1. Introduction
The common pulse comprises many different varieties and is the largest legume crop with an annual production of over 31 million tons worldwide [
1]. Of the pulses, the kidney bean, mung bean, adzuki bean, peas and chickpeas are the most commonly consumed in China [
2]. Pulses are generally characterized as rich sources of complex carbohydrates, vitamins and minerals, including folate, potassium and iron, in addition to having low lipid content [
3,
4]. They also contain significant amounts of protein (22–30% by weight) and are important plant-based sources of protein [
5]. Especially in developing regions, cereal grains and pulses are often the main component of the human diet, and provide a large proportion of the dietary protein intake [
6], with pulses providing proportionally more protein than the cereals. In comparison with cereal grains, pulses have higher protein contents. Generally, consumers are showing an increased interest in plant-based sources of protein, but accurate data on their digestible amino acid compositions is sparse.
An accurate assessment of the dietary protein quality and amino acid (AA) digestibility of food products are important [
7,
8], and the digestible indispensable amino acid score (DIAAS) has been endorsed by the Food and Agriculture Organization of the United Nations as the preferred method for determining protein nutritional quality [
9,
10]. A current barrier to the widespread use of DIAAS, however, is the limited amount of published data available for the true ileal AA digestibility of human foods [
10] and especially for foods in developing regions of the world. Ileal amino acid digestibility refers to measurement of digestibility at the end of the small intestine and replaces the less accurate measure of fecal digestibility [
11] while true digestibility denotes that the digestibility measure has been corrected for the potentially confounding endogenous amino acids found in ileal digesta [
12]. To calculate DIAAS, it is necessary to determine the true ileal digestibility (TID) of each AA, which is preferably determined in humans, however, if this is not possible, the growing pig is a suitable animal model for protein digestion in the adult human [
9,
13]. The physiology of protein digestion to the end of the small intestine is similar between the adult human and growing pig [
14] and the growing pig model has been shown to give true ileal amino acid digestibility values not statistically significantly different from those determined in the adult human [
15]. Therefore, the objective of the present study was to determine the apparent and true ileal AA digestibility values and DIAAS values of six cooked pulses commonly consumed in China using the growing pig as an animal model for protein digestibility in the adult human. The novelty of the present study is that true ileal amino acid digestibility was determined in pulses, prepared as for human consumption.
4. Discussion
According to the Food and Agriculture Organization of the United Nations (FAO) [
27], a pulse is a legume that is exclusively harvested for the dry grain and therefore excludes legumes such as peanuts and soybeans, which are harvested primarily for their oil. Pulses are also sometimes referred to as grain legumes or pulse grains. The published literature often refers to pulses as including kidney bean, mung bean, adzuki bean, broad beans, peas, chickpeas, cowpeas, hyacinth bean and lentils. The pulses are the major sources of dietary fiber, minerals and vitamins for many individuals, and have the potential to meet 10–20% of the recommended daily amount of certain nutrients for adults [
4,
5,
28,
29]. Additionally, they contain significant amounts of protein (22–24% by weight) and reflect important plant-based sources of this macronutrient [
4,
5,
28,
29]. In 2017, the world area of harvested beans, was 36.5 million ha, and the production of dry beans was 31.4 million tons. The production share of beans (dry by region) was: Asia 49.3%, Americas 25.2% and Africa 21.8%. Among them, India’s output is the first (6.4 million tons) and China’s is the fourth (1.3 million tons) [
1]. China has both temperate and subtropical zones and pulses comprise an important part of the agricultural production of China [
2].
Broad beans (
Vicia faba L.), adzuki bean (
Vigna angularis Ohwi & Ohashi), mung bean (
Vigna radiate L.) and kidney bean (
Phaseolus vulgaris L.) are all mainly distributed in Asia [
1,
2]. China ranks first, first, second and third in the production of broad beans, adzuki bean, mung bean and kidney bean in the world [
1,
2]. The annual production of them in China is about 1.8 million, 1 million, 0.4 million and 0.8 million tons, respectively [
1,
2]. Peas (
Pisum sativum L.) are mainly cultivated in Canada, the Russian Federation and China [
1]. Chickpeas (
Cicer arietinum L.) are mainly cultivated in India [
1]. All these pulses are widely consumed in China. Their production also plays an important part in the west semi-arid and arid areas of China. Recent investigations show that the consumption of pulses is beneficial, as they reduce the risk of acquiring chronic diseases [
30,
31,
32,
33,
34]. Pulses often provide a high proportion of the dietary protein for humans especially in developing countries [
6].
