Emulsifiers for the plant-based milk alternatives: a review

Abstract

Plant-based milk alternatives (PBMAs) have recently been receiving attention due to allergies, lactose intolerance, and environmental issues. PBMAs are manufactured by various sources from plants, but the emulsion stability of PBMAs is not good due to the different characteristics of the raw materials and manufacturing methods. To improve the stability of PBMAs, many researches were conducted by using various emulsifiers and different treatments. To help the development of new PBMAs with good stability for long shelf-life, the emulsifiers used in previous researches for PBMAs and their characteristics were reviewed.

Introduction

Milk has been consumed for an extended period due to its nutritional benefits; however, plant-based milk alternatives (PBMAs) have gained significant popularity in recent years. PBMAs refer to non-dairy milk products derived from plant sources, in contrast to traditional animal-based milk, such as cow’s milk. The preparation of these plant extracts typically involves soaking plant materials in water followed by milling, or milling the materials directly and subsequently extracting them with water. The resulting extract undergoes homogenization and heat treatment to yield the final PBMA products (Sethi et al., 2016). At this stage, various additives may be incorporated for specific purposes prior to homogenization and heat treatment, including emulsifiers for stability, nutrients for nutritional fortification, or flavorings for taste enhancement. The method of production for plant-based milk confers a range of advantages and disadvantages when compared to conventional milk.

PBMAs offer numerous benefits. Initially, they were chosen as means to circumvent issues related to milk allergies and lactose intolerance. In recent years, there has been a notable rise in the prevalence of milk protein allergies among both adults and children across various age demographics (Lajnaf et al., 2023). Furthermore, approximately 70% of the global population experiences lactose intolerance due to insufficient lactase production (Harvey et al., 1998). This trend has led to an increased preference for alternative mammalian milks or PBMAs. Additionally, a growing number of consumers are becoming increasingly aware of environmental concerns and animal welfare, resulting in heightened interest in PBMAs derived from diverse sources, which are now being developed and marketed (McClements et al., 2019).

Despite the increasing interest in PBMAs, several challenges hinder their production and marketability. From a nutritional standpoint, PBMAs generally exhibit low carbohydrate levels, and with the exception of soy milk, they typically contain significantly less protein than traditional dairy milk (Mehany et al., 2023). Furthermore, not all PBMAs are devoid of allergens; for instance, nut and soy milks can trigger severe allergic reactions, including anaphylaxis (Masiri et al., 2016). Additionally, the distinctive and often unappealing flavors associated with some PBMAs pose a barrier to consumer acceptance. Soy milk, for example, is frequently criticized for its beany taste, which deters certain consumers (McClements et al., 2019). Moreover, undesirable flavors may emerge as the product approaches its expiration date, primarily due to the oxidation of unsaturated fatty acids, necessitating meticulous attention during the manufacturing process (Sidhu and Singh, 2016). Finally, the inherent instability of PBMAs raises concerns regarding quality degradation prior to the expiration date, a situation exacerbated by factors such as the large size of extracted particles, unstable emulsion formation, intact starch granules, and the low solubility of proteins (Paul et al., 2020).

Numerous studies have been undertaken to enhance the inadequate stability of PBMAs by employing various treatments and additives. This review specifically concentrates on and synthesizes prior research that has successfully improved the stability of PBMAs through the incorporation of various emulsifiers, such as sugar esters, mono- and di-glycerides, as well as gellan and xanthan gum, rather than focusing on processes like ultrasound, pulsed electric fields, or ultra-high pressure homogenization. The aim of this review is to provide the information for the enhancement of the stability of PBMAs without necessitating further industrial capital investment in the facility.

Emulsifiers in general

An emulsion is defined as a mixture comprising two or more immiscible liquids, wherein one liquid is dispersed in the form of droplets within another liquid. Emulsions are typically categorized into three distinct types: (1) oil-in-water (O/W) emulsions, (2) water-in-oil (W/O) emulsions, and (3) water-in-oil-in-water (W/O/W) emulsions (Kralova and Sjöblom, 2009). Among these, O/W emulsions are the most prevalent in industrial applications, encompassing products such as flavored milk, ice cream, and mayonnaise. Additionally, PBMAs are classified as O/W emulsions, as they are primarily produced through aqueous extraction methods. The formation of emulsions generally occurs when one liquid is dispersed within another, facilitated by the use of emulsifying agents or additives that serve to stabilize the droplets and inhibit their coalescence or separation. These stabilizing agents are commonly referred to as emulsifiers.

Emulsifiers play a crucial role in the stability of emulsions by positioning themselves at the oil–water interface, thereby creating an interfacial film. This film reduces the surface tension between the two phases and contributes to emulsion stability through mechanisms such as electrostatic repulsion and steric hindrance (Arancibia et al., 2017; Deng et al., 2023). Electrostatic repulsion is a physical phenomenon wherein two similarly charged particles (either both positive or both negative) exert a repulsive force on one another, resulting in a separation of the particles. When two similarly charged droplets, each coated with emulsifiers, are brought into proximity, the electric fields generated by their charges repel one another, leading to a mutual distancing effect. Steric hindrance, on the other hand, refers to the physical obstruction caused by the presence of bulky groups surrounding the particles. This phenomenon inhibits the aggregation of particles that are encased in large emulsifiers, as their size prevents them from coming into close contact (Binks and Lumsdon, 2000; Deng et al., 2023).

