nutritive compounds but also significant amounts of bioactive compounds such as galactosides, phytates, protease inhibitors and phenolic compounds with the potential to contribute to human health and wellbeing. Notably, the health benefits reported for legumes including anticarcinogenic and antihypertensive effects, and improvements in cardiovascular disease, diabetes type 2 and obesity could be related to a synergistic combination of the bioactive components present in legume seeds. With the increasing innovations in food processing technology, the use of pulses in the development of new food products can meet the consumer’s requirements for more nutritious and healthier products. Finally, noteworthy that it is interesting not only to know the phytochemical content on the processed legumes but also it is necessary to know which amount of these compounds are available in the gut or once they are absorbed to exert their beneficial effect. In the near future, it would be necessary to carry out in vitro digestibility studies that allows to determine which is the amount that reach the gut and is available to exert their healthy effect.