An accurate assessment of the dietary protein quality and amino acid digestibility of food products is necessary [
7]. DIAAS has been recommended by the Food and Agriculture Organization of the United Nations as the preferred method for determining protein nutritional quality [
9,
10]. To calculate DIAAS, it is necessary to determine the true ileal digestibility of each AA, which is preferably determined in humans, but if this is not possible, TID can be determined in the growing pig [
13,
35]. The growing pig model provides a method that allows TID to be determined in foods routinely.
AA digestibility determination at the terminal ileum is more accurate than the traditional total tract (fecal) method [
35]. Although ileal digestibility may not be a perfect measure to determine net amino acid absorption, it is considerably better than the amino acid digestibility determined over the total digestive tract [
36]. Dietary protein evaluation can be improved by determining the TID values of AA where TID values have been corrected for the influence of the basal gut endogenous losses [
25].
In different physiological and age stages, the amounts of amino acids required daily are different. Based on the recommended amino acid scoring patterns for infants, children, older children, adolescents and adults given in the Report of the most recent FAO Expert Consultation [
17], the amino acids required in the highest amounts are leucine (96 mg/g protein requirement) and then the aromatic AA (phenylalanine + tyrosine) (94 mg/g protein requirement) for infants, leucine (66 and 61 mg/g protein requirement) and lysine (57 and 48 mg/g protein requirement) for children and older children, adolescents and adults, respectively.
The most limiting AA in all six cooked pulses differed according to which amino acid reference profile was used. As expected, the DIAAS values were also different. The presently determined DIAAS values for cooked pulses ranged from 36% for broad beans to 53% for mung beans (birth to 0.5-year-old infants reference pattern), from 53% for broad beans to 77% for kidney beans (0.5 to 3-year-old child reference pattern) and from 60 for broad beans to 88% for kidney beans (older children, adolescents and adults reference pattern) (
Table 6). The DIAAS values for the infant were particularly low, though it is recognized that pulses are unlikely to be sole foods for this grouping.
For children aged 0.5 to 3-year-old, the DIAAS values of cooked broad beans, peas and chickpeas were similar to values reported previously [
4,
5]. Kidney beans, however, had a higher DIAAS value than that previously published [
4]. In addition, the first limiting amino acid, determining the DIAAS value, was different from previous reports [
4,
5,
29]. The first limiting amino acid determining the DIAAS value of adzuki beans was demonstrated to be leucine, while for the other pulses in the present study the first-limiting AA was lysine. In previously reported studies the first-limiting AA was methionine + cysteine. Variation in DIAAS values may be a result of differences in the animal model used, analytical differences especially for the sulfur AA’s, the means of cooking the beans, or factors such as processing, bean variety, growing conditions of the pulses and the presence of anti-nutritional factors in the pulses [
4,
5,
29].
Based on the cut-off value for DIAAS from the Report of the FAO Expert Consultation [
9], the cooked kidney beans, mung beans and chickpeas are considered “good” protein sources for human consumption because their DIAAS is 88%, 86% and 76% (based on older children, adolescents and adults reference pattern). However, based on the 0.5 to 3-year-old child reference pattern, only kidney beans would be considered a “good” protein source.
From the current research and previous reports [
4,
5,
29,
37,
38], it is concluded that the range of DIAAS values for pulses according to recommended amino acid scoring patterns for children (0.5 to 3-year-old) [
9] is generally 55–83%, which is lower than that for animal proteins [
9,
24,
25,
37], but higher than that for cereal proteins [
8,
9,
24,
25,
26,
37,
38]. The DIAAS value of animal proteins is generally higher than 90% [
9,
24,
25,
37]. However, most of the DIAAS values for cereal proteins are less than 55% [
8,
9,
24,
25,
26,
37,
38]. Only in the work of Cervantes–Pahm et al. [
26] were higher DIAAS values of 77% and 64% observed for dehulled oats and polished white rice, respectively. In the Han et al. [
8] report, the DIAAS value of buckwheat was 68%. A novel aspect of the present work is that the materials were evaluated in their cooked form, as eaten by Chinese people. Cooking may alter nutritional quality.
In China, the main consumers of these cooked pulses are not infants or children aged 0.5 to 3-year-old, but older children, adolescents and adults. Currently, Dietary Guidelines for Chinese Residents recommend 250–400 g of cereal and potato-based foods per day, including 50–150 g of whole grains and edible beans. High-quality animal protein is often accompanied by high amounts of animal fat, while pulses have high-quality protein and a low amount of oil. For this reason, pulses are a good dietary choice. At the same time, when pulses are the main source of dietary energy and protein, protein complementation may be needed. Judicious combinations of pulses, cereals and relatively small amounts of other high-quality proteins can be used to maximize the supply of utilizable protein.