Emulsifiers can be categorized into several types, each functioning through distinct mechanisms (Wang et al., 2021). Consequently, it is essential to employ a combination of different emulsifiers to determine the optimal mixing ratio that ensures maximum stability for each emulsion. While a clear demarcation between emulsion types is not always evident, they are generally classified into three primary categories: (1) low-molecular-weight emulsifiers (or small-molecule surfactants), (2) amphiphilic biopolymers, and (3) solid or colloidal particles (Berton-Carabin et al., 2018). Low-molecular-weight emulsifiers are characterized by a hydrophilic head and a hydrophobic tail, which can be either nonionic or ionic in nature. These emulsifiers encompass a range of substances, including mono- and diglycerides, sucrose esters, sorbitan esters, polysorbates, as well as naturally occurring phospholipids such as lecithins. The second category, amphiphilic biopolymers, primarily consist of proteins and polysaccharides. Proteins, such as casein and whey, typically exhibit amphiphilic characteristics and possess emulsifying and foaming capabilities. Polysaccharides, including xanthan gum and guar gum, serve as thickening agents and may contain nonpolar lipid or protein components that enhance emulsification. The thickening agents contribute to the stabilization of emulsions by augmenting viscosity, which restricts the mobility of emulsion droplets and consequently reduces their tendency to separate (Dickinson, 2003). Lastly, solid or colloidal particles can enhance emulsion stability by minimizing coalescence, with starch and soy proteins serving as representative examples (Yang et al., 2017). A variety of emulsifiers are frequently employed in a synergistic manner within a single product to enhance the stability of an emulsion (Gülseren and Corredig, 2014).

Emulsifiers for peanut milk

The peanut is recognized for its abundant nutritional components, including proteins, minerals, and essential fatty acids. It comprises 22–30% protein and 44–56% oil (Adebiyi et al., 2002). The health advantages of peanut milk for both children and the elderly have been well-documented, as it is particularly high in proteins, minerals, and essential fatty acids such as linoleic and oleic acids (Diarra et al., 2005). Two emulsifiers were tested in the formulation of peanut milk: carrageenan (CGN) and mono- and diglyceride (MDG) (Hinds et al., 1997). The process began with the partial defatting and grinding of roasted and blanched peanuts. Subsequently, water was incorporated and the mixture was filtered. Following this, a small quantity of sugar and salt was introduced, and the temperature was elevated to 63 ℃ using a water bath. An emulsifier was then added, and the mixture underwent two rounds of homogenization before being cooled. The resulting peanut milk exhibited a chemical composition of 3.7% protein, 2.0% fat, and 11.8% total solids (Hinds et al., 1997).

Under identical homogenizing conditions, all beverages that included CGN exhibited a significantly higher viscosity (ranging from 8.0 to 17.5 cps) compared to those containing MDG, which showed viscosities between 3.7 and 9.5 cps. Both kappa- and iota-CGN are known to produce highly viscous solutions when mixed with water, attributed to the consistent ionization of their sulfate half-ester groups. This ionization imparts a negative charge to the molecules, which plays a crucial role in their gelling, thickening, and stabilizing characteristics (De Ruiter and Rudolph, 1997). The viscosities observed in peanut beverages that incorporate CGN demonstrated this characteristic, indicating that the hydrated polysaccharide-protein networks associated with CGN are more substantial than the hydrated lipid-protein complexes generated by MDG. And for reference, the elevated homogenization pressure likely induced more significant structural alterations in the colloidal framework of beverages containing CGN compared to those involving peanut protein molecules, resulting in the higher viscosity observed in the CGN-containing beverages (Hinds et al., 1997).

The stability of the beverage’s suspension was enhanced through the incorporation of CGN or MDG; however, increasing the homogenization pressure did not yield any improvements. The mean suspension stability indices (SSI) indicated that the control beverages exhibited significantly lower stability (0.4 ± 0.04) compared to those containing CGN or MDG, which had a stability index of 0.5 ± 0.02. The ratio of fat to protein in the peanut beverage closely resembled that found in commercially available low-fat (2%) cow’s milk; however, the latter exhibited the SSI of 1.0 ± 0.01. Consequently, the inadequate physical stability observed in peanut beverages is likely attributable to the denaturation of proteins that occurs during the roasting process. Subjecting the formulations to heating at approximately 71 °C prior to homogenization would promote further denaturation and increase the exposure of nonpolar groups within the protein molecules (Hinds et al., 1997).

Emulsifiers for coconut milk

Coconut (Cocos nucifera L.) is widely cultivated across over 90 nations, primarily centered in Asia and the Pacific region. The Philippines, Indonesia, India, Sri Lanka, Thailand, Malaysia, and Papua New Guinea collectively represent approximately 80% of the global coconut cultivation area (Pham, 2016). Coconut serves as a valuable source of diverse natural compounds that can be utilized in the creation of pharmaceuticals for treating various illnesses and in the production of industrial goods. Coconut also contains essential micronutrients and minerals that are crucial for human health. Consequently, coconuts are widely consumed as a food source across the globe, particularly in tropical regions (DebMandal and Mandal, 2011).

