Corroborating the 2-AFC and 2-AC Thurstonian models using both a model system and sparkling water

Two experiments were conducted to investigate the suitability of the Thurstonian models for the 2-AFC (2-alternative forced choice) and 2-AC protocols (2-AFC test with a “no difference” option). The Thurstonian approach predicts that if the same subjects and products are used, the degree of difference measured between the products (d^′) should be the same. Experiment I tested this prediction using solutions with different salt concentration. The average d^′ values for the 2-AFC and 2-AC were found to be 1.66 and 1.54, respectively, and were not significantly different (t-test, p=0.46). This indicated the suitability of the models for a taste stimulus. Experiment II used a trigeminal stimulus: sparkling water with different CO₂ levels as a food system. Three levels of carbonation were used and all three possible pairs of samples were compared. The d^′ values measured for the 2-AFC and 2-AC for each pair of products were: pair 1––1.7 and 1.7; pair 2––0.8 and 0.7; pair 3––1.2 and 1.0. For each pair of products, no significant difference was observed between the d^′ of each protocol (t-tests, p=0.65, 0.20, 0.30, respectively), giving further support to the Thurstonian models of these paradigms and indicating their robustness.     

A Thurstonian model for the degree of difference protocol

In the degree of difference methodology (DOD), subjects are presented with pairs of samples, either identical or different, and must indicate how different the samples are using a t-point category rating scale. In this article, the Thurstonian model for the DOD assuming independent assessments is derived. The model permits the estimation of the size of the underlying difference between the products (δ), the variance of this estimate, as well as the sizes of the t − 1 τ criteria (scale boundaries on the difference perceptual distribution). This model expands further the collection of Thurstonian models already available for many discrimination, rating and ranking methodologies.     

Statistical and Thurstonian models for the A-not A protocol with and without sureness

The A-not A protocol with sureness produce multinomial observations that are traditionally analyzed with statistical methods for contingency tables or by calculation of an R-index. In this paper it is shown that the Thurstonian model for the A-not A protocol can be written as a cumulative link model including the binormal unequal variances model. The model is extended to allow for explanatory variables and we illustrate how consumer differences can be modeled within the Thurstonian framework on a consumer study of packet soup conducted by Unilever. The extension also allows several test-product variations to be analyzed in the same model providing additional insight and reduced experimental costs. The effects of explanatory variables on the Thurstonian delta, the sensitivity (AUC), the ROC curve and the response category thresholds are discussed in detail. All statistical methods are implemented in the free R-package ordinal (http://www.cran.r-project.org/package=ordinal/).     

Accounting for no difference/preference responses or ties in choice experiments

The analysis of choice data in which no difference/preference responses, or ties, occur is considered in this paper. A key issue addressed in the paper is the need for “identicality norms” for difference and preference tests. These norms reflect the researcher’s expectation for the number of ties that would have occurred in the experiment had the products tested been putatively identical. Without these norms, the issue of how to account for ties can never be fully resolved. After this idea is developed, some methods from the statistics literature to account for ties are reviewed and the Thurstonian 2-AC (2-Alternative Choice) model is discussed. Common practices of equal or proportional redistribution of ties are noted to be either conservative or liberal, respectively, when the binomial distribution is used to evaluate results. In particular, the exact probability function for the equal allocation method is given as a particular type of mixing distribution, known as a convolution, of binomial probability functions. Regardless of which statistical method is used to test tied data, however, none of the current methods of analysis can account for segmentation or the effect of heterogeneity in individual assessors. To study the possible effect of heterogeneity, the data could first be tested against an identicality norm. Thus, this research clarifies the assumptions that are made when conducting tests on paired comparison data with ties and provides guidance on the choice of analytic method.     

Delta-tilde interpretation of standard linear mixed model results

We utilize the close link between Cohen’s d, the effect size in an ANOVA framework, and the Thurstonian (Signal detection) d-prime to suggest better visualizations and interpretations of standard sensory and consumer data mixed model ANOVA results. The basic and straightforward idea is to interpret effects relative to the residual error and to choose the proper effect size measure. For multi-attribute bar plots of F-statistics this amounts, in balanced settings, to a simple transformation of the bar heights to get them transformed into depicting what can be seen as approximately the average pairwise d-primes between products. For extensions of such multi-attribute bar plots into more complex models, similar transformations are suggested and become more important as the transformation depends on the number of observations within factor levels, and hence makes bar heights better comparable for factors with differences in number of levels. For mixed models, where in general the relevant error terms for the fixed effects are not the pure residual error, it is suggested to base the d-prime-like interpretation on the residual error. The methods are illustrated on a multifactorial sensory profile data set and compared to actual d-prime calculations based on Thurstonian regression modeling through the ordinal package. For more challenging cases we offer a generic “plug-in” implementation of a version of the method as part of the R-package SensMixed. We discuss and clarify the bias mechanisms inherently challenging effect size measure estimates in ANOVA settings.     

