11.3.10.1 Sweet Potato Protein
Besides plentiful starch and soluble sugars, sweet potato has 1.73-9.14 % protein (dry weight), which can be easily utilized by human body. As most of the protein is solubilized, sweet potato protein is not lost during starch preparing processes. As a result, effluent with waste water will finally lead to water pollution of rivers and lakes. Researches have demonstrated that sweet potato protein is one kind of plant protein of high quality and its amino acid ratio is relatively balanced (Tables 11.3.4 and 11.3.5). Meanwhile, the sweet potato protein has many excellent functional features, such as solubility, emulsibility, gelling property, water absorption, oil protection properties and good foaming, etc. (Mu et al., 2009b). By different technological procedures of extraction, separation and purification, sweet potato can be processed into sweet potato protein powder with varying content, which can be widely applied in various technological processing areas.
Table 11.3.4 Amino acid composition of protein derived from sweet potato variety 55-2a
Amino acid | Content (mg/g dry weight) | % of SPP
Aspartic acid (Asp) | 154 | 18.5
Threonine (Thr)b | 55.8 | 6.70
Serine (Ser) | 60.3 | 7.24
Glutamic acid (Glu) | 77.5 | 9.30
Glycine (Gly) | 34.3 | 4.12
Alanine (Ala) | 13.4 | 1.61
Cystine (Cys) | 24.3 | 2.92
Valine (Val)b | 62.0 | 7.44
Methionine (Met)b | 16.1 | 1.93
Isoleucine (Ile)b | 43.7 | 5.25
Leucine (Leu)b | 57.9 | 6.95
Tyrosine (Tyr) | 40.1 | 4.81
Phenylalanine (Phe)b | 54.3 | 6.52
Lysine (Lys)b | 43.3 | 5.20
Tryptophan (Trp)b | 5.85 | 0.71
Histidine (His) | 12.9 | 1.55
Arginine (Arg) | 44.2 | 5.30
Proline (Pro) | 32.9 | 3.95
Total | 833 | 100
% Essential amino acid | 339 | 40.7
a The values reported represent the average of three determinations.
b Essential amino acids.
Source: Mu et al. (2009a)
Table 11.3.5 Essential amino acid composition of SSP compared to the WHO "ideal protein"
Amino acid | WHO ideal protein (% of total protein) | SPP % of total amino acid | % amino acid/ ideal protein×100
Isoleucine | 2.8 | 5.25 | 188
Leucine | 6.6 | 6.95 | 105
Lysine | 5.8 | 5.2 | 89.7
Methionine + cysteine | 2.5 | 4.85 | 194
Phynylalanine + tyrosine | 6.3 | 11.3 | 179
Threonine | 3.4 | 6.7 | 197
Tryptophan | 1.1 | 0.71 | 64.5
Valine | 3.5 | 7.44 | 213
Source: Mu et al. (2009a)
Several techniques for recovering sweet potato protein have been developed, including biochemical, membrane separation and electrochemical technologies (Feng et al., 2009; Guven et al., 2012; Zhu et al., 2008). However, these technologies are expensive, time-consuming and complex. Therefore, it is of utmost importance to explore new technologies, which are simpler, inexpensive, non-polluting, and efficient and effective at recovering target compounds (e.g. proteins). If proteins can be recovered using a technology that can be used in food and pharmaceutical industries, the added value of sweet potato would increase.
Foam separation, also known as adsorptive bubble separation, uses foam bubbles to concentrate surface active materials (Stowers et al., 2009). During the early 20th century, foam separation was only used in mineral flotation and surfactant treatment. In the 1970s, it was used for proteins and enzymes recovery (Charm and Lemlich, 1972; Fenton and Hossain, 1998); the technology required only air or inert gas, making it a suitable method for processing diluted solutions (Backleh-Sohrt et al., 2005). Currently, foam separation is used in the metallurgical, fish, food and biochemical fields. Several studies have used foam separation to extract proteins, polysaccharides and bioactive compounds (Burghoff, 2012).
The technological process of protein recovery (Figure 11.3.14) from sweet potato starch waste water by foam separation is shown in Figure 11.3.15.
Figure 11.3.14 Sweet potato protein.
Figure 11.3.15 Technological process of protein recovery from sweet potato starch waste water by foam separation.
A foam separation apparatus, which consists of air compressor, gas flow meter, three-way valve, gas sparger, solution outlet, solution inlet and bent collection tube is shown in Figure 11.3.16.
Figure 11.3.16 Schematic representation of the foam separation apparatus.
11.3.10.2 Sweet Potato Dietary Fiber
As the living standards of people improve, the increased intake of high-calorie, high-protein, high-fat and, conversely, the gradually decreased intake of dietary fiber, has lead to the emergence of a series of “affluenza” diseases. Dietary fiber is capable of regulating human’s absorbing function of nutrients such as fat and sugar, and has great significance in nutritional balance in the human body. After starch extraction, the remained sweet potato residues have dietary fiber contents of over 20 %. Prepared by physical sieving, combined sieving and enzymatic hydrolysis, biotechnology, chemical separation, or combined chemical agents and enzyme methods, sweet potato dietary fiber can be applied to beverages, meat products, staple food products, seasonings and other foods.
The AO AC official method (Enzymatic-Gravimetric method) is commonly used for the extraction of total, soluble and insoluble dietary fiber in foods. The procedure of the method is continuous use of thermostable α-amylase, protease and glucoamylase at different temperatures and/or at different pH values to extract dietary fiber. However, the consecutive enzymatic extraction of dietary fiber has tedious steps, high costs and may result in interfusion of a lot of salt ions into the dietary fiber. Therefore, the single enzymatic method of extracting dietary fiber has been attracting the attention of the researchers gradually.
The technological process of extracting dietary fiber from sweet potato residues by the thermostable α-amylase method is shown in Figure 11.3.17 and a line of production is shown in Figure 11.3.18.
Figure 11.3.17 Technological process of extracting dietary fiber from sweet potato residues by the thermostable α-amylase method.
Figure 11.3.18 Production line for sweet potato dietary fiber.
11.3.10.3 Sweet Potato Pectin
Numerous research projects have shown that at least 20–30 % pectin remains in sweet potato residues. Nowadays, in China’s food industry, pectin is usually extracted from citrus skin and apple residues, while there is no report about preparing pectin by using sweet potato residues as raw materials and the corresponding applications in actual industrial manufacturing. High-purity sweet potato pectin can be prepared from sweet potato dietary fiber through the processes of acid extraction, concentration, precipitation and drying. Furthermore, sweet potato pectin has the physiological activities such as influencing nutrition and substance metabolism of fat and proteins, adjusting the growth of intestinal flora, promoting the apoptosis in cancer cells, tumor growth and transfer inhibition, and a stronger binding ability to metal ions. Therefore, sweet potato pectin can be added into preserved fruit, bread, frozen food, yogurt and beverages, etc.