Yeast Extract: Characteristics, Production, Applications and Future Perspectives

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Abstract

Yeast extract is a product prepared mainly from waste brewer’s yeast, which is rich in nucleotides, proteins, amino acids, sugars and a variety of trace elements, and has the advantages of low production cost and abundant supply of raw material. Consequently, yeast extracts are widely used in various fields as animal feed additives, food flavoring agents and additives, cosmetic supplements, and microbial fermentation media; however, their full potential has not yet been realized. To improve understanding of current research knowledge, this review summarizes the ingredients, production technology, and applications of yeast extracts, and discusses the relationship between their properties and applications. Developmental trends and future prospects of yeast extract are also previewed, with the aim of providing a theoretical basis for the development and expansion of future applications.

Introduction

As living standards in most countries have improved, consumer demand for healthy, nutritious and safe food has steadily increased. Yeast extract, which is safe and nutritious, is now considered a natural, high-quality product capable of meeting diverse food flavor requirements and supplying essential dietary nutrients [1, 2]. Yeast extract is usually defined as the water-soluble extract produced from yeast waste streams (e.g., baker's yeast, brewer's yeast, Candida utilis, Candida tropicalis, and Kluyveromyces marxianus) [3] following disruption of the cell membrane by various means [4]. Also known as “yeast hydrolysate” in the food industry, yeast extract is regarded as Generally Recognized as Safe (GRAS) by most food safety certification bodies around the world [5]. In particular, yeast extract has attracted increasing attention because of its low production cost, wide range of sources, and high content of vitamins, proteins, and minerals needed by the food industry. For example, Elsa et al.[6] obtained yeast extract by mechanical crushing and found the protein content to be as high as 64.1% (dw), the fat content at only 1.32% (dw), and the RNA content at 4%. Meanwhile, the essential amino acids accounted for 40% of total amino acid, while those with flavor-enhancing functions (glutamic acid, aspartic acid, glycine, and alanine) accounted for 34% of total amino acid. In addition to the great variety of physiologically valuable substances in yeast extract, its high antioxidant capacity is also very useful to the food industry [4].

The type and characteristics of yeast extract depend on the waste yeast source it is made from and the particular production process used. For industrial production, various methods are used to disrupt the yeast cells, such as mechanical disruption, enzymatic lysis, organic solvents, or autolysis using salt as the solubilizer, and other autolysis methods, depending on the intended application [7]. The yeast raw materials used in the industrial production of yeast extract are mostly brewer's yeast and baker's yeast ( Fig. 1 ), both of which come from completely different sources. Brewer's yeast is mainly obtained by fermenting waste yeast from breweries that produce beer, while baker's yeast requires special cultivation, is high in protein, with high safety and stability, so each has its own advantages and disadvantages. In terms of the frequency of use of these two yeasts, it is clear that brewer's yeast (Saccharomyces cerevisiae) is the most commonly used ( Table 1 ) [8]. The brewer's yeast cell-wall contains a high proportion (15-30% of the dry cell mass) of cross-linked polysaccharides, mainly mannose oligosaccharides and β-glucan [9, 10]. The β-glucan product obtained from brewer’s yeast has antibacterial, antioxidant and various other biological activities, so not only is it used as a food additive and dietary supplement to promote digestion, it also enhances human immunity and reduces liver and blood lipids [11]. Similarly, baker’s yeast extract is rich in free amino acids, minerals and vitamins, and is also often used industrially to enhance the flavor of soups and sauces [12]. At present, waste brewer's yeast is the main one of many by-products in the brewing industry. On average, 150-200 tons of waste yeast pulp will be produced for every 10,000 tons of beer brewed [4]. For this reason, waste brewer's yeast has become the main raw material used in the production of yeast extract. Although yeast extract is low in fat and carbohydrates, rich in fiber, and widely used in animal-feed additives [6], its application in other fields is very limited. However, with improved production technology, the applications for yeast extract have expanded from animal feed into amino acid and protein products for human consumption, immunostimulants for fish farming, meat flavoring agents [11], spice additives, plant growth regulation, and soil conditioning ( Fig. 2 ) [13].