Rheological characteristics of healthy sugar substituted spreadable strawberry product

Full replacement of sucrose with healthier sugars such as fructose and/or isomaltulose is possible in spreadable strawberry products. These products formulated with different sugars (sucrose, isomaltulose, sucrose–glucose, and fructose–isomaltulose) were rheologically analysed. Static tests characterized them as Herschel–Bulkley fluids. The values of the consistency index (k) and yield stress (τ₀ were influenced by the type of sugar, the elaboration method and the pectin levels; while the fluidity index (n) was not affected by the different sugars, but by the elaboration method and the levels of pectin. The dynamic tests permitted classification of some of the products as weak gels. The strength of the network “A” increased with the pectin level, while the “coordination number” “z” did not show a clear trend depending on the different process variables.     

Development of an improved two-alternative choice (2AC) sensory test protocol based on the application of the asymmetric dominance effect

The goal of this study was to demonstrate that participants in a 2AC test can be induced to select the no-difference alternative by modifying the design of test ballots. Four different protocols (2AFC, 2AC, and two modified versions of 2AC) were evaluated; the two modified versions consisted of one where the judges were told that samples were the same and another where an “I do not know” alternative was included. Aqueous sucrose solutions were employed to evaluate the discriminative efficiency of the tests. Both 2AC modified versions effectively increased (p ? 0.05) the proportion of judges choosing the “no-difference” option. The discriminative efficiency of the “I do not know” 2AC version was about three times higher than that of the other three protocols. This study demonstrated that it is possible to modify the proportion of judges choosing the no-difference option through ballot design, and that free-choice protocols may show much higher discriminative power than forced-choice tests when the no-difference alternative is properly used.     

Initial evaluation of a field-friendly incubation procedure for the colorimetric assay of cassava linamarase

The Cooke colorimetric assay of cassava linamarase activity is temperature- and time-inflexible, making “real time” monitoring of linamarase activity in remote cassava-processing sites practically impossible. A modified incubation procedure is described, in which the 30 °C linamarase incubation step is terminated through acidification, yielding a stable cyanohydrin solution. Using partially purified linamarase as a “standard extract”, the solution – held for up to 21 days at ambient/refrigeration temperatures before colorimetry – showed reductions of up to 21% compared with the standard Cooke assay. In a separate trial, a strong linear relationship (r² > 0.95) was observed between recorded linamarase activity values and incubation temperature in the 25–40 °C range, indicating that incubation may take place in remote processing sites without a water bath, and resulting data may reliably be adjusted in keeping with the standard 30 °C incubation assay. The novel procedure thus appears to offer a satisfactory “field-friendly” means of assaying linamarase activity.     

Proton NMR transverse relaxation measurements to study water dynamic states and age-related changes in Mozzarella di Bufala Campana cheese

Proton transverse magnetization decay curves of Mozzarella di Bufala Campana samples were measured and analysed for the presence in the sample of four components, namely “serum water”, “entrapped water”, “junction zone water” and “fat”, characterised by different transverse relaxation times, T₂. T₂ values were interpreted on the basis of the diffusive and chemical exchange model that allowed information to be obtained on the size of diffusive domains and dynamics of the water molecules. Furthermore, T₂ values were measured as a function of the aging time of Mozzarella samples, as produced by different cheese-makers. A decrease of “serum water” T₂ value with aging time was found and this may be used to monitor the evolution of Mozzarella samples up to seven days after manufacture.     

