Encyclopedia
About 35-45 % of all soluble potato proteins are fractions of albumin and globulin characterised by high molecular weight in the range of 40-45 kDa and high nutritive value, comparable to proteins of animal origin. They are soluble in water and in water and salt solutions, respectively; these are patatin glycoproteins, with a pIs in the range of 4.8–5.2. The second large group of potato proteins comprise fractions of protease inhibitors with a molecular weight in the range of 7–21 kDa, mostly acid soluble; they are often heat-resistant, showing a wide spectrum of healthpromoting effects, such as antimicrobial or antitumor, as well as antioxidant and antiradical. Properties of potato proteins depend strictly on the method of isolation and the applied factors, such as pH, ionic strength and temperature.
- potato protein
- fractions of patatin and protease inhibitors
- nutritional value
- functional properties
- biological properties
1. Introduction
The importance of isolating potato protein from potato industry by-products, mainly starch, was recognized by various authors over 65 years ago. This was a result of issues connected with the management of huge amounts of waste products, potentially posing a threat to the human environment due to the high concentration of nutrients that feed microorganisms and enable unfavorable changes in water and soil. Initially, attempts were made to separate proteins from juice water in the starch industry and then, after the introduction of pulp centrifuges, from potato juice. Research on the nutritional value and structure of potato proteins, including the identification of fractions differing in molecular weight, solubility or isoelectric point, conducted during this period proved the high nutritional value of most potato protein fractions, especially the patatin fraction, and their significant differentiation in terms of functional characteristics. Moreover, depending on the factors used, such as temperature, pH, type of acid used, ionic strength of the protein solution, and the properties of the preparations obtained, the efficiency of their production and the directions of potential use were differentiated. The introduced techniques for the separation of protein fractions, as well as the attempts to modify the obtained preparations, e.g., through their enzymatic hydrolysis, led to the increased possibility of obtaining preparations with specific compositions and properties. The specific, rich chemical composition of potato juice, including the presence of non-starchy carbohydrates, polyphenols and natural toxic substances, such as glycoalkaloids, significantly hindered the production of potato protein preparations; the different features of foodstuffs, with high purity, good digestibility and optimal functional characteristics, thus necessitated the use of several different isolation techniques in parallel, which significantly increased the price of such preparations.
A new, interesting direction in this field is the development of research on methods of obtaining and modifying low-molecular-weight proteins due to their previously unknown or insufficiently documented antimicrobial properties, or the use of certain potato protein preparations as technological factors, such as in filtration processes in winemaking. Moreover, the wider use of potato pulp for obtaining potato protein preparations, especially with the use of multi-enzyme systems, seems interesting. Therefore, the purpose of this article was to trace the results of research relating to the nutritional, functional, pro-health, antimicrobial and technological properties of potato proteins, as well as the directions of their use resulting from the structure, molecular weight and fractionality of proteins, the raw material used and the methods of isolation and modification.
The processing of potatoes in the starch industry requires the management of large quantities of protein by-products. These include the potato juice separated from the potato pulp and the pulp left over after washing the starch from the potato cells. Annually, the European Union processes around 8 million tons of potatoes in this way, which generates around 6 million m3 of potato fruit juice (PFJ) and potato pulp in large quantities [1,2]. Potato fruit juice as a by-product in the starch industry contains only around 1–2% of crude protein. Obtaining proteins from such large-volume by-products is a challenge for the industry but, at the same time, enables the production of protein with high nutritional value and significant added value due to the use of raw material with a potential threat to the human environment [3,4,5]. The management of waste by-products from the starch industry contributes also to the acquisition of relatively inexpensive and non-allergenic protein preparations [6] with favorable functional properties, allowing for the replacement of soy or milk proteins traditionally used in food production. Depending on the conditions of the isolation process of proteins from waste products in the starch industry, especially potato fruit juice, the obtained preparations have different properties and there are different potential directions for their use.
Potato proteins, isolated under industrial conditions by acid-thermal coagulation, are primarily used as a valuable feed. In 2017, 63.900 tonnes of fodder potato protein were produced in Europe [7]. In the starch industry, traditionally obtained dried potato protein for fodder purposes has an important position in the market, being a relatively cheap source of protein for pigs and young calves [8]. Some studies in this area also show a positive effect of potato proteins on the health of farmed pigs [9].
