Methodology for assessing the performance of urine absorbing aids in controlling malodour release
Politecnico di Milano, Department of Chemistrym, Materials and Chemical Engineering ‘Giulio Natta’, Milano, Italy
Correspondence: Selena Sironi, Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering ‘Giulio Natta’, Piazza Leonardo da Vinci 32, 20133 Milano, Italy. Tel.: +390223993292; fax: +390223993291; e-mail: firstname.lastname@example.org
The aim of this study was to investigate the possibility of comparing the performance of different absorbent aids in terms of odour control by discussing a suitable methodology for product evaluation. To overcome the problems of low test reproducibility owing to biological urine variability, the first step of the work consisted of the identification and the production of artificial urine having a constant and stable composition over time, moreover preventing sensorial assessors from any risk of biological contamination. Sensorial measurements were performed to optimize the similarity between artificial and biological urine, especially as far as the composition of the volatile component and therefore of the odour properties are concerned. The assessment of absorbent articles performance to control urine malodour includes both the concentration and the hedonic tone of the odour released by the article itself loaded with synthetic urine. Analyses were run on different products, which can be grouped into two different classes: absorbing aids with or without odour control technology (OCT) respectively. Results show that, despite of the presence or absence of OCT on absorbing products, their odour concentrations are almost identical, being comprised between 10 000 and 12 000 ouE m−3. For this reason, it is evident that odour concentration is not suitable as the sole parameter for comparison of different absorbing products. Instead, the hedonic odour tone (odour pleasantness/unpleasantness) relevant to the different product typologies (that is products with and without OCT) should be used as an additional discriminating factor for this kind of comparative tests.
Le but de cette étude est d'étudier la possibilité de comparer la performance des différentes aides absorbants en termes de contrôle des odeurs, en décrivant une méthodologie appropriée pour l'évaluation des produits. Afin de surmonter les problèmes de la faible reproductibilité du test en raison de la variabilité biologique des urines, la première étape du travail a consisté en l'identification et la production d'urine artificielle, ayant une composition constante et stable dans le temps, et par ailleurs permettant à prévenir les évaluateurs sensoriels de tout risque de contamination biologique. Des mesures sensorielles ont été réalisées afin d'optimiser la similitude entre l'urine artificielle et biologique, en particulier en ce qui concerne la composition du composant volatil et donc des propriétés odorantes. L'évaluation de la performance des articles absorbant pour contrôler les mauvaises odeurs d'urine comprend à la fois la concentration et la tonalité hédonique de l'odeur dégagée par l'article lui-même chargé d'urine synthétique. Les analyses ont été effectuées sur des produits différents, qui peuvent être regroupés en deux catégories différentes: des absorbants avec ou sans la technologie de contrôle des odeurs (OCT), respectivement. Les résultats montrent qu'en dépit de la présence ou de l'absence de la OCT sur les produits absorbants, les concentrations d'odeurs sont presque identiques, étant comprises entre 10 000 et 12 000 ouE m−3.
Pour cette raison, il est évident que la concentration d'odeur ne convient pas comme seul paramètre pour comparer des différents produits absorbants. Au lieu de cela, le ton hédonique des odeurs (odeur agréable/desagréable?) pertinents aux différentes typologies de produits (i.e. produits 'avec et sans OCT) devrait être utilisé comme un facteur supplémentaire de discrimination pour ce genre de tests comparatifs.