Six emulsifiers have been applied to coconut milk: sodium caseinate (SC), whey protein isolate (WPI), sodium dodecyl sulfate (SDS), polyoxyethylene sorbitan monolaurate or Tween 20 (TW20), polyoxyethylene sorbitan monostearate or Tween 60 (TW60), and sucrose fatty acid ester (SE) (Ariyaprakai et al., 2013; Tangsuphoom and Coupland, 2008, 2009). Coconut milk was prepared by mixing coconut flesh pulp and distilled water at a weight ratio of 2:1, filtering it through double-layered cheese cloth, and pressing it. Before or after adding the emulsifiers, the mix was homogenized 3 to 5 times at 20/2 MPa, and 0.02 wt% of thimerosal or sodium azide was used into that mix to control microorganisms (Ariyaprakai et al., 2013; Tangsuphoom and Coupland, 2008, 2009).

Coconut milk is very unstable and easily separated into a lower water-soluble portion and an upper oil layer. The reason is that the quantity and quality of protein in coconut milk is not sufficient to stabilize fat globules (Monera and Del Rosario, 1982; Tangsuphoom and Coupland, 2005). Application of 4 emulsifiers such as SC, WPI, SDS, and TW20 to the coconut milk has been studied (Tangsuphoom and Coupland, 2008). Incorporation of low-molecular-weight emulsifiers such as SDS and TW20 subsequent to the homogenization process has the capability to displace coconut proteins from the interface and disrupt the flocculation. Conversely, introducing these emulsifiers prior to homogenization enhances the effectiveness of the homogenization process and results in the formation of stable emulsion droplets at a submicron scale. In other words, adding stabilizers to the emulsion and then homogenizing it significantly improved its effectiveness. All four added emulsifiers showed a tendency for decreasing particle size into the submicron range as the concentration of each emulsifiers increased, because there was sufficient amount available to coat the new surface created during homogenization. There was a slight difference in the case of creaming index, where only low levels of SDS (0.1 wt%) showed no difference in creaming kinetics, whereas similar levels of other emulsifiers tended to delay the onset of creaming by approximately 60 h. All samples began to stabilize on creaming at 0.25 wt%, and stability of coconut milk was excellent in this order: TW20, WPI, SC, and SDS (Tangsuphoom and Coupland, 2008). On the other hand, there was a large difference in zeta potential between 4 emulsifiers in coconut milk. SC and WPI, which are solid or colloidal particle type emulsifiers, had the effect of lowering the zeta potential by about − 10 to 15 mV when added after homogenization, but had little effect on improving zeta potential when added after homogenization. Low-molecular-weight emulsifiers such as SDS and TW20 added before homogenization had slightly better stability than those added after homogenization. In particular, when 1%wt of SDS was added, it showed an excellent ability to lower zeta potential by more than − 20 mV (Tangsuphoom and Coupland, 2009).

Application of SE and TW60 to the state of homogeneous coconut milk was observed after heat treatment at room temperature (28 °C), 121 °C for 10 min, 100 °C for 20 min, and − 20 °C for 14 h (Ariyaprakai et al., 2013). The control with no emulsifier had an average particle size of 10 μm at room temperature, but the heat-treated product at 100 °C for 20 min and at 121 °C for 10 min had 1.6 times and 2.2 times larger particles, respectively. Coconut milk with TW60 revealed the particle size of 5.5 μm at room temperature, and became 1.3 times and 2.3 times larger after heat treatment at 100 °C for 20 min and at 121 °C for 10 min, respectively. On the other hand, coconut milk with SE showed the particle size of 8.8 μm, which was a little larger than that of TW60 at room temperature, but there was no change in particle size after heat treatment at 100 °C and 121 °C, showing more heat stability than TW60. This result was consistent with the previous studies that SE is less sensitive to temperature changes compared to other nonionic surfactants with oxyethylene groups such as TW60 (Stubenrauch, 2001).

Emulsifiers for walnut milk

Walnut (Juglans regia L.) is a longstanding tree nut variety that has been cultivated for centuries. Originating from southeastern Europe, eastern Asia, and northern America, walnuts are known for their high content of vitamins B and E, which have been shown to have anti-aging properties, delay cellular deterioration, and boost cognitive function. Research has indicated that walnuts may offer therapeutic benefits for various health conditions, including neurasthenia, hypertension, coronary heart disease, emphysema, and gastrointestinal discomfort (Sze‐Tao and Sathe, 2000).

Three emulsifiers have been applied to the walnut milk: xanthan gum (XG), glycerol monostearate (GMS), and decaglycerol monolaurate (DML) (Liu et al., 2016a, 2016b). To produce walnut milk, the shelled walnut kernels were mixed with deionized water and made into a slurry. This initial slurry was then further processed using a colloid mill and subsequently filtered to eliminate any solid remnants. This slurry is mixed with water to make 0.85% protein and 1.97% fat. In order to attain a high-quality emulsification, an initial pre-homogenization step was conducted at a rotational speed of 10,000 rpm for 3 min. Subsequently, the mixture underwent processing through a two-stage valve homogenizer for three cycles at a pressure of 80 MPa and a temperature of 65 °C. The resulting emulsion was promptly cooled to 25 °C and subsequently dispensed into containers (Liu et al., 2016a, 2016b).