Kinetic modelling: A tool to predict the formation of acrylamide in potato crisps

Three empirical models were used to fit the formation of acrylamide in crisps of three different cold-sweetened potato genotypes, fried under the same experimental conditions. Statistical methods were used to compare the performance of the models, with the “Logistic-Exponential” model performing the best. The obtained model parameters for the formation of acrylamide showed improvement in precision compared to an earlier study, the precision of the parameter estimates for the degradation of acrylamide was still problematic. Nevertheless, the predictive capacity of the “Logistic-Exponential” model was tested, as this model showed a strong correlation between parameter a and the reducing sugar content of the raw potato. The predictions from this model for the formation of acrylamide in potato crisps were close to earlier reported experimental values. Therefore, the use of the “Logistic-Exponential” model as a tool to predict acrylamide in potato crisps seems promising and should be developed further.     

Use of Carnobacterium maltaromaticum cultures and hydroalcoholic extract of Lippia sidoides Cham. against Listeria monocytogenes in fish model systems

Minimally processed refrigerated ready-to-eat fishes may offer health risk of severe infection to susceptible individuals due to contamination by the psychrotolerant bacterium L. monocytogenes. In this work, inhibition of L. monocytogenes by a plant extract and lactic acid bacteria (LAB) was studied in model fish systems kept at 5 °C for 35 days. For that, fillets of tropical fish “surubim” (Pseudoplatystoma sp.) and hydroalcoholic extract of the plant Lippia sidoides Cham. (“alecrim pimenta”) were used. Fish peptone broth (FPB), “surubim” broth and “surubim” homogenate were inoculated with combinations of L. monocytogenes and bacteriocin-producing Carnobacterium maltaromaticum (C2 and A9b⁺) and non bacteriocin-producing C. maltaromaticum (A9b^-), in the presence or absence of extract of “alecrim pimenta” (EAP). In all model systems, monocultures of L. monocytogenes and carnobacteria reached final populations ≥ 10⁸ CFU/ml after 35 days, except for L. monocytogenes in “surubim” homogenate (10⁴ CFU/ml). In FPB, EAP alone and combined with cultures of LAB inhibited L. monocytogenes but carnobacteria without EAP were only weakly antilisterial. In “surubim” broth, EAP alone did not prevent L. monocytogenes growth but cultures of carnobacteria combined or not with EAP inhibited L. monocytogenes, with more pronounced effect being observed for C. maltaromaticum C2, which produced bacteriocin. In “surubim” homogenate, EAP alone and combined with cultures of C. maltaromaticum A9b⁻ and A9b⁺ were strongly inhibitory to L. monocytogenes, while C. maltaromaticum C2 with EAP caused transient inhibition of L. monocytogenes. No significant inhibition of L. monocytogenes was observed for carnobacteria in “surubim” homogenate without EAP. In conclusion, it was observed that the use of EAP and cultures of carnobacteria have potential to inhibit L. monocytogenes in fish systems and the applications should be carefully studied, considering the influence of food matrix.     

Estimation and inference in the same–different test

Inference for the Thurstonian δ in the same–different protocol via the well known Wald statistic is shown to be inappropriate in a wide range of situations. We introduce the likelihood root statistic as an alternative to the Wald statistic to produce CIs and p-values for assessing difference as well as similarity. We show that the likelihood root statistic is equivalent to the well known G² likelihood ratio statistic for tests of no difference. As an additional practical tool, we introduce the profile likelihood curve to provide a convenient graphical summary of the information in the data about δ.On the basis of simulations, we show that the coverage probability of the Wald-based 95% CI for δ is often far from 95%, whereas the coverage probability of the profile likelihood-based CI is close to the desired 95%. We also show how the likelihood framework can be used to combine information from independent experiments possibly using different discrimination protocols to obtain inference for a common δ. Finally, we provide a free R package with an implementation of the likelihood methodology presented in this paper.     

Viscoelastic properties of high pressure and heat induced tofu gels

Tofu gels were rheologically examined to determine their storage or elastic (G′) and loss or viscous (G″) moduli as a function of frequency within their linear viscoelastic limits. The tofu gels were made using either glucono-δ-lactone (GDL) or calcium sulphate (CaSO₄·2H₂O), followed by either heat treatment (heated soymilk at ?97 °C prior to coagulation and subsequently held at 70 °C for 60 min, HT) or high pressure treatment (400 MPa at 20 °C for 10 min, HP). The overall moduli values of the GDL gels and CaSO₄·2H₂O gels of both physical treatments were similar, each gave frequency profiles expected for weak viscoelastic materials. However, although both temperature and high pressure treatments could be used to produce tofu gels, the final products were not the same. Pressure formed gels, despite having a higher overall “consistency” (increasing values of their moduli), had a proportionately higher contribution from the loss modulus (increased tan δ). Differences could also be observed using confocal scanning laser microscopy. While such treatment may give rise to differing systems/structures, with new or modified organoleptic properties, the more “open” structures obtained by pressure treatment may well cause processing difficulties if subsequent reworking or moulding is required.     