As the adult ages, the risk of urinary incontinence increases with significant impact on the physical and emotional health of the patient. Risk factors include advancing age, female gender and it is thus an important clinical and social problem in the elderly today, expected to grow further. There are varying reports of the female population suffering from incontinence. In a survey conducted on 2400 people, a prevalence of urinary incontinence of 48.4% is reported . Malodour reduction is one of the most important factors as the perception of urine malodour from the worn absorbent product by users or others has a significant impact on self-esteem and social interaction and might represent an additional psychological burden to the incontinence itself. Odour control is one of the top unmet needs of the Adult Care Category; 94% of people suffering from incontinence are looking for a specific product that controls the malodour. To confirm the importance of the benefit, Odour control remains the 1° top claim of the new launches observed in 2011 equal to the 44% [2-8]. Absorbent articles are being developed to help managing urine loss and also decreasing the social impact. Many types of urine absorbent products are available on the market. They can have different shape, material, technology or design. The International Standard ISO 15 621:2011  reports how to evaluate different urine absorbing products. This International Standard supplies general guidelines on evaluation of urine-absorbing aids, lists the most important factors for users and caregivers of absorbent incontinent products, gives guidance for how these factors can be evaluated, and finally gives an overview of testing methodologies and interpretation of test results.
International Standard ISO 15 621:2011 specifies that a possible way of evaluating malodour reduction is to use sensory analysis methods, as described in the International Standard ISO 6658:1985 . This Standard is a general introduction to the methodology of sensory analysis for odour evaluation. However, it does not specify the parameters to be assessed for odour characterization and neither it refers to specific standards relevant to odour assessment. This study describes an approach to define the parameters to be considered to evaluate the odour released from different urine absorbing products using an objective sensorial evaluation executed by expert sniffers.
Sensorial evaluations can be of different types and can be classified into:
The assessment of absorbent articles performance to control urine malodour includes both the concentration and the hedonic tone of the odour released by the article itself loaded with synthetic urine. Intensity measurements were not considered, because odour intensity is proved to be correlated to the concentration by different relations, such as the Weber Fechner law or the Steven law .
As mentioned above, International Standards (ISO 15 621:2011 and ISO 6658:1985) do not include specific procedures to perform odour assessment nor indications about the panel selection. As panel selection is a crucial aspect of sensorial analysis, it was executed according to the requirements of another standard for the regulation of sensorial analyses, i.e. the European Norm EN 13 725 [11, 15]. This European Standard refers to the measurement of odour concentration of pure substances, defined mixtures and undefined mixtures of gaseous odorants in air or nitrogen, using dynamic olfactometry with a panel of human assessors being the sensor. The unit of measurement is the European odour unit per cubic metre: ouE m−3. The odour concentration is measured by determining the dilution factor required to reach the detection threshold. The odour concentration at the detection threshold is by definition one ouE m−3. The odour concentration is then expressed in terms of multiples of the detection threshold.
Hedonic tone measurements were carried out according to the German Guideline VDI 3882- part 2 , which specifies the methodology to be used to evaluate the odour pleasantness or unpleasantness.
The data presented in this study are averaged from a set of analyses run on a large number of products, which can be grouped into two different classes: absorbent aids with or without odour control system respectively.
The proposed methodology may effectively be applied for the comparison of single commercial products both for pair or multiple evaluations.
Materials and methods
Odour concentration measurement:Dynamic olfactometry allows to determine the odour concentration (cod) of an odorous air sample, which is expressed in European odour units per cubic metre (ouE m−3), and represents the number of dilutions with neutral air that are necessary to bring the odorous sample to its odour detection threshold concentration.
The olfactometric method is based on the identification of the so-called ‘odour detection threshold’, i.e. of the threshold at which an odour is perceived by 50% of the assessors (panel).
To bring a sample to this threshold, a suitable dilution device must be used, which allows to dilute the gas sample according to given ratios with neutral air, i.e. with air made odour- and humidity-free by filtration through active carbon and silica gel. This dilution device is called ‘olfactometer’.
Odour concentration measurements are carried out in a specific room, called ‘olfactometric chamber’, at the Department of Chemistry, Materials and Chemical Engineering ‘Giulio Natta’ at the Politecnico di Milano. The olfactometric chamber is specifically equipped and meets the requirements of the EN 13 725:2003. The atmosphere inside the olfactometric chamber is controlled to minimize the risk that undesired external factors may affect the measurement. Odours coming from the equipment and other materials are eliminated and the room is ventilatedr to keep the environment odour-free. The ventilation air is filtered through active carbon before introduction into the olfactometric chamber to be deodorized. A minimum of 20 air exchanges/hour are guaranteed inside the olfactometric chamber. An olfactometer Ecoma Mod. TO8 equipped with four odour sniffing ports controlled by a PC was used as a dilution device (Fig. 1).
