1. Introduction

Most medicinal and health-promoting properties of plants are due to secondary metabolites which exist in forms and concentrations which vary with species of plants and plant parts. Geographical, environmental, and climatic conditions also affect the types and amounts of phytochemicals ^[1]. Some of the most common classes of phytochemicals are terpenoids, saponins, tannins, flavonoids, and alkaloids ^[2][3].

Most medicinal and health-promoting properties of plants are due to secondary metabolites which exist in forms and concentrations which vary with species of plants and plant parts. Geographical, environmental, and climatic conditions also affect the types and amounts of phytochemicals [1]. Some of the most common classes of phytochemicals are terpenoids, saponins, tannins, flavonoids, and alkaloids [2,3].

Modern toilet soaps and detergents trace their origin to the ancient use of plants, commonly referred to as soapy plants, which possess foaming ability when they are agitated in water. The soapy properties of these plants are attributed to the presence of saponins and their scientific nomenclature arose as a result of this; for example, the genus Saponaria is made up of saponin-rich plants commonly called soapworts ^[4]. However, there are other genera such as Sapindaceae ^[5][6], Aceraceae ^[7][8], and Hippocastanaceae ^[9][10], whose saponin concentrations are also high. Saponins are identified by their ability to rupture erythrocytes or form colloidal solutions that can produce a stable lather when they are shaken in the presence of water ^[11].

Modern toilet soaps and detergents trace their origin to the ancient use of plants, commonly referred to as soapy plants, which possess foaming ability when they are agitated in water. The soapy properties of these plants are attributed to the presence of saponins and their scientific nomenclature arose as a result of this; for example, the genus Saponaria is made up of saponin-rich plants commonly called soapworts [4]. However, there are other genera such as Sapindaceae [5,6], Aceraceae [7,8], and Hippocastanaceae [9,10], whose saponin concentrations are also high. Saponins are identified by their ability to rupture erythrocytes or form colloidal solutions that can produce a stable lather when they are shaken in the presence of water [11].

Various studies have been published that reported the use of plants for oral hygiene purposes ^[12][13]. Moreover, there is a surge of sites on the internet that focus on formulation of plant-based shampoos, and this is probably due to the increase in the demand for commercial shampoo ^[14]. However, there is very little literature on the use of plants for hand hygiene. This highlights the fact that this is a neglected area of plant utilization. It may be that there is an assumption that hand and toilet soap are accessible to everyone in the world. This is not the case because the UN reported that about three billion people (which is 40% of the world’s population) survive without basic handwashing facilities with soap and water available in their homes ^[15]. Thus, a focus on the application of ethnobotany to hand and toilet hygiene is a unique approach on the role plants can play in enhancing community health. This paper tries to integrate what is known about saponins, their utility as soaps and how they can be applied in the community, either directly by communities which have no access to commercial soap, or by companies or non-governmental organizations formulating soaps using local resources.

Various studies have been published that reported the use of plants for oral hygiene purposes [16,17]. Moreover, there is a surge of sites on the internet that focus on formulation of plant-based shampoos, and this is probably due to the increase in the demand for commercial shampoo [18]. However, there is very little literature on the use of plants for hand hygiene. This highlights the fact that this is a neglected area of plant utilization. It may be that there is an assumption that hand and toilet soap are accessible to everyone in the world. This is not the case because the UN reported that about three billion people (which is 40% of the world’s population) survive without basic handwashing facilities with soap and water available in their homes [19]. Thus, a focus on the application of ethnobotany to hand and toilet hygiene is a unique approach on the role plants can play in enhancing community health. This paper tries to integrate what is known about saponins, their utility as soaps and how they can be applied in the community, either directly by communities which have no access to commercial soap, or by companies or non-governmental organizations formulating soaps using local resources.

A possible strategy to increase soap availability and affordability would be the use of the floristic heritage in many poor and rural communities. Soapy plants present natural sources of soaps with putative antimicrobial and disinfectant properties ^[16] which can be used in their natural forms as extracts or formulated into finished low-cost products. In order to do this, we saw the need for an in-depth review of literature and synthesis of knowledge on saponin-rich plants generally and Southern African flora in particular.

A possible strategy to increase soap availability and affordability would be the use of the floristic heritage in many poor and rural communities. Soapy plants present natural sources of soaps with putative antimicrobial and disinfectant properties [20] which can be used in their natural forms as extracts or formulated into finished low-cost products. In order to do this, we saw the need for an in-depth review of literature and synthesis of knowledge on saponin-rich plants generally and Southern African flora in particular.

2. Saponin-Rich Plants

As shown in

Figure 1, saponins are high molecular weight organic molecules, which form glycone (saccharide) and aglycone (non-saccharide) moieties upon hydrolysis ^[17][18]. The glycone part of saponins consists of one or two sugar moieties. The saccharide moiety is usually in the form of either pentoses or hexoses and is water soluble. It has been noted that arabinose, xylose, glucose, ribose, glucuronic acid, and rhamnose are the most commonly occurring saccharide moieties in saponins ^[18][19]. The part which is non-soluble in water, also referred to as the sapogenin or genin, is characterized by 27 to 30 carbon atoms ^[14] and is either a triterpene or a steroid ^[18]. Depending on the type of aglycone that they have, saponins are classified into triterpenoid or steroid saponins, the former of which is more common in the plant kingdom ^[2].