Table 1

Composition analysis of baker's yeast and brewer's yeast after treatment.

Despite these broad application prospects of yeast extract, most related reviews are limited to its practical applications, which, however, do not necessarily fully exploit the unique characteristics of yeast extract, and there exists a disconnect between application and theory. In addition, there are very few reports on the pros and cons of the wide diversity of yeast extract production processes. In this review, the characteristics of yeast extract are summarized, the different production processes are compared and comprehensively reviewed, and recent research findings on yeast extract are also outlined and discussed.

Composition and Characteristics of Yeast Extract

Chemical Composition

Yeast extract is a very complex product, the main components of which are cell wall material and cell contents [6]. The cell wall is mainly composed of structural polysaccharides, such as mannose oligosaccharides and β-glucans, which are extensively cross-linked, and there are also small proportions of chitin and glycogen [10]. Most of these polysaccharides are water-insoluble and they make up as much as 83% of the total carbohydrate content of yeast cells [14]. The cell lysate contains a high proportion of essential and nonessential amino acids, ribonucleotides, minerals, vitamins, peptides, and other water-soluble substances ( Table 2 ) [15]. The complexity of yeast extract is not only manifested in the different types of macro-molecules and small molecules it contains, but also in the diversity of the nutrient content. For example, for yeast extracts obtained from the same raw materials and production conditions, but with different processing times, there can be major differences in the product composition, as different production processes and raw materials result in even greater differences. In fact, it is these very differences in the production methods of yeast extracts that lead to the diversification of yeast extract products capable of meeting the needs of different industries and applications [16].

Table 2

Types and contents of trace elements in yeast extract [6, 7, 121, 123].

Nutritional Characteristics

Yeast extract is high in nucleic acid, protein, B vitamin and fiber content [17]. As such, it is an important ingredient in animal feed as well as in dietary supplements to meet human nutritional requirements. Glucans, mannans, chitin, protein and other macromolecular substances derived from yeast extract provide more balanced nutritional supplementation to animal feed than plant-sourced supplements [18]. Moreover, ribose, the major reducing sugar in yeast extract, is an important precursor for cellular energy metabolism.

The addition of a suitable amount of yeast extract to poultry feed can strengthen the immunity of birds and reduce the incidence of disease [19, 20]. Many countries have banned the use in pig feed of spray-dried animal plasma (SDPP), which is a safety risk that is also expensive as a protein supplement [21], so many pig farmers have switched to yeast extracts that are safer and relatively inexpensive. Yeast extracts are also used in the daily feed of weaner piglets to meet their nutritional needs and enhance their immunity [22]. The addition of yeast β-glucan to human dietary supplements can lower cholesterol and liver fat levels, as well as promote the proliferation of beneficial intestinal microflora [17, 23]. Yeast β-glucan has useful functional properties that can enhance food products, such as fruit drinks, biscuits, yogurt, chocolate, and jelly [11]. Although yeast extracts are rich in beneficial nutrients and are widely used in various industries, there are restrictions on the use of high nucleic acid content ingredients. Yeast has a nucleic acid content of up to 15%, 10 times that of human tissues. Excessive nucleic acid intake increases uric acid levels and can lead to hyperuricemia and gout [24]; the United Nations Protein Advisory Group recommends limiting nucleic acid intake to 2 g per day in the adult diet [25]. One way to reduce nucleic acid intake is to remove purines from foods by using silver complexes, or cuprous salt precipitation [26].

Antioxidant Properties of Polysaccharide Structures in Yeast Cell Walls

The polysaccharide components (mannan and β-glucan) in the yeast cell wall make a major contribution to the antioxidant properties of yeast extract, through their ability to scavenge hydroxyl free radicals and superoxide anions [27]. In particular, modification of β-glucan, by sulfation [28], or phosphorylation [29], can markedly change its physicochemical properties and biological activities ( Table 3 ), thereby further improving its antioxidant capacity. Mannan also has excellent antioxidant properties in humans and has immunostimulatory, anti-aging, anti-tumor and other health-beneficial effects [30]. These two polysaccharides with antioxidant function are both extracted from yeast cells. Industrial production of β-glucan and mannan from yeast is an ideal choice due to the abundance of raw materials and the product having less pollution and high purity [31].