The measurement sessions start with a high dilution rate, as to keep the first sample presentations below the odour threshold concentration. The assessors smell the gas from a sniffing port and at each presentation, they shall indicate if they perceive an odour (‘yes/no’ method). After each round, the concentration of the sample is increased by reducing the dilution ratio. A button shall be pressed as soon as the sample odour is detected. The assessors are not asked to recognize or to describe the perceived odour, but just to indicate the moment at which they perceive a difference from neutral air. In some cases (at least 20%), reference air is presented instead of the sample (blanks), as to keep the assessors' concentration high. The measurement finishes when all the panel members have answered ‘yes’ correctly for at least two consecutive times.
Hedonic tone measurement: The hedonic tone (H.T.) describes the pleasantness/unpleasantness of an odour sample on a 9-point scale, ranging from −4 (extremely unpleasant) to +4 (extremely pleasant) .
Hedonic tone measurements are performed using the same dilution apparatus used for the determination of the odour concentration, i.e. the olfactometer.
The range of concentrations that has to be presented to the panel members comprises six dilution steps, which differ by a factor of two, the lowest dilution step corresponding to the sample odour threshold concentration. For this reason, it is mandatory to know a sample odour concentration before evaluating its hedonic tone.
In the case of low odorant concentrations, a smaller number of dilution steps may be presented. If necessary, the test may be confined to the undiluted test gas. The presentation of odour stimuli is done at random. Adaptation effects must, moreover, be minimized by ensuring that an above-threshold odour stimulus is not presented for longer than 15 s, with an allowance of an additional decision time of 5 s. The minimum break between two different stimuli should be at least 1 m. To check possible contamination of the olfactometer or to minimize possible guessing tendencies on the part of the panel members, the presentations are interspersed with blank samples (neutral air samples).
A series of measurements must not commence either with a blank sample or with a maximum stimulus.
For every sample presentation at the different dilution steps, the panel members shall evaluate the hedonic tone of the perceived concentration in accordance with the above-mentioned 9-point scale (Fig. 2).
Panel selection: For odour concentration measurements, the panel members are selected by means of specific sensitivity tests, in conformity with the criteria of the EN 13 725 . According to the norm, the panel sensitivity must be tested towards a reference odorant (n-butanol) as to choose a panel representative of the population average sensitivity. The following requirements must be satisfied: the sensitivity towards n-butanol must be included within a given range; the standard deviation with respect to the reference value must not exceed a given value and no errors on ‘blanks’ shall be committed. According to the EN 13 725, assessors having an odour threshold to n-butanol between 20 ppb and 80 ppb can be accepted as examiners. At our Laboratory, as a higher quality criterion, a narrower range is adopted, thus selecting as panel members, only people having an odour threshold to n-butanol comprised between 30 ppb and 60 ppb.
For hedonic tone measurements, the panel member's sensitivity (level of individual odour threshold) should not be a criterion for panel selection, as it is the case for the determination of odour concentration. The procedure for panel selection described in the German VDI, which was adopted by our Laboratory, provides to test the assessors using specific reference odorants.
The odorants employed for panel selection are vanillin (1 g to 200 ml dipropylene glycol) and guaiacol (1 μl to 200 ml distilled water), representing a pleasant and an unpleasant odour respectively. More in detail, panel screening is done by offering each panel member reference odorants in bottled and asking him or her to evaluate them according to the above-mentioned scale (Fig. 2), and the mean value of the results obtained from the entire panel must lie within a given range, i.e. between +1.9 and +2.9 for vanillin (pleasant) and −0.8 and −2.0 for guaiacol (unpleasant)) .