, saponins are high molecular weight organic molecules, which form glycone (saccharide) and aglycone (non-saccharide) moieties upon hydrolysis [21,22]. The glycone part of saponins consists of one or two sugar moieties. The saccharide moiety is usually in the form of either pentoses or hexoses and is water soluble. It has been noted that arabinose, xylose, glucose, ribose, glucuronic acid, and rhamnose are the most commonly occurring saccharide moieties in saponins [22,23]. The part which is non-soluble in water, also referred to as the sapogenin or genin, is characterized by 27 to 30 carbon atoms [18] and is either a triterpene or a steroid [22]. Depending on the type of aglycone that they have, saponins are classified into triterpenoid or steroid saponins, the former of which is more common in the plant kingdom [2].

Figure 1.

Possible structures of steroid saponins, with a steroid aglycone (

a) (source ^[19]) and a triterpenoid aglycone (

) (source [23]) and a triterpenoid aglycone (

b) (source: ^[20]).

) (source: [24]).

Saponins are present in different plants, in different quantities ranging from low to high.

Table 1

gives information on plants that were found to have various concentrations of saponins, through different quantification techniques, by different researchers around the world. It is also worth noting that a 40 mg/ g concentration of saponins in plants was the benchmark for inclusion in this review.

Table 1.

Apart from foaming properties, saponins are typically identified by their ability to exhibit hemolytic activity, which also renders them the ability to disrupt microbial cells. Thus, plants that exhibit high hemolytic activity are more likely to have more saponins.

As highlighted in

Table 1, saponin-containing plants fall under a variety of taxa, though Fabaceae and Lamiaceae families were the most common in this review. We also noted that the quantities of saponins in plants still vary, even within families. There is evidence that environmental conditions, tissue type, age, physiological state, and genetic profiles of plants also impact on the concentration of phytochemicals present within a plant ^[18]. Saponin content in different parts of a single plant may also vary ^[29]. The results that are shown in

, saponin-containing plants fall under a variety of taxa, though Fabaceae and Lamiaceae families were the most common in this review. We also noted that the quantities of saponins in plants still vary, even within families. There is evidence that environmental conditions, tissue type, age, physiological state, and genetic profiles of plants also impact on the concentration of phytochemicals present within a plant [22]. Saponin content in different parts of a single plant may also vary [46]. The results that are shown in

Table 1

suggest that researchers use different solvents for extracting saponins, and this is more likely to affect the amount of quantifiable saponins that are extracted. 

3. Use of Saponin-Rich Plants

3.1. Traditional Uses

Some plants have been documented to be used traditionally as natural soaps and shampoos as shown in

Table 2

. Only a small sub-set of saponin-rich plant species are traditionally utilized.

Table 2

shows saponin-rich plants that are being traditionally utilized, as compared to a larger number of saponin-rich plants, as shown by

Table 1

, that are not being used for their soapy characteristics. This may be because of lack of knowledge about, and access to, the plants.

Table 2.

3.2. Topical Use

Saponins have been reported to be active ingredients in some patented dermatological products which are topically applied for haircare.

3.3. Antimicrobial Use

Numerous studies show evidence that saponins possess potent antiviral, ^{[41][42][43][44][45][46]}, antibacterial, and antifungal activities ^{[47][48][49][50]}.

Numerous studies show evidence that saponins possess potent antiviral, [93,94,95,96,97,98], antibacterial, and antifungal activities [99,100,101,102].

3.4. Activity against Viruses

An in vivo experimental study on saponin extracts from

Quillaja saponaria Mollina demonstrated antiviral activity of the extracts against rotavirus in mice ^[44]. Triterpenoid saponins from P. anserine inactivated the Hepatis B virus by inhibiting the replication of its DNA. 

Mollina demonstrated antiviral activity of the extracts against rotavirus in mice [96]. Triterpenoid saponins from P. anserine inactivated the Hepatis B virus by inhibiting the replication of its DNA. 

3.5. Activity against Bacteria

Saponin extracts that were prepared from

Sorghum bicolor

L Moench were tested for their antimicrobial activity against

Escherichia coli

,

Candida albicans

, and

Staphylococcus aureus

. The results showed inhibition of

S. aureus ^[47].

[99].

3.6. Activity against Fungi

Saponins are active against various types of fungi, including commercially important yeasts. Another piece of research revealed the antifungal properties against

Saccaromyces cerevisiae

by saponins from

M. sativa

L. and

Medicago arborea L. ^[50]. Much more evidence on the antifungal activity of saponins has been reported from various studies ^{[51][52][53][54]}.

L. [102]. Much more evidence on the antifungal activity of saponins has been reported from various studies [116,117,118,119].