A “successful allele” at Campylobacter jejuni contingency locus Cj0170 regulates motility; “successful alleles” at locus Cj0045 are strongly associated with mouse colonization

Campylobacter jejuni is an important foodborne pathogen of humans and its primary reservoir is the gastrointestinal (GI) tract of chickens. Our previous studies demonstrated that phase variation to specific “successful alleles” at C. jejuni contingency loci Cj0045 (successful alleles carry 9G or 10G homopolymeric tracts) and Cj0170 (successful allele carries a 10G homopolymeric tract) in C. jejuni populations is strongly associated with colonization and enteritis in C57BL/6 IL-10 deficient mice. In the current study, we strengthened the association between locus Cj0170, Cj0045, and mouse colonization. We generated 8 independent strains derived from C. jejuni 11168 strain KanR4 that carried a Cj0170 gene disruption and these were all non motile. Two randomly chosen strains with the Cj0170 gene disruption (DM0170-2 and DM0170-6) were gavaged into mice. DM0170-2 and DM0170-6 failed to colonize mice while the control strain that carried a “successful” Cj0170 10G allele was motile and did colonize mice. In parallel studies, when we inoculated C. jejuni strain 33292 into mice, the “unsuccessful” Cj0045 11G allele experienced phase variation to “successful” 9G and 10G alleles in 2 independent experiments prior to d4 post inoculation in mice while the “successful” 9G allele in the control strain remained stable through d21 post inoculation or shifted to other successful alleles. These data confirm that locus Cj0170 regulates motility in C. jejuni strain KanR4 and is a virulence factor in the mouse model. The data also support a possible role of locus Cj0045 as a virulence factor in strain 33292 in infection of mice.     

Barcode High Resolution Melting (Bar-HRM) analysis for detection and quantification of PDO “Fava Santorinis” (Lathyrus clymenum) adulterants

Legumes considered as one of the most important crops worldwide. Due to high price as a PDO product, commercial products of “Fava Santorinis” are often subjected to adulterations from other legume products coming from other Lathyrus or Vicia and Pisum species. Using plant DNA barcoding regions (trnL and rpoC) coupled with High Resolution Melting (Bar-HRM) we have developed a method allowing us to detect and authenticate PDO “Fava Santorinis”. Bar-HRM proved to be a very sensitive tool able to genotype Lathyrus and its closed relative species and to detect admixtures, being sensitive enough to as low as 1:100 of non-“Fava Santorinis” in “Fava Santorinis” commercial products. In conclusion, Bar-HRM analysis can be a faster, with higher resolution and cost effectiveness alternative method to authenticate PDO “Fava Santorinis” and to quantitatively detect adulterations in “Fava Santorinis” with other relative commercial “Fava” food products.     

A comparison of regimen methods for the removal and inactivation of bacteria, fungi and Acanthamoeba from two types of silicone hydrogel lenses

Purpose

To compare the antimicrobial efficacy of commercial contact lens solutions when used according to the manufacturers’ recommended regimens with two types of silicone hydrogel lenses.

Methods

Four multipurpose contact lens care solutions were examined, representing manufacturer recommended regimens of “rub & rinse”, “no rub, rinse” or “no rub, no rinse”. Test organisms were Pseudomonas aeruginosa, Serratia marcescens, Staphylococcus aureus, Fusarium solani, Candida albicans and Acanthamoeba castellanii (trophozoites and cysts). Organisms, in the presence of organic soil, were inoculated on to Acuvue Oasys or Air Optix lenses and subjected to the solution manufacturer's recommended regimen. The number of surviving organisms on the lenses and in the soak solution was enumerated in accordance with ISO 14729.

Results

ISO 14729 dictates that for a given organism the combined average number of surviving microbes from the lenses and disinfectant soaking solution must be ≤10 colony forming units (cfu)/ml. By this criterion, only Complete Easy Rub (“rub & rinse” regimen) gave satisfactory results for all bacteria, fungi and Acanthamoeba with both lens types. Solutions employing “no rub, rinse” were less satisfactory but significantly better than “no rub, no rinse”. Significant differences were found in organism survival on the lenses with greater numbers remaining on the Air Optix compared to Oasys (p < 0.01-0.0001).