The inclusion of GMS and DML resulted in a reduction in droplet size and polydispersity index (PdI), while causing an increase in the absolute value of zeta potential. This indicated that emulsifiers could have the potential to enhance the surface activity of droplets by being absorbed at the oil–water interface through hydrogen bonding, thereby reducing the interfacial tension and preventing droplet coalescence (Isaac et al., 2013). The presence of XG led to an increase in the absolute value of zeta potential of droplets in the emulsion compared to the control group. This indicated the adsorption of the anionic polysaccharide onto the surface of the droplets, suggesting the formation of protein-polysaccharide complexes (Liu et al., 2016a, 2016b). Despite both walnut protein and polysaccharides carrying a net negative charge at a pH greater than the isoelectric point, the interaction involved attraction between positively charged local patches on the protein and the anionic polysaccharide (Dickinson, 1998). When the mixture of GMS + DML + XG was introduced into the walnut milk emulsion, a reduction in droplet size occurred, similar to the walnut milk emulsion stabilized by the mixture of GMS + DML. This led to an enhanced electrostatic repulsion effect. Consequently, the walnut milk emulsion stabilized by the mixture of GMS + DML + XG exhibited greater stability compared to emulsions stabilized by the mixture of GMS + DML or XG alone. The GMS and DML likely adhered to the water–oil interface, resulting in steric repulsion (Liu et al., 2016a, 2016b). Conversely, XG may have formed protein-polysaccharide complexes, thereby increasing both steric and electrostatic repulsion between droplets and diminishing van der Waals attraction (Chen et al., 2017).

The zeta potential of droplets in walnut milk emulsion stabilized by the mixture of GMS + DML + XG exhibited a higher value compared to the control, suggesting that the anionic polysaccharide had the ability to adhere to the positively charged regions on the protein surface within a pH range of 3–10. The presence of XG led to a significant decrease in the zeta potential of the walnut milk emulsion as the pH was adjusted from 5 to 3, because the XG’s pKa was approximately 3.1. At pH levels of 3 and 4, the carboxyl group of XG became protonated while the walnut protein carried a positive charge, resulting in insufficient electrostatic attraction to deposit XG onto the protein-coated droplets (Liu et al., 2016a, 2016b). Emulsions stabilized by the mixture of DML + GMS exhibited a higher negative charge at pH values of 3 and 4 compared to those stabilized by XG, possibly due to the acid-resistant nature of the nonionic surfactants present in the mixture of GMS + DML (Malhotra and Coupland, 2004). Overall, these findings indicate that GMS and DML can enhance stability of the walnut emulsion by adsorbing onto the oil–water interface within a pH range of 3–10. Conversely, XG was found to be ineffective in preventing droplet aggregation in highly acidic environments, but it significantly improved stability by enhancing viscosity, electrostatic interactions, and steric repulsion within a pH range of 6–10 (Liu et al., 2016a, 2016b).

In the presence of XG alone, an increase in the number of freeze–thaw cycles led to an increase in droplet size. This observation suggested that the ice crystals ruptured the interfacial membrane formed by the polysaccharide. However, the emulsion containing XG demonstrated improved physical stability during freeze–thaw cycles, possibly due to the polysaccharide increasing the emulsion’s viscosity and reducing droplet migration rates as per the Stokes formula (Danov et al., 2012). Conversely, in emulsions stabilized by the mixture of GMS + DML, there was no significant change in droplet size during freeze–thaw cycles. This indicates that nonionic surfactants could adsorb onto the oil–water interface, forming a robust and thick interfacial membrane that prevented droplet coalescence during freezing. A single emulsifier did not offer effective protection against partial coalescence due to thinner interfacial layers (Thanasukarn et al., 2004). When the mixture of GMS + DML + XG were introduced in the emulsion, there was minimal increase in droplet size and physical stability after the first freeze–thaw cycle. Subsequent cycles showed similar droplet size and physical stability in emulsions stabilized by XG, which was more stable than those without XG. These findings suggested that the mixture of GMS + DML + XG effectively protect emulsion droplets from coalescence and flocculation during freeze–thaw treatment. The crystallization of water in the freezer caused droplets to come closer together, as the non-frozen droplets in the emulsion remained in the aqueous phase (Ghosh et al., 2006; Surh et al., 2005). Consequently, the combination of polysaccharides such as XG and nonionic surfactants such as GMS and DML might offer enhanced resistance to environmental stresses (Liu et al., 2016a, b).