Table 3

Comparison of different properties of β-glucan derivatives [28, 29]. [a]

[a] The values in the table are all improved values over unmodified β-glucan.

There are various methods for extracting the polysaccharide components from yeast cell walls, and the method can be selected and/or modified to meet particular application requirements. Common polysaccharide extraction methods are alkaline, enzyme, ultrasonic, and microwave extraction ( Table 4 ) [32]. The extracted cell wall polysaccharides are often combined with other antioxidants, such as selenium, amino acids, vitamins and their derivatives, for use in skin-care products that can increase stratum corneum hydration and reduce skin roughness. This method of formulating yeast extract polysaccharides has become the mainstream direction of choice for the development of antioxidant skin-care products [31].

Table 4

Different extraction methods for polysaccharides from yeast cell walls.

Special Antioxidant Properties

The antioxidant properties of yeast extract are not limited to the polysaccharide components of yeast cell walls, as the cellular contents of yeast also have antioxidant functions under specific environmental conditions [33]. For example, when live yeast is subjected to oxidative stress, the cells can absorb phenolic compounds (such as syringic acid, ferulic acid, caffeic acid, chlorogenic acid, cinnamic acid, gallic acid and (±) catechin) from the environment [6], to enhance their antioxidant defenses, which can improve the antioxidant properties of yeast extract to some extent [34]. This approach has been used to optimize the production of glutathione (GSH) (an important antioxidant in yeast extracts) by yeast cells [35], potentially enabling mass-production of GSH and reducing the production cost of yeast extract for antioxidant purposes for the food and beverage industry [36].

Research on the antioxidant properties of yeast extracts has also been extended to the cosmetics industry; yeast extract is usually combined with other cosmetic ingredients to formulate sun protection, moisturizing and exfoliating products, which also protect the skin from oxidative stress [31]. For comparison, the antioxidant capacity of yeast extract is ten times that of blueberries [31].

Organoleptic Properties

Organoleptic properties are another important property of yeast extract. In fact, the flavors of yeast extract as a condiment mainly include meat flavor and barbecue flavor, but inevitably, bitterness and yeast taste remain after processing, which is not acceptable to everyone [5].

Aroma Properties of Yeast Extract as a Flavoring Agent

Yeast extract has become the fourth most important natural food-flavoring agent, after monosodium glutamate, nucleotides and hydrolyzed protein [37]. Treatment of yeast extract with the Maillard reaction (a complex series of reactions between heat-treated sugars and amino acids), enables production of a variety of flavors, such as umami, salty, meaty and other flavors, mainly derived from the amino acids and peptides in the lysate [5]. The chemical compounds responsible for some of the various flavors of yeast extracts have been identified, for example: meat flavor is derived from 2-methyl-3-furanmethanol, 2-methyl-3-methyldithiofuran and nitrogen-containing compounds such as pyrazine and furan; baking aroma from 2-furan-methyl-mercaptan and 4-hydroxy-2,5-dimethyl-3-furanone [38]; creamy flavor from 2,3-butanedione; nutty flavor from trimethylpyrazine; and chocolate flavor from 3-methylbutyraldehyde [39]. The aroma characteristics of 48 flavor compounds, including aldehydes, ketones, alcohols, furans, and pyrazines in yeast extracts have been reported [40].

Sensory Properties of Nucleotides

Nucleotides in yeast extract are one of the three major flavoring substances in yeast extract, in addition to amino acids and peptides [24]. Although nucleotides do not have much flavor, they make a major contribution to the taste of yeast products by interacting with other components. Nucleotides based on 5'-adenosine phosphate (AMP), 5'-inosine phosphate (IMP), and 5'-guanosine phosphate (GMP) are 100 times more taste-active than seasonings such as monosodium glutamate [41, 42], so nucleotides play an important role in yeast extract food-flavoring agents.