Measurement sessions: For the odour concentration evaluation, three consecutive measures for each sample have to be performed, with a minimum of four panellists for each measure, as a minimum of 12 individual detection thresholds are necessary to obtain the odour concentration of each analysed sample . The geometric mean of these 12 individual detection thresholds is calculated at the end of each test session. If one of the values obtained by one panellist is too far from the geometrical mean (according to EN 13 725, the ratio between an individual threshold estimate and the geometrical mean of all individual threshold estimates in a measurement should be between +5 and −5), the data from that panellist are excluded, and the geometric mean is recalculated using only the remaining measures .
For hedonic tone evaluation, the panel has to consist of at least 15 persons on account of the interindividual differences which may occur . The hedonic odour tone is expressed as the arithmetic mean of the hedonic odour tone values evaluated by each of the 15 panel members at the six different concentrations, thus representing the average of 90 assessments. This procedure is not reported in the above-mentioned German guideline, which limits the presentation of the hedonic tone measurements to a graphical representation of the panel responses with a statistical evaluation of their dispersion. We considered this procedure not to be suitable for comparing the odour pleasantness/unpleasantness released from different products, whereby the use of an averaged value, supported by suitable statistical analysis, is more significant.
All statistical analyses were conduced applying the Z-test method, considering a confidence interval of 95%. In general, the Z-test compares two groups of values (A and B) to determine if there is a significant difference between them . The Null Hypothesis considered is that the difference between A and B is not significant, whereas the Alternative Hypothesis is that the two mean values are significantly different. Using data of each group of values, the variances are calculated. Successively, relevant ‘z’ value is calculated. The obtained ‘z’ value is compared with the critical ‘z’ value relevant to the confidence interval chosen (95%) to establish if the Null Hypothesis has to be rejected or not. If the Null Hypothesis has to be rejected, the Alternative Hypothesis is proved and the two mean values (A and B) are significantly different. Moreover, the uncertainty related with the averaged data was calculated considering a confidence interval of 95%. Calculated uncertainties are reported on graphs as error bars.
Biological urine is proved to have extremely variable composition [17-21]. Urine composition may be affected by several different factors, such as the studied subject (sex, age), its diet and health conditions (e.g. infections or other pathologies), and the hour of the day (typically, the first morning urine is the most concentrated, then concentration decreases during the day).
Such variability is in contrast with the requirement of reproducibility of the tests for the sensorial evaluation of the products to be tested. This requirement can be fulfilled by the use of a reproducible synthetic urine-like fluid having a constant and stable composition over time. For this reason, the first step of the work consisted of the identification and the production of artificial urine, which, besides being stable, shall also be similar to real urine from the point of view of olfactory perception.
The identification of a synthetic urine-like fluid to be used for the tests is also necessary to prevent the sensorial assessors (panel) from any risk of biological contamination.
The preparation of artificial urine was based on a wide bibliographical research focused on the scientific studies describing the volatile and odorous substances contained in biological urine [22-24]. The recipe of artificial urine was thus identified as a mixture of volatile compounds to be added to a solution of water and salts.
Moreover, microbiological activity occurring in the biological fluid was simulated by addition of an enzyme (urease) to the mixture.
The enzyme is added to the liquid solution just before the pad loading, so that the enzymatic reaction starts as soon as the products are filled with the artificial urine.
Sensorial measurements were performed to optimize the similarity between artificial and biological urine, especially as far as the odour properties are concerned.
The dosage of the single substances is therefore a crucial aspect associated with the preparation of artificial urine for sensorial odour testing. The artificial urine used for this study was prepared as to be comparable, from the point of view of the emitted odour (quality and quantity), to biological urine after 24 h ageing. This time interval was chosen as it may reasonably represent the worst-case scenario for the final disposal of an absorbent pad.
Tests for the comparison of synthetic urine vs. biological urine
The comparison of synthetic vs. biological urine was based on sensorial measurements of both odour concentration and hedonic tone.