Emulsifiers for soy milk

Soy milk is a conventional plant-derived protein beverage prevalent in Asia, particularly in China. It is produced through the processes of soaking, grinding, and filtering soybeans. This beverage is not only abundant in protein, fats, and carbohydrates, but it also provides essential nutrients such as calcium, phosphorus, iron, and vitamins (Giri and Mangaraj, 2012). Furthermore, soy milk is a source of compounds like soy saponin, isoflavones, and lecithin, which may offer various health benefits, including cholesterol reduction, liver protection, anti-cancer properties, antioxidant effects, and potential activity against the human immunodeficiency virus (Omoni and Aluko, 2005). In case of soy milk, only one emulsifier was applied: konjac glucomannan (KGM) (An, 2019). Fresh soy milk was produced using a soy milk maker, resulting in a composition of 3.8% protein, 3.5% fat, 4% other components, and 88.7% water (w/w). Subsequently, KGM powder was dissolved in distilled water to create a series of KGM solutions ranging from 0.1 to 2.5% (w/v). The experimental procedures did not include processes such as enzymolysis, sterilization, or homogenization, but various formulations of soy milk were tested (An, 2019).

To examine the phase behavior, the KGM-soybean milk mixture was allowed to stand at a temperature of 5 ℃ for a duration of 12 h. Typically, when the concentration of KGM was elevated, the mixture remained in a homogeneous state, exhibiting no substantial variation before and after standing. And the phase separation diagram indicated that the region of stable phases increased as the concentration of KGM increased. In mixed systems created with low concentrations of KGM solution, specifically within the range of 0.1–0.5% (w/v), phase separation was only observed at minimal concentrations of soy milk. This phenomenon might be explained by the incomplete coverage of soy proteins by KGM molecules, leading to the agglomeration and settling of colloidal particles as a result of protein interactions (An, 2019).

The viscosity of the mixed solutions also increased corresponding to the increasing concentration of KGM. Polysaccharides are typically characterized by their high viscosity, which can consequently enhance the viscosity of the mixture system. Additionally, incorporating polysaccharides can enhance the water retention capacity of the emulsion, thereby minimizing water evaporation or loss over short periods and subsequently improving the stability of the mixture (Le et al., 2017; Onsaard et al., 2014). The incorporation of KGM into food products has the potential to alter the texture, and to enhance the rheological characteristics of the food, which is attributable to the colloidal stability provided by KGM (Devi et al., 2017). The viscosity is closely associated with the rate of phase separation (Chun et al., 2014). KGM contributed viscosity to the soy milk mixture, resulting in the establishment of a network within the soy milk, which effectively inhibited precipitation and aggregation (Ni et al., 2018). Consequently, with the increase in KGM concentration, the stability of the mixed system improved, attributed to the elevated viscosity of the mixture, a finding that also was substantiated in the confocal laser scanning microscope (CLSM) (An, 2019).

The notable rise in turbidity can be attributed to the clustering of mixtures (Turgeon et al., 2003). At equivalent volume percentages of soy milk and low concentrations of KGM, the influence of KGM concentration on the turbidity of the mixture was minimal. Typically, the turbidity of mixtures with elevated KGM concentrations was lower compared to those with reduced KGM levels. This observation may be attributed to the ability of KGM to enhance the dispersion of aggregated protein particles, and it also has been demonstrated that KGM with higher viscosity can stabilize protein granules (An, 2019).

The particle size in soy milk varied between 10 and 1000 nm. The particle size distribution exhibited considerable breadth in mixtures with low KGM concentration. However, as the KGM concentration increased, the particle size distribution diagram revealed relatively narrow double peaks, suggesting that the KGM concentration in the mixture significantly affected the particle size distribution (An, 2019). In a mixed system characterized by a low concentration of KGM, the quantity of KGM molecules was inadequate to fully envelop the proteins, potentially resulting in the aggregation of proteins with adjacent molecules. Conversely, in a mixed system with a high concentration of KGM, the increased viscosity contributed by KGM could restrict the mobility of droplets, thereby inhibiting the aggregation of particles (Jafari et al., 2008).

When the concentration of KGM was held constant, the zeta potential of the mixture with a greater volume ratio of soymilk exhibited a higher value, suggesting enhanced stability. This finding contrasts with the outcomes observed in the earlier phase separation diagram. This phenomenon can be elucidated by the fact that a higher proportion of soy milk resulted in insufficient KGM to fully encapsulate the protein particles, leading to their aggregation and subsequent sedimentation (An, 2019).

Emulsifiers for flos sophorae immaturus beverage

Flos sophorae immaturus (FSI), also known as the pagoda tree, refers to the dried flower buds of Sophora japonica L., primarily cultivated in East Asia, especially China. For approximately 2000 years, FSI has been extensively utilized in China as a significant traditional Chinese medicine and herbal tea, recognized as a nutritious food due to its rich flavonoids content (Wang et al., 2022). Flavonoids are natural plant compounds known for their various health advantages, including anti-inflammatory, anti-diabetic, and antioxidant properties (Abotaleb et al., 2018; Dong et al., 2019; He et al., 2016). Four emulsifiers has been applied to FSI beverage: gellan gum (GeG), microcrystalline cellulose (MCC), glycerol monostearate (GMS) and sucrose fatty acid ester (SE) (Li and Fan, 2020). To produce FSI beverage, the FSI underwent micronization using a vertical batch-type planetary ball mill. Emulsifiers were introduced into the FSI beverage system and agitated continuously for a duration of 20 min at a temperature of 70 °C. Subsequently, the FSI beverages underwent homogenization through two cycles at a pressure of 30 MPa. Finally, the FSI beverages were subjected to sterilization at a temperature of 121 °C for 20 min.

