RHEOLOGICAL AND SENSORY PROPERTIES OF PETROLEUM-BASED COSMETIC CREAM 161 viscosity measured at high shear rates (27). Studies show that lotions with high yield stress but low yield strain are desirable to consumers, because the high yield stress allows the cream to maintain its structure upon pickup when transferring from the container to the skin while the low yield strain allows quick break down and easy spread out of the product for fast absorption (29). Results obtained from oscillatory rheological mea- surements were shown to be signifi cantly correlated to fi rmness, thickness, resistance, spreadability, stickiness, and slipperiness. Perceptions of these qualities typically increase with the storage modulus (G′) at 100% strain (24,30). Integrity of shape and penetra- tion force of skin lotions were also predicted by a combination of rheological and tex- tural parameters (24). Most rheological characterizations of skin creams were focused on linear viscoelastic properties [small-amplitude oscillatory shear (SAOS)]. Although they were shown to be correlated to sensorial properties of skin creams, they may not be suffi cient to fully char- acterize the skin feel upon the application of skin lotions at high shear rates (500–5000/s) (27). Nonlinear viscoelastic properties [large-amplitude oscillatory shear (LAOS)] of cos- metic materials under high shear rates have not been well characterized. The objective of this work is to compare the rheological properties between a commercially available petroleum-based skin cream and a novel petroleum-free skin cream, focusing on the non- linear viscoelastic properties of the samples. MATERIALS AND METHODS SAMPLE PREPARATION The petroleum-free skin cream was prepared by following the method used by Wang and Marangoni (14). It is an o/w emulsion structured with glycerol monostearate (GMS) using SSL as the coemulsifi er. The water phase contained 69.83% (w/w) of water, 0.1% (w/w) potassium sorbate, and 0.07% (w/w) xanthan gum. The oil phase was composed of 25% (w/w) oil and 5% (w/w) emulsifi ers mixed with 19:1 (w/w) MG: SSL. The oil phase used in the emulsion was Neobee® M-5 oil from Stepan Company (Northfi eld, IL) kindly supplied by Charles Tennant & Company Limited (Weston, ON, Canada). The GMS used was Αlphadim 90 SBK and the SSL used was Emplex Sodium Stearoyl Lactylate, both provided by Caravan Ingredients (Lenexa, KS). The xanthan gum used was FASTir® Xanthan EC (TIC GUMS, White Marsh, MD), and the potas- sium sorbate was purchased from Sigma-Aldrich Canada Co. (Oakville, ON, Canada). The petroleum-based skin cream was a Vaseline™ fragrance-free hypoallergenic intensive rescue repairing moisture lotion (Unilever Canada Inc., Toronto, ON, Canada) purchased from a local drugstore. Its ingredients are aqua, glycerin, petrolatum, stearic acid, glycol stearate, dimethicone, isopropyl isostearate, dihydroxypropyltrimonium chloride, hydroxy- ethyl urea, tapioca starch, cetyl alcohol, glyceryl stearate, magnesium aluminum silicate, stearamide AMP, carbomer, isopropyl myristate, cedrol, triethanolamine, disodium ethyl- enediaminetetraacetic acid, phenoxyethanol, methylparaben, and propylparaben. The petroleum-free skin cream will be referred to as “experimental” while the petroleum- based skin cream will be called the “control” in this article.

JOURNAL OF COSMETIC SCIENCE 162 DETERMINATION OF THE WATER CONTENT IN THE COMMERCIAL SKIN CREAM The water content of the petroleum-based skin cream was determined by placing it in an oven set at 105°C for 24 h. Samples were weighed before and after drying in the oven and the water loss was calculated from the difference between the initial and fi nal weights. Samples were prepared and measured in triplicate and the average percentage weight loss was reported as the water content. BRIGHT-FIELD LIGHT MICROSCOPY The microstructure of the experimental and control skin cream was observed using bright-fi eld light microscopy. Samples were observed with a Leica DMRXA2 microscope (Leica Microsystems Canada Inc., Richmond Hill, ON, Canada), and images were taken with a CCD camera (RETIGA 1300i, Burnaby, BC, Canada) controlled by Volocity 6.2.1 software (PerkinElmer, Woodbridge, ON, Canada). The average emulsion droplet size of the two samples was computed from thresholding the images with Image J software (National Institution of Health, Bethesda, MD). OSCILLATORY RHEOMETRY The linear and nonlinear viscoelastic properties of the skin creams were determined using an Anton Parr MCR 302 dynamic rheometer controlled with RheoCompass software (Version 1.13, Anton Parr, Graz, Austria). The temperature of the plate was set at 25°C. The geometry used for the measurements was 2° plate and cone geometry with 50 mm diameter (Model CP50-2, Anton Parr). Strain sweep from 0.01% to 5000%, covering the linear and non-linear viscoelastic region (LVR) of the samples, was performed at frequen- cies of 0.05, 0.25, 0.5, 0.75, and 1.0 Hz. Storage modulus (G′), loss modulus (G”), and yield stress of samples were determined at all the frequencies. The yield stress was spe- cifi cally determined as the value of stress at which G′ deviated from the LVR. The over- shoot of G” was calculated from the difference of the value of G” before it deviated from the LVR and the local maximum G” before crossing over with G′. The cage modulus of was calculated from the slope of the stress–strain curve at zero stress, following the method used by Rogers et al. (31). CONSUMER ACCEPTANCE TESTS Consumer liking tests between the two samples were conducted using randomized double- blind tests in November 2014 when the weather is cold and dry. The use of human subjects for consumer acceptance and skin condition was approved by the research ethics boards of University of Guelph. Coded samples were presented to 60 panelists who were University of Guelph students and employees age between 18 and 50 years. Volunteers were informed and agreed to participate the study. Panelists were asked to apply the skin cream samples on the back of their left and right hands, and the side to apply each sample was randomized to eliminate contribution of hand dominance. Panelists were also required to score how much they liked the appearance, texture, and skin feel of each skin