Formulating solid shampoos presents unique technical challenges. The bar should be hard enough to resist melting when used and especially not break after multiple uses. The hardness of a solid bar is obtained by mixing natural waxes, butters, and fatty alcohols. Cetyl alcohol or cetearyl alcohol are the most common fatty alcohols used to achieve the desired hardness, while shea or cocoa butter are commonly used natural butters. Kaolin clay and other natural powders may be included for their oil-absorbing and scalp-purifying benefits, especially in products targeting oily hair or scalp conditions.

Due to this oil-rich base, powdered surfactants like SCS and SCI are preferred (7). APGs, typically available in liquid form, are valued for their mildness and biodegradability (8). However, their water content can complicate incorporation into solid bars and pose preservation challenges. The development of solid or powder-form APGs would be highly advantageous, enabling easier formulation and improved stability. Reliable sensory evaluation methods are also needed to assess their performance in solid shampoo formulations and would greatly support product development and optimization.

The objective of this study was to develop a reliable protocol for evaluating the sensory qualities of foam in solid shampoos, including the impact of a new powdered decyl glucoside. To achieve this, a systematic approach was followed, beginning with the selection of representative solid shampoo products to serve as examples. Sensory descriptors relevant to foam performance were then defined, enabling a structured evaluation framework. The panel conducted consensus-based evaluation to generate detailed sensory profiles.

Selection of representative solid shampoo products

Around 20 solid products available on the European market were selected. Among these:

• 30% used a Sodium Coco Sulfate (SCS) base
• 20% used a Sodium Cocoyl Isethionate (SCI) base
• 20% used a SCI + decyl glucoside (APG) base
• 30% used other surfactants, alone or in combination

These shampoos exhibited wide diversity in visual appearance, ranging in color and gloss, as well as in texture, with noticeable variations in roughness and hardness. This variability highlights the broad formulation spectrum.

Sensory descriptors relevant to foam performance

To define the appropriate descriptors, the team based their approach on consumer gestures when using solid shampoos, which typically involved holding the bar between the hands to create foam before applying it to the hair.

Hand gesture

The foam evaluation on hands begins by activating the solid shampoo previously dipped in hot water through 10 rotations between wet hands. This method ensures consistent foam generation for sensory assessment. The following key attributes are then rated to characterize the foam quality:

• Hand coverage: How uniformly the foam creates a white layer over the hands; a more uniform white effect corresponds to a higher rating.
• Bubble size: Larger bubbles receive higher scores, reflecting specific sensory preferences.
• Foam homogeneity: The uniformity in bubble size throughout the foam; greater uniformity indicates better foam quality. (Figure 1-1)

Strand gesture

Table 1 below presents the key sensory foam attributes of solid shampoos along with their definitions and standardized protocols for evaluation on hair strands.

Table 1. Sensory foam attributes on strands

Figure 1. Foam homogeneity (1), strand coverage (2) foam volume (3)

Sensory profile of four formulations

A consensus profile was developed with six participants. In consensus methods, assessors evaluate the products individually and then discuss the attributes and intensities to reach a shared assessment (9) Four products were studied: three currently on the market (A,B,C) and one product under development D:

• Product A: SCS-based
• Product B: SCI-based
• Product C: SCI + decyl glucoside (liquid APG)
• Product D: SCI + solid decyl glucoside (powdered APG)

The results are presented in Figure 2. The developed protocol enables to observe clear differences between the products across all descriptors, except for the cushion effect, which remains a challenging attribute to evaluate. The product A based on SCS and shea butter showed the lowest performance in terms of foam quality. In contrast, the solid APG-based product D, despite being a first, non-optimized formulation, already produced a homogeneous foam

Figure 2. Consensus sensory profile of foam performance

Many surfactants contain water, which complicates waterless formulation. Powdered surfactants are rare, limiting formulation flexibility. However, spray-dried decylglucoside, as studied here, appears promising due to its dry form, mildness and performance.

In 2018, the team had already developed a framework for evaluating liquid shampoos and conditioners (10). However, a specific protocol for bar hygiene products was missing. The sensory evaluation method developed proved effective in distinguishing formulations. It provides a structured, objective way to evaluate full sensory experience from visual appearance to post-rinse feel. This approach supports future innovation in waterless formats by informing improvements in foam, texture and consumer perception.

The development of a straightforward and standardized method for evaluating foam in solid shampoos represents a significant advancement for the cosmetic industry. By clearly defining essential criteria such as foam coverage, bubble size, homogeneity, and cushion effect, this method enables effective comparison across various formulations. This study introduces a practical sensory evaluation protocol tailored for solid shampoos, using the consensus profile method. This approach allows for reliable assessments with a limited number of trained panelists and does not require statistical analysis, making it efficient and reproducible. By clearly defining key sensory attributes, the method enabled the identification of performance differences between formulations, particularly those using spray-dried alkylpolyglucosides. These solid surfactants show strong potential to support the development of mild, effective, and sustainable waterless products. The protocol also provides a foundation for future research on innovative ingredients and formulation strategies adapted to solid formats.