The inclusion of MCC or GeG in the FSI beverage led to a reduction in particle volume frequency for larger sizes ranging from 20 to 150 μm, as supported by the d₄₃ and d₃₂ values. This outcome indicates that the addition of emulsifiers can effectively inhibit particle aggregation within the FSI beverage. On the other hand, a notable disparity in d₃₂ values was observed with the addition of MCC + GeG. This discrepancy can be attributed to the gel-forming property of GeG, which hinders droplet movement and enhances beverage stability (Vilela and da Cunha, 2016), as well as the formation of a three-dimensional network structure by MCC, which prevents small particles from coalescing into larger, insoluble particles (Yaginuma and Kijima, 2006). The application of MCC + GeG has been demonstrated that they positively influence the stability of beverages (Taherian et al., 2008). In the sample containing mixed emulsifiers (GMS + SE as well as MCC + GeG + GMS + SE), the d₄₃ and d₃₂ values were decreased compared to the control group. This reduction may have implications for the thickening properties of the beverage system. And the introduction of MCC + GeG as well as GMS + SE appeared to have a protective effect on the particles within the beverages, potentially preventing aggregation during thermal processing. This protective mechanism could be attributed to the adherence of these additives at the particle interface, leading to an enhancement in system viscosity (Liu et al., 2018; Taherian et al., 2008). Furthermore, these emulsifiers were observed to mitigate the denaturation of the respective proteins in FSI beverages, consequently improving the thermal stability of the beverage system (Lu et al., 2019).

The inclusion of MCC led to a noticeable increase in the absolute value of zeta potential, suggesting that the negatively charged component of MCC engaged with the interface protein (Xu et al., 2016). The introduction of GeG had a minimal impact on zeta potential in comparison to MCC, potentially due to the low GeG concentration and its thermal instability. The zeta potential magnitude of GMS + SE increased to 45.5 mV, surpassing that of the MCC + GeG. This observation implies that the mixed emulsifiers potentially absorbed around the droplets, leading to the formation of electrostatic repulsion among them (Li and Fan, 2020). The highest absolute value of zeta potential (51.9 mV) was observed in the sample containing MCC + GeG + GMS + SE. This phenomenon occurred as a result of the reduction in the size of liquid droplets followed by heat sterilization, which was achieved by inhibiting the aggregation of sample particles through electrostatic repulsion (Hong et al., 2018; Liu et al., 2016a, 2016b).

The viscosity at the given shear rate (between 0.01 to 100 s⁻¹) exhibited an increasing trend in the following order: Control < GeG < GMS + SE < MCC < MCC + GeG < MCC + GeG + GMS + SE. With an increase in shear rate, the viscosity decreased for all samples while the shear stress increased, demonstrating shear-thinning rheological characteristics. The sample containing GeG exhibited reduced viscosity compared to samples incorporating alternative emulsifiers at equivalent shear rates. GeG is capable of gel formation at low concentrations, yet this does not notably elevate the viscosity of the beverage (Li and Fan, 2020). This property is advantageous as it helps maintain the beverage at a lower viscosity level (Yan et al., 2022). The rheological properties of GMS + SE exhibited similarities to those of MCC and GeG, displaying characteristics of a non-Newtonian fluid (Hong et al., 2018). The stabilizer concentration in the sample containing MCC + GeG + GMS + SE exceeded 0.15%, yet the measured viscosity was below 100 mPa. This indicated that the utilization of 4 mixed emulsifiers not only enhanced the stability of the FSI beverage but also successfully mitigated the adverse impacts of excessive beverage viscosity (Li and Fan, 2020).

In Turbiscan analysis, the control, GeG, and GMS + SE exhibited significantly elevated positive values of change of backscattered light intensity (△BS) indicating a high degree of instability and disorder. This observation suggests the occurrence of irreversible phase separation within the beverage system in a short time. The sedimentation of the lower layer ceased upon the introduction of MCC + GeG, indicating that MCC may shield GeG during thermal sterilization processes. The structural framework created by MCC facilitated the suspension of particles, while the gel-like structure formed by GeG augmented the viscosity, thereby improving the overall stability of the beverage (Li and Fan, 2020). The sample with MCC + GeG + GMS + SE exhibited the lowest value of △BS (not exceeding 0.5%), signifying that the beverage system achieved optimal stability (Li and Fan, 2020).

Emulsifiers for ginkgo milk

Ginkgo (Ginkgo biloba), an ancient tree species with a history dating back 300 million years, is characterized by its female trees producing fruit containing edible, soft, yellow-green kernels. These ginkgo nuts have been traditionally consumed in China both as a food source and for their medicinal properties. Despite China being the primary producer of ginkgo nuts, their full potential remains under-utilized (Zhang et al., 2007). Two emulsifiers was applied to the ginkgo milk: gellan gum (GeG) and microcrystalline cellulose (MCC) (Ni et al., 2021). To produce walnut milk, ginkgo nuts were finely ground in deionized water at a ratio of 1:10 (w/v) utilizing a colloid mill. Starch hydrolysis was facilitated by the amylase. Subsequently, each hydrocolloid was incorporated into the beverage mixture and agitated continuously at a temperature of 80 °C. The resultant beverage samples underwent homogenization at a pressure of 30 MPa for two cycles. The beverage samples were subjected to sterilization at a temperature of 121 °C for 25 min. Different stabilizer formulations were utilized in the following concentrations: 0.3% MCC (w/w), 0.1% GeG (w/w), 0.3% MCC + 0.05% GeG (w/w), 0.3% MCC + 0.08% GeG (w/w), 0.3% MCC + 0.12% GeG (w/w), and 0.3% MCC + 0.15% GeG (w/w) (Ni et al., 2021).

