Nowadays, food is recognized as a source of dietary substances and biologically active compounds that improve human health and the general conditions of the organism. The consumers' increasing awareness of the influence of diet on health is reflected in their selection of natural products, abundant in vitamins, minerals, and other bioactive compounds like carotenoids [], anthocyanins [], polyphenols [], or peptides [, ].
Bioactive peptides are protein fragments that benefit the body systems and overall human health. Most bioactive peptides range between two [dipeptides] and 20 amino acid residues and have a molecular mass of 0.4–2 kDa []. Longer peptides have also been reported in rare cases. Lunasin, for example, is a peptide formed by 43 amino acids produced from soy, which demonstrates anti-cancer and hypocholesterolemic properties [].
Bioactive peptides generated from food possess an excellent potential for creating functional foods and/or nutraceuticals to prevent or treat some chronic diseases []. Many articles on the generation and characterization of bioactive peptides with antimicrobial, anti-inflammatory, antihypertensive, anti-obesity, and antioxidant attributes have been published []. Herein, we focused on bioactive peptides from different foods and their by-products, their effects on health, and possible applications.
In this investigation, eligible studies [in English] were acknowledged during an electronic search of the PubMed database [1991–2021] [//www.nlm.nih.gov/] and Google. We employed the chief search word “bioactive peptides” along with the words “sources,” “by-products,” “extraction,” “purification,” “identification,” “bioactivities,” “health effects,” “pharmaceutical applications,” “food applications,” “cosmeceutical applications,” “feed applications,” and “safety” to find the relevant articles. We selected the titles, keywords, and abstracts of the articles collected from the database. Several review articles were omitted in favor of the primary sources cited.
The Sources of Bioactive Peptides
Peptides and proteins are critical macronutrients as they provide the necessary raw materials for protein production and serve as a source of energy. Bioactive peptides have been isolated or produced from various plant and animal sources [–]. Food proteins are chosen as a reference for bioactive peptides based on two factors: [i] a desire to add value to abundant underused proteins or protein-rich industrial food waste, and [ii] the use of proteins with particular peptide sequences or amino acid residues with specific pharmacological benefits [].
Table 1. Peptides from milk and by-products and their bioactivity.
Table 2. Peptides from meat and by-products and their bioactivity.
Table 3. Peptides from plants and by-products and their bioactivity.
Table 4. Peptides from marine and by-products and their bioactivity.
Extraction of Bioactive Peptides
Bioactive peptides are conventionally isolated by chemical or enzymatic hydrolysis and fermentation. To enhance the degree of hydrolysis in the generation of bioactive peptides, new approaches, such as microwave, ultrasound-assisted extraction, ohmic heating, pulsed electric fields, and subcritical water hydrolysis, have been investigated []. Physical processes are at the core of these techniques [].
Figure 1. Scheme for extracting bioactive peptides.
Chemical Methods
Chemical techniques using alkalis, such as sodium hydroxide, are the most typical and conventional method for protein extraction from plant sources [, ]. It can effectively break hydrogen and amide bonds to solubilize rice bran proteins. Although this process is highly effective in obtaining most proteins in a soluble form, it creates specific structural changes that cause a protein to lose its original function [].
Enzymatic Methods
Enzymatic hydrolysis is another common approach for separating proteins and hydrolysates/peptides from various food sources []. Enzymes are employed in diverse ways to facilitate protein extraction from food, such as cell wall degradation, starch-bond protein release, and protein solubility improvement []. In this regard, Wang et al. [] utilized phytase and xylanase to isolate protein from rice bran and noticed that the use of carbohydrates could be helpful to improve the yield of soluble protein.
Physical Methods
Physical methods are often favored over chemical or enzymatic treatments for food production because they have fewer changes []. These techniques are more economical and easy to adapt and use in the industry. Conventional physical procedures, such as colloidal milling, homogenization, high-speed blending, freeze-thaw, and high pressure, have been utilized for protein extraction [].
Microwave-Assisted Extraction
Microwave heating is a novel technology based on electromagnetic waves with wavelengths and frequencies ranging from 1 mm to 1 m and 300 MHz to 300 GHz, respectively. It has gained popularity in the food processing industry because of its uniform heating, high heating rates, safety, simple, quick, and clean operation, and low maintenance. Furthermore, this kind of heating has a lower impact on food products' flavor and nutritional quality than conventional heating. By shattering disulfide and hydrogen bonds [non-covalent bonds], this approach can cause protein unfolding, which affects the secondary and tertiary structures of proteins [, ]. In this respect, the microwave process was shown to assist the chia seed protein enzymatic hydrolysis with enhanced bioactivity [antioxidant activity], and functionality [emulsification and foaming properties] gained in a shorter time in comparison to traditional hydrolysis techniques [].
Ultrasound-Assisted Extraction
Sonication is a green, novel, innovative and sustainable strategy based on high sound waves of frequencies [>16 kHz] undetectable by the human ear. This approach has several benefits compared with traditional thermal processes, including higher efficiency, higher rate, more accessible and cheaper application and operation, lower equipment contamination, and higher quality and functionality of processed foods [, ]. In this context, Zhao et al. [] demonstrated that sonication with power levels of 200, 400, or 600 W for 15 or 30 min altered the secondary and tertiary structure of walnut protein isolate without any impact on its primary structure since the process could not break the covalent bonds. Further, Vanga et al. [] indicated that ultrasonic treatment [25 kHz, 400 W, 1–16 min] reduced soymilk protein trypsin inhibitor activity by 52% and enhanced its digestibility.