Flavor Modification Using the Maillard Reaction

Although yeast extracts made by different methods each have characteristic tastes and flavors, these properties appear to be closely related to the various nitrogen-containing compounds produced by the Maillard reaction [43]. The Maillard reaction is normally a by-product of cooking and heat treatment, but the resulting taste/flavor can be modified by changing the reaction conditions, such as the pH, salt concentration, the peptide concentration and composition, and the type of sugar (glucose, fructose, or sucrose) [44]. The intermediate products made from yeast extract using the Maillard reaction commonly include both volatile and non-volatile compounds. The non-volatile substances are usually amino acid derivatives, whereas the volatile substances include derivatives of alcohols, ethers, sulfur compounds, and aldehydes. The sulfur-containing volatiles generally make the greatest contribution to the overall flavor of most condiments [45]. Gas chromatography mass spectrometry (GC-MS) can be used to identify and characterize the key aroma-active substances produced by the Maillard reaction and the factors influencing their formation [5].

Flavor Properties and Production of Glutathione

Along with the increasing application of the antioxidant and immune-stimulatory properties of glutathione (GSH), its properties as a flavor compound are becoming better known to the condiment industry. In recent years, industrial production of GSH using recombinant yeast cells obtained through genetic modification has become increasingly important [36]. As a precursor of a variety of flavor compounds, GSH is also gaining in importance for flavor modification over conventional yeast extracts [46].

Improving Taste and Odor Defects of Yeast Extract

Although yeast extract is used as a food flavoring and seasoning, according to consumer surveys, there is an undesirable odor associated with it, which is repellant to some consumers and may limit sales of products containing yeast extract [40]. The challenge of odor removal from products made with yeast extract, such as nutritional supplements and condiments, is attracting increasing attention. Sensory evaluations of yeast extract have characterized its odor notes as burnt, sour, smoky, musty, gasoline and fatty [40], with most of these resulting from heat treatment at excessive temperature since the concentration of these odors increases with increased processing temperature [47]. The compounds mainly responsible for these odors are o-xylene, styrene, n-octanal and acetic acid; their relative concentrations vary depending on the yeast strain the extract was made from, treatment methods, and other factors [40].

The "yeast taste" in yeast extract is due to an important substance that affects its sensory evaluation and is related to one of its main odors, which is mainly composed of propionic acid and butyric acid. Ma et al. [48] used the mixed fermentation method of Saccharomycopsis fibuligera and Lactococcus lactis to completely remove propionic acid and butyric acid, the main sources of the "yeast taste." Under the action of these two bacteria, the lactic acid content with flavor and taste in the yeast extract increased by 6.27 g/l, so that the yeast extract showed improved flavor and taste, without having the "yeast taste."

In common with many protein hydrolysates, yeast extract has a bitter component to its taste and market surveys indicate that the bitterness is undesirable to most consumers [5]. The source of the bitterness is peptides resulting from hydrolysis of yeast proteins [49] and the intensity of the bitterness is generally proportional to the length of the peptide chain. Generally, heat treatment of foods can degrade long peptide chains, but heat treatment of yeast extract can strengthen the bitterness as the treatment temperature increases because the bitter peptides are very stable and heat- resistant [5]. However, limiting the heat treatment temperature to less than 120°C not only masks the bitterness but also strengthens the umami taste to produce a condiment with a much-improved taste [5]. Therefore, it is necessary for the food industry to strictly control all aspects of yeast extract production to meet food safety and flavor requirements. Future research on the sensory attributes of yeast extract should focus on further enhancing taste/flavor and eliminating odor and taste defects to maximize the market potential of yeast extract and make the most of its many positive characteristics.

Yeast Extract Production Technology

Yeast cells have strong cell walls, so lysing the cells to release their contents is the main challenge in producing yeast extract. There are four main process types used to produce yeast extract ( Fig. 3 ): autolysis, plasmolysis, enzymatic lysis, and physical methods [12], with each one having its own advantages and disadvantages ( Table 5 ) [49, 50].

The production process of yeast extract with pretreatment, cell lysis, separation, inspissation, and evaporation.

Table 5

Comparison of different production methods of yeast extract.