The incorporation of MCC and GeG respectively into the beverage resulted in a reduction in the aggregation of suspended particles, as indicated by a decrease in the peak height around 50 μm. Additionally, the particle size measurements of the beverage samples containing MCC and/or GeG were smaller compared to the control sample. This protective effect is likely attributed to the development of a three-dimensional network structure by MCC (Yaginuma and Kijima, 2006) or the gel formation of GeG (Sworn and Stouby, 2021). In relation to the beverage samples subjected to a combination of MCC and GeG, alterations in particle size were found to be closely linked to the proportion of MCC to GeG utilized. Notably, the beverage samples containing 0.3% MCC and 0.05% GeG (w/w) demonstrated the least significant rise in particle size, whereas the particle size of the beverage containing 0.3% MCC and 0.15% GeG (w/w) increased by 150% after sterilization. These findings indicate the critical importance of maintaining an appropriate MCC to GeG ratio to prevent an increase in particle size (Ni et al., 2021).

Proteins present in ginkgo beverages demonstrate a negative charge as a result of the beverages’ pH level of 7.0, which exceeds the isoelectric point (pI) of ginkgo proteins (pH 4.62) (YANG et al., 2011). Additionally, the magnitude of the zeta potential in beverage samples containing MCC or GeG was found to be higher compared to the control group. This can be attributed to the anionic nature of both MCC and GeG (Sworn and Stouby, 2021; Xu et al., 2016), as they are polysaccharides that introduce negatively charged particles. Following the sterilization process, the zeta potential values of all beverages increased, potentially due to a decline in the effective surface charge of suspended particles caused by particle aggregation (Song et al., 2013).

All beverages exhibited characteristics of shear thinning fluids, where the shear stress was observed to rise in conjunction with an increase in shear rate. After undergoing sterilization, the decrease in perceived thickness of the beverage sample containing solely GeG (35%) was more pronounced compared to the sample containing only MCC (22%). This suggests that the impact of sterilization on the fluid gel created by GeG may be greater than its effect on the three-dimensional network of MCC (Ni et al., 2021). Through an analysis of the MCC to GeG ratio and the consequent alterations in viscosity, it was deduced that a combination of MCC with a minor quantity of GeG could be advantageous in safeguarding beverages against viscosity decline during the sterilization process. Conversely, the combination of MCC with a larger proportion of GeG might yield adverse outcomes. The MCC system demonstrates notable thermal stability, with minimal impact on its functionality and viscosity of the MCC dispersion in response to temperature variations (Wang et al., 2023). These findings imply that a minimal GeG (0.05% and 0.08%) enhances the thermal resilience of the network structure established by MCC, thereby impeding viscosity reduction in beverages. Conversely, a higher GeG (0.12% and 0.15%) could disrupt the formation of the MCC network structure due to GeG’s propensity to self-gel, leading to diminished thermal stability in beverages (Ni et al., 2021).

In the analysis of Turbiscan results, it was observed that beverages containing only MCC or GeG displayed lower △BS values compared to the control beverage. This suggests that MCC and GeG can enhance the physical and thermal stability of cloudy beverages by forming a network structure or self-gelation structure. The varying ratios of MCC and GeG in the beverage samples showed that those with a smaller amount of GeG (0.05% and 0.08%) had better Turbiscan Stability Index (TSI) outcomes than those with a higher amount of GeG (0.12% and 0.15%) (Ni et al., 2021). This finding was supported by rheology analysis, indicating that a small quantity of GeG combined with MCC synergistically reinforce three-dimensional network structures, likely due to the emulsifying properties of GeG (Vilela and da Cunha, 2016).

Emulsifiers for oat milk

Oats are inherently rich in dietary fiber, especially beta-glucan, a type of soluble fiber recognized for its positive effects on cardiovascular health (Sethi et al., 2016). The production of oat milk preserves a portion of this fiber, contingent upon the processing techniques employed, potentially aiding in digestive wellness and enhancing satiety. In the study of oat milk, three emulsifiers were applied: guar gum (GuG), xanthan gum (XG), and soluble soybean polysaccharides (SSPS) (Zhang et al., 2023). These emulsifiers were incorporated into oat milk produced through enzymolysis followed by ultra-high temperature (UHT) sterilization, prior to homogenization at 60 MPa. During the preparation of the oat milk, the use of alpha-amylase, amyloglucosidase, and proteinase for the degradation of starch and protein contributed to a reduction in particle size, while the UHT treatment effectively maintained stability by preventing degradation from high temperatures. Different stabilizer formulations were utilized in the following concentrations: 0.16% SSPS (w/w), 0.1% GuG (w/w), 0.14% XG (w/w), 0.1% GuG + 0.14% XG (w/w), and 0.1% GuG + 0.14% XG + 0.16% SSPS (w/w) (Zhang et al., 2023).