2. Structure and Fractions of Potato Proteins (Solanum tuberosum L.)
Most potato proteins are fractions of albumin and globulin, soluble in water and in water and salt solutions, respectively; these are patatin glycoproteins, with a pIs in the range of 4.8–5.2 [4,5,22,23]. According to various authors [5,19,24,25], globular proteins constitute 75–85% of all soluble protein fractions found in potatoes, while approximately 25% of all potato proteins are insoluble proteins that build the potato cell walls. Potato juice also contains smaller amounts of glutelin protein fractions (around 9%) soluble in dilute alkali and prolamines (2–4%) soluble in aqueous alcohol. Research conducted by various authors over the course of around 40 years allowed for a detailed characterization of the protein fractions present in potato juice. This showed that the main proteins dissolved in potato fruit juice (PFJ) can be classified into three groups. These include the patatin group proteins with a molecular weight in the range of 40–45 kDa [4,5,14,18,22,26], including the 41 kDa family of glycoproteins, occurring in the form of dimers with a molecular weight of around 88 kDa. They constitute 35–40% of soluble proteins and coagulate in an acidic environment. The second, no less numerous group of potato proteins is acid-soluble fractions with a molecular weight in the range of 16–25 kDa [4,5,19,25], predominantly the protease inhibitor fractions of 7–21 kDa. The remaining proteins found in potato fruit juice (PFJ) are of high molecular weight (above 87 kDa) [27] and form the third group of protein fractions. The proteins of the protease inhibitor family have been divided into seven groups due to the active site of the enzymes [20,28,29,30]. Pouvreau et al. [31], analyzed the inhibitor proteins from Elkana cultivar tubers, which released the highest amounts of potato serine protease inhibitor (PSPI/PI-2), which accounted for around 22% of total nitrogen; potato cysteine protease inhibitor (PCPI), whose share in total nitrogen was on average 12%; and a large group of inhibitory proteins present in a total amount of around 12%, such as starch synthetase, polyphenol oxidase or potato multicystatine. In the amount of 4–6% were such protease inhibitors as potato aspartate protease inhibitor (PAPI) (6%), potato inhibitor I (PI-I) (5%) and potato Kunitz-type protease inhibitor (PKPI) (4%). The individual fractions of potato proteins showed significant differences in terms of nutritional value and functional properties [5,19,32,33,34,35] and biological activity [18,36]; therefore, it is preferable to separate the protein fractions of the patatin group and protease inhibitors [19,37,38]. In addition, potato proteins intended for food purposes must be subjected to heat treatment, which affects their structure and biological activity, including individual protein fractions [18]. Fluorescence spectroscopy allowed the determination of the temperature at which the conformation of proteins of protease inhibitors clearly changed. As shown by the research of Sun et al. [18], a temperature of 80 °C not only damaged the hydrophobic interactions but also caused the proteins to stretch, as evidenced by the presence of a greater number of tryptophan residues located on the proteins’ surfaces. These studies showed that the inhibitory protein family fractions showed mainly a β structure [18].
2. Nutritional Value of Potato Proteins
Potato proteins are characterized by high nutritional value, comparable to proteins of animal origin. This has been confirmed by the research of many authors over several decades [4,5,10,32,35,39,40,41,42,43,44]. Despite the beneficial nutritional, health-promoting and functional properties of potato proteins, they are relatively rarely used in food production. This is due to a number of technological and economic limitations; however, the constant progress in methods of isolating proteins from plant materials, including potatoes, and in increasing the purity and homogeneity of the obtained preparations creates the basis for the development of food preparations with high nutritional and health-promoting potential, as well as technological potential.
The nutritional value of proteins is related to the content of amino acids, their mutual proportions and digestibility. In potatoes, apart from protein-building amino acids, there are significant amounts of free forms of amino acids, which increase the nutritional value of potatoes as a food product [39,40]. The high nutritional value of potato protein and its good digestibility has been confirmed by many studies [4,5,10,32,35,41,42,43,44]. Basic fractions that build potato protein, i.e., albumin, globulins, glutelins and the so-called residual proteins, are characterized by high nutritional value, as evidenced by the values of indicators such as chemical score (CS), the index of essential amino acids or PER at the level of 57–69, 48–83 and 0.95–2.3, respectively [5]. The rather wide ranges of values of these indicators showed, first of all, the varietal differences. With respect to nutritive value, potato proteins are similar to proteins of animal origin and exceed most plant-based proteins [35]. On the other hand, the low amounts of the prolamine fraction proteins (around 4% soluble protein) show lower nutritional value. The studies presented by Pęksa et al. [45] showed that the nutritional value of potato proteins was influenced by the potato variety, regardless of the color of the tuber flesh. Storage conditions such as time and temperature, especially in the interaction with the cultivar, also influenced the differentiation of potatoes in terms of the content of specific proteins and selected amino acids [39,40]. Potato protein is characterized by a particularly high content of the essential amino acid lysine, which is distinguished among plant proteins, while the limiting amino acids are usually tryptophan, methionine and cysteine; in colored potatoes, leucine is found [45]. The most abundant in potatoes are the amino acids aspartic and glutamic acid and their amides.