Ohmic Heating
Ohmic heating is a thermal processing technology that applies alternating electric currents directly into a semi-conductive media. It was initially employed for milk pasteurization in 1920. According to Joule's law, direct or volumetric heat is generated in products by passing a moderate and alternating electric current through them, which functions as resistance in an electrical circuit [, ]. In this way, Li et al. [] evaluated the structure and techno-functionality of proteins in soybean milk when using ohmic heating against traditional heating. Their findings revealed that ohmic heating effectively reduced heating time and enhanced the protein's emulsifying capacity. The protein's foaming ability, on the other hand, reduced as its surface hydrophobicity dropped.
The Pulsed-Electric Field [PEF]
The pulsed-electric field [PEF] technique has been employed as a non-thermal process for microorganisms and enzymes inactivation. In this technology, the food sample is subjected to short high-power electrical pulses [μs or ms] between electrodes []. A PEF system consists of a chamber, electrodes, a high-voltage pulse generator, and a computer for monitoring and controlling devices. A strong electric field is formed between two electrodes because of their electrical potential difference. During the PEF process, the generated electrical energy might cause protein unfolding and enhanced interactions with the solute. This can impact the peptides/protein's functional characteristics by increasing its solubility []. In this regard, PEF treatment of canola seeds enhanced the extracted protein's solubility, emulsifying, and foaming capabilities, according to Zhang et al. []. Nevertheless, depending on the strength and duration of the PEF process, it can result in denaturation and aggregation, resulting in decreased solubility. The PEF method can change plant-derived peptides and proteins' secondary and tertiary structures. Changes in the secondary structure of peptides derived from pine nut protein were also informed, along with their antioxidant effect [].
Purification and Identification of Bioactive Compounds
All the methods for purifying and identifying bioactive peptides are very similar. Purification of active peptides is required to produce a commercially viable product. Ultrafiltration, RP-HPLC, size exclusion chromatography, and ion-exchange chromatography, can all be used to purify bioactive peptides. Additionally, for protein identification, analytical techniques such as mass spectrometry [MS], electrospray ionization MS, matrix-assisted laser desorption ionization time-of-flight MS, liquid chromatography-MS/MS, and hydrophilic interaction liquid chromatography [HILIC] are widely utilized [].
Bioactivities of Bioactive Peptides and Their Impact on Health
Proteins are necessary for the growth and the preservation of many biological processes. The awareness regarding physiologically active peptides is growing quickly, as they may serve as possible modifiers for several regulative functions in the body. Bioactive peptides have different biological actions depending on the amino acid class, net charge, secondary structures, sequence, and molecular mass []. Multiple studies have determined the bioactivities of peptides, which were linked to improved overall health and a lower risk of specific chronic diseases, such as cancer, diabetes, and heart diseases [].
Figure 2. Bioactivities of bioactive peptides.
Antioxidant Activity
Reactive oxygen species cause cell damage, leading to cancer, diabetes, cardiovascular disease, and hypertension []. The antioxidative characteristics of bioactive peptides are associated with their composition, formation, and hydrophobicity. Histidine, glutamic acid, proline, tyrosine, cysteine, methionine, and phenylalanine are all amino acids with antioxidant properties []. Amino acids bind pro-oxidant metal ions to perform their activity, scavenge the OH radical and/or inhibit lipid peroxidation. As a result, each amino acid contributes as an antioxidant uniquely, depending on its type []. Most antioxidant peptides include 4–16 amino acid residues and have a molecular mass of 0.4–2 kDa. Peptide molecular size influences both the pathways to target locations and the gastrointestinal digesting process, potentially increasing antioxidant activity in vivo []. Tyrosine-containing peptides work primarily through hydrogen atom transfer, whereas cysteine, tryptophan, and histidine-containing peptides work mainly through single electron transfer []. Aromatic amino acids like Tyr and Phe are excellent at donating protons to electron-deficient radicals. This characteristic enhances the bioactive peptides' radical-scavenging abilities. The antioxidant capacity of His-containing peptides is confirmed to be linked to hydrogen donating and lipid peroxyl radical trapping []. The sulfhydryl group in cysteines, on the other hand, is endowed with an antioxidant effect because of its primary reaction with radicals []. Plant-based proteins derived from industrial food and its by-products, such as soybean, wheat germ, hemp seeds, rice bran, sesame bran, wheat bran, and rapeseed, possess bioactive peptides with antioxidant characteristics [].
Antimicrobial Activity
Antimicrobial peptides possess an antimicrobial activity that protects mammals from various bacteria, fungi, and viruses. Antimicrobial activity is also a coveted feature in prepared foods since it directly impacts the product's shelf life. Antimicrobial peptides are divided into three categories: short [20–46 amino acid residues], basic [rich in Lys or Are], and amphipathic. They are commonly abundant in hydrophobic residues, such as Leu, Ile, Val, Phe, and Try []. Multicellular organisms create antimicrobial peptides as defensive strategies against pathogenic microorganisms. Antimicrobial peptides can alter the cell membrane and biological processes, including cell division []. Their action is assumed to create channels or pores within bacterial membranes, inhibiting anabolic activities, changes in gene expression and signaling transduction, and promoting angiogenesis. For example, the antimicrobial action of milk is demonstrated by extensive research. Lactoferrin, which is hydrolyzed into lactoferricin in the gastrointestinal tract, is an essential contributor to the synthesis of various other bioactive peptides and has antimicrobial ability in and of itself []. Antimicrobial peptides have also been discovered in marine products. Many microorganisms, like Staphylococcus aureus, Escherichia coli, Bacillus subtilis, Shigella dysenteriae, Pseudomonas aeruginosa, Salmonella typhimurium, and Streptococcus pneumoniae, were inhibited by the peptide GLSRLFTALK, isolated from anchovy cooking wastewater []. Moreover, Aguilar-Toalá et al. [] found that adding chia protein hydrolysate [ 10 kDa; 3–10 kDa, and