Yeast extracts produced by different production processes from the same raw material can have marked differences in some of their properties, and therefore the choice of process must be carefully matched to the desired properties of the product. The standards commonly used to match processes and properties are measurements of the degree of yeast cell lysis (determined via cell morphology and cell viability assays) and the degree of protein/polysaccharide hydrolysis (determined via total soluble solid content, soluble protein content, and total carbohydrate content) ( Fig. 4 ) [8].

The detection index of yeast extract via the degree of yeast cell lysis and the degree of protein/polysaccharide hydrolysis.

Pretreatment of Yeast for Extract Production

Under normal circumstances, pretreatment of waste yeast is required before large-scale industrial production of yeast extract, especially for food applications. Food-grade yeast extract has strict requirements for removal of toxic substances, or prevention of their formation, as well as removal of undesirable tastes and odors; therefore, pre-treatment is essential. The pretreatment methods generally involve washing and debittering. The first step is to wash the waste yeast to remove residues on the surface of yeast cells that may reduce product quality [51](mainly hop components, such as resin and tannin [52, 53] produced during the fermentation of beer by yeast)[54]. Spent yeast cells need to be washed multiple times with distilled water, diluted to 10% dry matter, filtered through a yeast sieve (mesh size 0.5 mm) and centrifuged to recover the cells, before undergoing quality and hygiene tests and the production process [4]. The second step is the debittering of the yeast cells. Most of the yeast used for extract production is waste yeast from beer fermentation. Beer yeast has a strong bitter taste due to the presence of humulones and isohumulones from the hops added to the fermentation to give the beer its characteristic bitter taste. These bitter compounds are mainly bound to the cell wall and therefore difficult to separate from the yeast by washing, especially at the low pH of the completed fermentation. The conventional method of debittering is to use high pH washing treatments or organic solvent/water mixtures [55]. Although the conventional method is simple to perform and low cost, it produces a large amount of toxic waste water.

A new yeast extraction process combines debittering with extraction, avoiding alkali-debittering, by taking advantage of the binding of bitter compounds to the cell wall at low pH [55] and using a combination of homogenization, autolysis and rotating microfiltration technology. This approach maximizes the release of cell contents, but the bitter components mostly remain attached to the cell-wall fragments, enabling high debittering efficiency along with a high yield of protein and other substances; this method has great application potential in industrial production.

Yeast Extract Production via Autolysis

Autolysis involves endogenous yeast hydrolytic enzymes (mainly proteases, nucleases, and glucanases), which are activated by artificial methods and degrade the cell-wall polysaccharides, DNA, RNA, and cellular proteins [12]. During autolysis, there is a physiological change in which the activity of cellular respiratory enzymes decreases and that of the hydrolytic enzymes increases [56]. Common enzyme activators include ethanol, hexamethylenediamine, sodium chloride, Triton X-100 detergent, diethyl ether, digitonin, and sucrose. In the industrial context, although the use of organic solvents can increase product yield and permits efficient solid-liquid separation, there are also disadvantages, such as high cost and higher generation of polluting waste materials [12].

Autolysis usually involves suspending the yeast cells, addition of an activating agent, or heat treatment (40-60°C), stirring for 24 h at 50°C, centrifugal concentration, and recovery [7]. Although autolysis is much simpler than other cell lysis methods, it also has disadvantages, for example, low nutrient retention; autolysis is more destructive of antioxidant substances, i.e., amino acids, B vitamins, polyphenols, and glutathione. However, if the yeast extract product application does not require a high content of nutrients, autolysis is the easiest, lowest-cost process for industrial production [57] and is the major production process for food-flavoring agents, which do not require a high nutrient content, but do need a high content of peptides and amino acids [11].