Biological urine was collected from 20 healthy individuals, aged in a range between 50 and 90 years, over a 24-h period. Samples were combined from each person over a 24-h collection and analysed individually. This procedure aimed to obtain a reasonable representation of inter- and intra-individuals variability of urine composition.
The samples to be analysed were prepared by storing a known quantity of fluid into special sampling bags at a fixed temperature. In more detail, 200 ml of fluid was used for each sample, and stored at a temperature of 37°C, this being the body temperature. The sampling bags are in Nalophan™, with a Teflon inlet tube and a volume of about 6 litres (Fig. 3).
The olfactometric evaluations were run on the gaseous phase above the liquid (headspace).
The analyses for the determination of both the odour concentration and the hedonic tone were repeated at different time intervals after sample preparation, to account for the variability of urine over time. The first test was performed at 15 min after sample preparation, then at 6 h and finally at 24 h.
These time intervals were chosen as to account for three different moments of the use of urine absorbing products: urination (time ‘0’, 15 min was left as to allow the realization of equilibrium conditions between liquid and gas phase), change (6 h wearing) and final disposal (24 h).
Tests for the comparison of different urine absorbing products
As previously explained, the sensorial tests for the comparison of different urine are performed using synthetic urine.
The absorbent articles are prepared flat on the bench and loaded with an amount of synthetic urine that is representative of the average loading of urine that can be present in urine absorbent products of a specified size. For instance, for products used for light incontinence, usual loading is below 30 mL of urine, for products specific for severe incontinence, it is common that the users load for each product is around 300 mL.
The chosen amount of synthetic urine is loaded in the centre of each product sample. Each sample is then positioned into a sampling bag in Nalophan™ and in contact with 6 L of neutral air. Also in this case, it is important to fix suitable storage conditions, i.e. time and temperature, to simulate the conditions of use of the products to be tested.
For this reason, a storage temperature representative of the body temperature (37°C) was chosen, and a storage time in a range between 4 and 8 h, which is estimated to be the average usage time of the products.
After storage, the bags are wrapped into additional black bags to prevent prejudice against the products and proceed to the olfactometric analyses for the evaluation of both the odour concentration and the hedonic tone [11, 13].
Also in this case, the tests were repeated on three different samples prepared in triplicate and stored for three different times: 15 min, 6 h and 24 h after sample preparation, for the same reasons discussed above.
Results and discussion
Comparison of synthetic urine vs. biological urine
As previously mentioned, both odour concentration measurements and hedonic tone measurements were performed for the comparison of the samples of biological and synthetic urine respectively.
Figure 4 shows the results of the odour concentration determinations performed in accordance with the European Standard EN 13 725.
The numbers in abscissa indicate the time after sample preparation at which the sensorial analysis was run, whereby ‘0’ stands for the first test, which was run 15 min after sample preparation, as to leave a minimum time to allow the realization of equilibrium conditions between the liquid and the gas phase at the chosen temperature (37°C). The ordinate reports the odour concentration values in ouE m−3.
The results shown in Figure 4 allow formulating some interesting considerations. The odour concentration of synthetic urine is almost constant over time, with values ranging from about 40 000 ouE m−3 to 60 000 ouE m−3. In contrast, the odour concentration of biological urine increases with time, from an initial value of about 10 000 ouE m−3 to a final value of about 40 000 ouE m−3, this increase being attributable to the microbiological ageing processes occurring in biological urine.
Synthetic and biological urine are proven to have comparable odour concentrations at 24 h after sample preparation. Synthetic urine is representative of 24-h aged biological urine, thus representing the worst-case scenario of an absorbent pad at the moment of its final disposal, which may occur up to 24 h after wearing (especially as far as the use in hospitals or rest homes is considered).
One important advantage of synthetic urine is that the fact of using a fluid with a stable odour concentration over time makes olfactometric tests more reproducible.