The protein fractions of albumin and globulin, glycoproteins with a degree of glycosylation of around 4% [46], belong to the group of proteins typical of potato and are referred to as patatin or tuberin [22]. These proteins, due to their presence in significant amounts (40–60% soluble potato proteins), are considered as storage proteins. They are distinguished from other fractions by their high nutritional value, good solubility in water or aqueous salt solutions and enzymatic and antioxidant activity [14,17,23,27,46]. Patatin proteins are characterized by a good balance of the amino acid composition and a higher content of the essential amino acid, lysine, than other plant-based proteins. For these reasons, their nutritional value exceeds most plant proteins, including lysine-poor cereal proteins, and is close to the nutritional value of hen egg proteins [34], considered to be one of the reference proteins. Isolated from potato juice in its natural or only partially denatured form, it can be a potentially beneficial food additive [10,19,47,48].
The second-largest group of proteins, accounting for around 30% of all potato proteins, are protease inhibitor proteins. They show great diversity in terms of structure, molecular weight, isoelectric point pH and biological activity [31]. Their nutritional importance is limited to exerting a positive therapeutic effect in the feeling of satiety in weight loss therapy. This may be related to the lower digestibility of these proteins, e.g., with the participation of trypsin. It is believed that the potential anti-obesity properties of proteins of protease inhibitors result from the release of the cholecystokinin peptide under the inhibitory effect of trypsin [49,50]. For these reasons, potato protein inhibitors, such as the protease inhibitor II-proteins (21 kDa), are considered a possible dietary supplement [3,4,18].
3. Functional Properties of Potato Proteins
The functional properties of proteins are determined by factors such as hydrophobicity, the presence of cross-links, spatial structure (secondary, tertiary and quaternary) and molecular flexibility/rigidity [4,9,51]. According to the definition presented by Kinsella [11], functional properties of proteins are “those physical and chemical properties that influence the behaviour of proteins in food systems during processing, storage, as well as preparation and consumption of food products”. Due to the favorable functional properties, proteins are often used as ingredients to impart rheological and structural properties to products with their participation, to influence the water binding or solubility of products. As it is believed that potato protein allergies are much less common, they may be a replacement for the traditionally used wheat, soybean, egg, fish or milk proteins, with proven high allergenicity [4].
Preparations of potato proteins in non-denatured form, obtained under laboratory conditions by many authors [10,12,13,52], starting from the 1970s, were characterized by favorable functional properties, especially solubility, emulsifying and foaming properties [27,34,38,53]. Stable emulsions with the participation of all potato proteins were obtained by Holm and Eriksen [12], who showed that, in terms of emulsion properties, native potato proteins are comparable to commercial soy protein preparations. Cheng et al. [52] showed that potato protein hydrolysates are active in inhibiting lipid oxidation in soybean-oil-in-water emulsions. Separated fractions of patatin proteins and inhibitory proteins have been shown in studies by various authors [4,9,19,51,53] to possess more favorable functional properties than preparations containing all fractions of potato proteins.
Due to the low denaturation temperature of patatin proteins (in the range of 50–55 °C), the method of their isolation from the raw material influences the development of these properties [46]. Thus, the preservation of the beneficial functional properties of patatin is associated with the use of mild temperature conditions in the method used to obtain them. Van Koningsveld et al. [20], analyzing the functional properties of potato proteins, showed that the activity of protease inhibitor proteins in the formation and stabilization of foams and emulsions is relatively low compared to patatin proteins, which is related to the higher denaturation temperature of inhibitory proteins, i.e., in the range of 55–70 °C.
This entry is adapted from the peer-reviewed paper 10.3390/app11083497