Autolysis has unique advantages over other yeast extraction processes for production of flavoring agents because the synergistic actions of a variety of proteases and peptidases increases the degree of protein hydrolysis and improves the amount and variety of free amino acids [58]. Autolysis not only produces better food-flavoring agents than other extraction methods, but careful control of the process conditions also allows the production of a much wider variety of flavors [59]. Among the many solubilizers, saponins are particularly typical [60]. They are natural emulsifiers that occur widely in leguminous plants. For example, in recent years, the use of quillaja (or “soapbark”, a tree native to Chile) saponins for efficient autolysis has been developed [61]. Saponin is a safe substance which has been officially approved for use in the food industry. Moreover, saponin can effectively avoid the disadvantage of promoting solvent in the yeast autolysis process. Taking salt- promoting autolysis as an example, in the production of yeast extract condiments or nutritional supplements, high salt content will seriously affect sensory characteristics and nutritional performance of the product. Using saponins to lyse yeast cells is effective, inexpensive, simple to implement, and provides a high yield of extract [61]. Saponins increase the permeability of the yeast cell membrane, allowing efficient release of cellular contents under mild conditions, and can promote protein degradation and the release of nitrogen-containing compounds at very low dosages. Compared with conventional autolysis, saponins can increase the release of cellular contents such as protein and the degradation of macromolecules by nearly 400 times [62]. A comparison of saponin/ethanol and saponin/sodium chloride mixtures to autolyze yeast cells shows that the addition of saponin markedly improves the preservation and release of nutrients from the cells [63] more so than the conventional high salt process [61].

In a radically different approach, the yeast cells are suspended in pure water and water absorption under the action of osmotic pressure causes the cells to rupture and release their contents; however, this method requires more development to improve cell lysis speed and nutrient preservation [64].

Production via Plasmolysis

Plasmolysis involves the application of a cell membrane-disrupting agent (ethyl acetate, toluene, or ethanol) to the yeast cells to disrupt the integrity of the lipid bilayer and greatly increase the permeability of the cell membrane, permitting complete release of the cell contents into the external medium. Currently, the most common plasmolysis reagent is 1.5% ethyl acetate, the use of which is also referred to as an improved autolysis process and which works in a very similar manner to that of saponin-autolysis, as described above. In addition to promoting the autolysis of yeast, ethyl acetate has an inhibitory effect on the contamination of the yeast raw material [65]. Importantly, given the same raw material, the yield of extract obtained by plasmolysis with ethyl acetate is greater than that by the autolysis method [65].

The ethyl acetate plasmolysis method usually involves mixing of yeast cell suspension with ethyl acetate, adjustment of pH and temperature, further mixing for about 48 h, centrifugation, and finally analysis of the extract [8]. A comparison of the autolysis and plasmolysis methods, using the same spent yeast feedstock and analysis by cell viability, protein content, leakage analysis, and carbohydrate detection, found that plasmolysis produced a higher extract yield and higher solids content [8]. Similarly, a comparison of extract yield between plasmolysis with ethyl acetate/sodium chloride [58] and physical cell disruption by ultrasonic sonotrode found little difference in the total protein and residual ash contents [7]. This improved autolysis method (i.e., plasmolysis) is significantly more efficient than the conventional autolysis process.

Production via Enzymatic Degradation

Enzymatic degradation is very similar to autolysis, with both using mild conditions, and enzymes to lyse the cells. The difference is that enzymatic degradation uses exogenous enzymes, whereas autolysis uses endogenous yeast enzymes. The principle of enzymatic degradation is to allow the enzyme to digest the cell wall proteins, subjecting the cell to osmotic shock, or precipitating the cell wall protein to obtain the lysate [8]. The main types of enzymes used are protease, zymolyase, flavourzyme, helicase, pancreatin, and protamex. The most effective enzymes are fruit-sourced proteases, such as papain, ficin, and bromelain; however, since all proteases cleave peptide bonds with some degree of selectivity and produce mainly peptides, it is important to add just enough enzyme to complete the conversion to peptides. Once the enzyme has hydrolyzed all of the peptide bonds it is selective for, the reaction cannot proceed further, so additional enzyme has no effect on the hydrolysis and just increases the production costs [50]. Enzymatic hydrolysis with trypsin was compared with autolysis under the same conditions; trypsin had a synergistic effect with various cellular enzymes and greatly increased the rate of cell degradation. Similarly, in a comparison of autolysis, plasmolysis, and enzymatic degradation, enzymatic degradation released the most soluble substances and proteins from yeast cells. Enzymatic degradation also has advantages, such as rapid cell lysis, low salt content, and less product odor [8]. Industrially, enzymatic degradation often uses a mixture of several exogenous enzymes, resulting in faster cell lysis and macromolecule degradation, and a higher recovery of soluble substances. However, the use of such enzyme mixtures requires thorough optimization to maximize extraction efficiency and minimize enzyme consumption and costs [50].