Figure 5 reports the results of the hedonic odour tone measurements run, in analogy with the odour concentration measurements, at 15 min, 6 h and 24 h after sample preparation respectively. In this case, the ordinate reports the hedonic tone (H.T.) values in a scale from −4 (extremely unpleasant) to +4 (extremely pleasant), as indicated by the German VDI .
As previously explained, each reported hedonic odour tone value represents the arithmetic mean of the single values evaluated by the 15 panel members at six different concentrations, thus representing the average of 90 assessments.
The results obtained show synthetic and biological urine to have very similar hedonic odour tones, except for the test run shortly after sample preparation (15 min). As previously discussed for the odour concentration, for olfactometric evaluations on urine absorbing aids, it is important for the synthetic urine to be representative of the biological urine aged 24 h, which is the time interval for the pad final disposal.
The combination of odour concentration and hedonic odour tone measurements thus proved the identified synthetic urine to be suitable for the setup of olfactometric evaluations for the comparison of different pads.
Olfactometric tests on urine absorbing aids
Odour concentration measurements: Odour concentration analyses were run on different products, which can be grouped into two different classes: absorbing aids with or without odour control technology (OCT) respectively.
Figure 6 shows the results of the odour concentration measurements on the tested product typologies at the different time intervals after sample preparation.
Some considerations can be extrapolated from these results. Even though for several other applications, the comparison of odour concentrations is the best way for the comparative evaluation of odour control systems, this is not sufficient in this case. The results show that, despite of the presence or absence of OCT on absorbing products, their odour concentrations do not present significant variations, thus showing almost identical values in all the different tests (10 000–12 000 ouE m−3). It can be observed that the odour concentration values measured in the samples containing the absorbing pads loaded with urine are much lower than the odour concentrations of urine alone (10 000 ouE m−3 vs. 40 000 ouE m−3), thus indicating that the presence of the absorbing pad already produces a reduction of the emitted odour.
Still, given the similarity of the odour concentrations relevant to the different product typologies, it is evident that the sole odour concentration is not a suitable parameter for the comparison of different absorbing products.
However, even though the odour concentration is not directly affected by the OCT, a modification of the odour quality in the presence of the OCT may be noticed. For this reason, hedonic odour tone measurements seem to be more appropriate for product comparison.
Hedonic tone measurements: Figure 7 shows the results of the hedonic odour tone measurements on the tested product typologies at the different times.
Unlike for odour concentration, the hedonic tone values relevant to the different product typologies (with and without OCT) are significantly different.
Hedonic tone values relevant to products without OCT range from −1.9 to −1.3, thus indicating the presence of an unpleasant odour (malodour) over the whole-product life cycle. These values are still higher than those of urine alone (below −2), indicating that also odour pleasantness is slightly improved by the sole presence of the absorbing material.
Instead, hedonic tone values of products with OCT are much higher, i.e. close to neutrality (zero value representing a neither pleasant nor unpleasant odour), ranging from −1.2 at 15 min from sample preparation to +0.2 at 24 h. The increase in the hedonic tone values is owing to the time required by the OCT to exploit its odour control action.
These results show the hedonic tone measurement suitability for discriminating the effectiveness of the different products tested in reducing urine malodour.
This paper proposes an effective test methodology to evaluate the efficacy of different absorbent products in reducing urine malodour.
To make sensorial tests reproducible and free from any biological risk for expert sniffers, preliminary studies were conducted to identify an artificial urine-like fluid with odour properties (odour concentration and hedonic tone) similar to those of biological urine.
The proposed method provides the evaluation of both odour concentration and hedonic tone to discriminate between different absorbing products. The similarity of the odour concentrations relevant to the different product typologies (with and without odour control system) proves the sole odour concentration not to be a suitable parameter for the comparison of different absorbing products.
On the contrary, hedonic odour tone (odour pleasantness/unpleasantness) turned out to be more suitable for this purpose, allowing to effectively discriminate different absorbing products.
The study was carried out at and founded by the Olfactometric Laboratory of Department of Chemistry, Materials and Chemical Engineering ‘Giulio Natta’, Politecnico di Milano, Italy.