Conclusion

The findings of this study demonstrate that the use of a manual rubbing step is more effective than rinsing or soaking alone in removing pathogenic microbes from silicone hydrogel lenses. Accordingly, it would seem prudent to recommend that contact lens care systems include a rub step as part of the hygiene regimen.     

The interrelationship between sensory tenderness and shear force measured by the G2 Tenderometer and a Lloyd texture analyser fitted with a Warner–Bratzler head

A comparison of peak shear force results for a Lloyd texture analyser fitted with a Warner Bratzler type of shearing head and a G2 Tenderometer was undertaken using sheep meat. The G2 is a new version of the Tenderometer that uses an electric linear motor to compress the sample, but still retains the blunt wedge-shaped “tooth”. Analysis of sheep meat samples (n = 121) revealed that the average G2 Tenderometer shear force results were approximately 1.2 times those for the Lloyd based on the following model; Lloyd = 1.561 Tenderometer^0.84. Both instruments explained low amounts of the variation (less than 20%) in the sensory traits tenderness and overall liking. The high values for the sensory traits indicate that a wider range of samples, including samples with lower sensory scores, is required to develop robust threshold estimates so that either instrument could be use as an auditing instrument for the processing industry.     

Decision strategies for the A Not-A, 2AFC and 2AFC-reminder tasks: Empirical tests

Signal detection theory provides an approach to modelling sensory difference tests that separates estimates of discriminability from the effect of response bias. However, assuming an incorrect decision strategy can also lead to inaccurate estimates of sensitivity, the most common index of which is d′. Using signal detection theory, Hautus, van Hout, and Lee (2009) describe and develop a number of models for the two-alternative forced choice with reminder (2AFCR) task; a task whose trial structure is identical to that of the constant-reference duo-trio task. There are alternative decision strategies (e.g., β or τ), when judging stimuli presented in the 2AFCR, to the “comparison of distances” strategy assumed to be used in the duo-trio task in food science. We investigated the decision strategies adopted by judges in the 2AFCR task using recently developed detection-theoretic models tested on purposefully collected discrimination data. The results indicate that, in the experimental context employed, the optimal β and τ strategies are more likely to be adopted by judges in the 2AFCR task than the “comparison of distances” strategy. Findings have implications for sensory difference testing and the validity of the sensitivity indices they provide.     

Fatty acid compositions of selected varieties of Spanish dry ham related to their nutritional implications

Five varieties of Spanish dry cured ham were studied to assess their nutritional value in relation to fatty acids. Ten hams of the Traditional Speciality Guaranteed (TSG) “Jamón Serrano”, and the Protected Designations of Origin (PDO) “Jamón de Teruel”, “Dehesa de Extremadura”, “Jamón de Huelva” and “Guijuelo”, were analysed. Iberian hams (“Dehesa de Extremadura”, “Jamón de Huelva” and “Guijuelo”) were characterised by a lower proportion of saturated fatty acids (SFA) and a significantly higher percentage of monounsaturated fatty acids (MUFA) than white hams (“Jamón Serrano” and “Jamón de Teruel”). The Iberian varieties also showed a high proportion (approximately 50%) of C18:1 n − 9, while “Jamón Serrano” showed the highest percentage of C18:2n−6. The PUFA/SFA (P/S) ratio of the five varieties was ? 0.19, with the highest ratio corresponding to “Jamón Serrano” (0.3). The n−6/n−3 ratio was in the order of 13/1 in “Jamón Serrano” and “Jamón de Huelva”, and ranged from 9.3/1 to 10.3/1 in the other varieties. The most favourable hypocholesterolemic/Hypercholesterolemic (h/H) ratio (?2.5) was found in the Iberian varieties. TSG “Serrano” was shown to supply the lowest percentage of the recommended daily intake of MUFA, the Iberian varieties showed the highest percentage of the daily intake of long-chain PUFA, and PDO “Dehesa de Extremadura” showed the highest percentage of the intake of C18:3n−3. The higher MUFA proportion and h/H ratio observed in the Iberian hams, together with their contribution to the recommended daily intake of fatty acids, would make these products more suitable for healthier diets, although consumption must be recommended in moderation.