Production via Physical Disruption

The common methods of physical disruption of yeast cells include high-pressure homogenization, ultrasound, bead milling, and overweight method. The overweight method is new and derived from the osmotic shock crushing method. It mainly uses the osmotic pressure changes of different phases to exert pressure on the cells to cause breaking [4]. There are many types of equipment and methods available for industrial-scale physical disruption of yeast and all have strict requirements for their operating environment, but they are widely used, effective, relatively inexpensive to operate, and produce a high yield of nutrients [4]. Another advantage is avoidance of the damaging effects of organic solvents and salts on yeast cell components and nutrients, as well as minimal waste production. The polysaccharides β-glucan and mannan in the yeast cell wall have the beneficial biological activities of scavenging free radicals, delaying aging, and lowering blood cholesterol and lipid levels. Mechanical disruption is particularly effective for obtaining these products from yeast in good yield and high quality [6, 66].

Taking mechanical crushing with glass beads as an example, the crushing process roughly requires the following steps: first, the cell suspension is mixed with with glass beads of different specifications (1:2 mass ratio), then mixed in a vortex mixer at 4°C for 1 min and repeated 10 times, and finally centrifuged to obtain the precipitate and supernatant [67]. Mechanical disruption is superior to autolysis in a number of ways. The free long-chain fatty acid yield from physical cell disruption was higher than from autolysis, probably because of fatty acid degradation by the solvent used for autolysis [6]. In the industrial production of trehalose by yeast, physical cell disruption produces a higher yield of trehalose than autolysis [68]. Autolysis of yeast cells results in a much greater loss of vitamins, especially folic acid and antioxidants, such as phenolics and glutathione [69], compared with mechanical disruption, which appears to be the best choice for production of high nutrient/bioactive content extracts [7].

Mechanical disruption, however, does not promote proteolysis and the extracts it produces contain little peptide, or free amino acid, meaning that it is well-suited to producing extracts high in vitamins and antioxidants, but not for flavorings, which require a high content of amino acid [58].

Other Factors Affecting Yeast Extract Production

Given that each of the yeast extraction methods discussed above has both advantages and disadvantages, a single method is often unable to produce an extract with the required composition and properties. Therefore, in industrial production, a combination of two or more methods may be applied to produce the desired product [70].

There can be significant variability in the yeast raw material depending on the supply source. For example, yeast extract produced using delayed yeast (a waste yeast raw material with longer fermentation and growth time than other waste yeasts) is significantly lower than waste yeast in ash and protein content [7], because of differences in fermentation time and consequent differences in the age and growth stage of the yeast. Therefore, selecting yeast of a suitable age and metabolic state can significantly improve the quality of the final product [71].

Yeast cells contain two main useful components; the cell wall, which is used in health products and cosmetics, and the small molecules and proteins in the cytoplasm. Relevant studies have shown that when extracting the polysaccharide component in the yeast cell wall, some extraction methods will cause the loss and destruction of some nutrients (such as vitamin B6, vitamin B9, and vitamin B12) in the yeast cell. On the contrary, the extraction of certain nutrients in yeast cells will also cause different degrees of damage to the polysaccharide structure of the yeast cell wall. So, to fully release the cell contents or maintain the physiological function and structure of polysaccharides, it is necessary to separate the cell wall and cytoplasm [6].

Applications of Yeast Extract

Yeast extracts have become increasingly prominent on the global market due to their unique nutritional and biochemical properties, low production costs, and abundant raw material supply from beer brewery wastes. They are widely used in animal feed, food, cosmetics, pharmaceuticals, health products, and biotechnology. Here, we summarize recent developments in application and the potential future research direction related to yeast extract ( Fig. 5 ).