The mean particle size of GuG exceeded that of XG and SSPS, which can be attributed to their nature as non-adsorbed polysaccharides (Zhang et al., 2023). In a neutral environment, XG and SSPS exhibit a negative charge, while GuG remains with no charge. XG and SSPS have the capacity to adsorb certain positively charged protein molecules, leading to the formation of a neutral carbohydrate surface layer. This interaction diminishes the rejection of flocculation within the system and inhibits droplet aggregation, resulting in a reduction of the average particle size (Patino and Pilosof, 2011). In comparison to SSPS and XG, the incorporation of GuG exhibited minimal influence on the absolute zeta potential value. The absolute zeta potential value of GuG + XG + SSPS peaked at approximately 32.12. Consequently, the electrostatic repulsion of GuG + XG + SSPS was more pronounced than that observed between GuG and XG alone (Zhang et al., 2023). An increase in the electrostatic repulsion among droplets correlates with a higher absolute value of the potential, thereby enhancing the stability of the beverage (Dickinson, 2009).

SSPS did not demonstrate a significant impact on viscosity; however, the incorporation of GuG and XG resulted in an increase in apparent viscosity. SSPS exhibits minimal stickiness, whereas GuG demonstrates a significant ability to interact with water molecules via hydrogen bonding, leading to the formation of a network structure that contributes to its high-viscosity properties (Martín-Alfonso et al., 2018). XG is recognized for its remarkable shear thinning properties, its rigid rod-like molecular structure, and its elevated molecular weight, along with various other structural attributes (Yadav et al., 2018). Subsequent to the SSPS, which exhibited minimal variation, the viscosity levels were notably high in the order of GuG, XG, GuG + XG, and SSPS + GuG + XG. Oat beverages possess a significant starch concentration, which is believed to work synergistically with hydrophilic colloids, thereby improving their thickening capabilities (Zhang et al., 2023).

In the Turbiscan analysis, the introduction of XG resulted in a significant reduction in the TSI. This phenomenon can be attributed to the ability of XG polysaccharide molecules to form hydrogen bonds with water, thereby creating a gel-like network structure that effectively inhibits particle aggregation (Hemar et al., 2001). However, due to the absence of hydrophobic bonds in XG, it is unable to interact with oil, leading to the formation of an oil slick. Conversely, GuG exhibits a weaker capacity for sedimentation inhibition compared to XG, yet it demonstrates superior efficacy in mitigating oil slick formation. Although both XG and GuG showed improvements relative to the control group, the presence of precipitation and clarification layers indicates that the oat beverage possesses limited physical stability. The TSI of GuG + XG decreased to 0.8%, suggesting a significant degree of stability, which can be ascribed to the synergistic interaction between the galactose moieties present in both XG and GuG (Morris et al., 1977). Furthermore, in the presence of GuG + XG + SSPS, the sample exhibited the lowest change in ΔBS (not exceeding 0.2%) and TSI (stayed almost at 0). This outcome is due to the negatively charged nature of both XG and SSPS, which enhances electrostatic repulsion and steric hindrance among droplets (Ren et al., 2022), thereby preventing flocculation. Additionally, SSPS rapidly adsorbs to the surface of oil droplets, with its sugar chain extending into the aqueous phase to form a thick hydration film around the oil droplets, thus stabilizing them through steric hindrance. Consequently, the interaction between SSPS and hydrocolloids such as GuG and XG has the potential to enhance the stability of PBMAs (Zhang et al., 2023).

Consequently, stability assessments were performed on a total of 16 emulsifiers across 7 PBMAs. These studies indicated that low molecular weight emulsifiers, such as GMS and SE, enhanced stability by minimizing the size of droplet without causing a significant increase in viscosity. They adhered to the water–oil interface, resulting in steric repulsion. In contrast, amphiphilic biopolymer emulsifiers, including XG and GeG, contributed to stability by increasing viscosity and facilitating network formation. They increased both steric and electrostatic repulsion between droplets and diminishing van der waals attraction. The most impressive finding was that the emulsion stability was best when different types of emulsifiers were used in combination. On the other hand, solid or colloidal particle emulsifiers, such as SC and WPI, yielded some positive effects in the coconut milk. However, it is advisable to limit their use due to the inherent objective of excluding animal-derived ingredients from PBMAs.

In the context of establishing an industrial shelf-life, it is crucial to ensure not only microbial stability but also physicochemical stability. The appearance of a product holds significant importance for consumers, as they directly influence both the perceived value and the sensory experience associated with the product. Thus, research on the stability of PBMAs is imperative. Given the significant variability in the characteristics of each PBMAs based on their source and ingredient composition, further research is warranted to explore additional nutrients, such as fat and protein content, that align with consumer preferences. Furthermore, as there has been a lack of research on emulsifiers for almonds—an increasingly popular ingredient globally—future investigations in this area appear to be necessary.

Funding

This study was funded by Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry, RS-2024-00403609, Moo-Yeol Baik.

Declarations

Conflict of interest

The authors have declared no conflict of interest.