Crunchiness Loss and Moisture Toughening in Puffed Cereals and Snacks

Upon moisture uptake, dry cellular cereals and snacks loose their brittleness and become soggy. This familiar phenomenon is manifested in smoothing their compressive force–displacement curves. These curves’ degree of jaggedness, expressed by their apparent fractal dimension, can serve as an instrumental measure of the particles’ crunchiness. The relationship between the apparent fractal dimension and moisture content or water activity has a characteristic sigmoid shape. The relationship between the sensorily perceived crunchiness and moisture also has a sigmoid shape whose inflection point lies at about the same location. The transition between the brittle and soggy states, however, appears sharper in the apparent fractal dimension compared with moisture plot. Less familiar is the observation that at moderate levels of moisture content, while the particles’ crunchiness is being lost, their stiffness actually rises, a phenomenon that can be dubbed “moisture toughening.” We show this phenomenon in commercial Peanut Butter Crunch® (sweet starch‐based cereal), Cheese Balls (salty starch‐based snack), and Pork Rind also known as “Chicharon” (salty deep‐fried pork skin), 3 crunchy foods that have very different chemical composition. We also show that in the first 2 foods, moisture toughening was perceived sensorily as increased “hardness.” We have concluded that the partial plasticization, which caused the brittleness loss, also inhibited failure propagation, which allowed the solid matrix to sustain higher stresses. This can explain other published reports of the phenomenon in different foods and model systems.     

CRISPNESS AND CRUNCHINESS JUDGMENT OF APPLES BASED ON CONTACT ACOUSTIC EMISSION

Contact acoustic emission (AE) measurement, together with a puncture test, were used to evaluate the crispness and crunchiness of three apple cultivars. A storage program was used to induce changes in apple crispness and crunchiness. The instrumental measurements were compared with the results of sensory evaluation.
It was found that crispness and crunchiness could be evaluated instrumentally by measuring firmness, AE events, mean AE amplitude or the product of AE events and mean AE amplitude. For one cultivar, however, firmness was not correlated with any sensory attribute. Generally, the best correlation with sensory attributes was found for the exponential value of AE events. Contact AE allows the measurement of sensory crispness or crunchiness for a given apple cultivar or independently for any apple cultivar. Contact AE can be considered as a useful method for the evaluation of sound properties of wet-crisp foods.

PRACTICAL APPLICATIONS

Contact acoustic emission method can be used for instrumental evaluation of crispness and crunchiness of apples. The method allows more accurate evaluation of these attributes than firmness measurement. There is a chance to extend the application to other wet-crisp foods and to use the method together with other destructive tests, like texture profile analysis or single edge notched bend.     

Texture concepts for consumers: a better understanding of crispy–crunchy sensory perception

Differences in the definitions and terms used by consumers to describe their sensory sensations of crispness/crunchiness were studied, highlighting the relevance of using well-defined terms in consumer studies. A questionnaire was presented to consumers from two different Spanish-speaking countries (Spain and Uruguay, n = 200). The answers showed that the terms crispy and crunchy had different meanings or evoked different perceptions depending on the country. The crispy/crunchy food items most frequently mentioned by consumers were dry-crisp, processed products; vegetables or other wet-crisp products were not often mentioned by the consumers interviewed. The main difference between the two countries was that 38% of Spanish consumers did not know the Spanish term for “crunchy” and 17% thought that “crispy” and “crunchy” had the same meaning. The results contribute to a greater knowledge of the terminology used by the consumers to describe oral sensations related to crispness and crunchiness.     

基于穿刺法库尔勒香梨果肉脆度评价研究

本研究采用二阶微分法和峰值法对不同成熟度库尔勒香梨果肉的穿刺力-变形曲线锯齿度进行度量计算,结合不同成熟度香梨果肉组织显微组织观察,分析了锯齿度指数CI、平均穿刺力Fcr、结构破裂空间频率Nsr、结构破裂平均比力fsr和脆功Wc随香梨贮藏时间的变化规律,并分析了这5个锯齿度度量参数与香梨可溶性固形物含量及含水率的相关性,比较了对香梨果肉脆度变化的敏感度。研究结果表明,不同成熟度香梨果肉穿刺力-变形曲线锯齿度有明显差异,随着香梨脆度减小,锯齿度指数CI、平均穿刺力Fcr、结构破裂平均比力fsr、脆功Wc呈逐渐减小趋势,而结构破裂空间频率Nsr呈逐渐增大趋势,这5个锯齿度度量参数都与可溶固形物含量、含水率显著相关,但是峰值分析法的结构破裂平均比力fsr不仅与可溶性固形物含量及含水率的相关系数都很高,而且对香梨脆度敏感度值最高,最适宜作为评价香梨果肉脆度的指标。     

ESTIMATION OF THE MECHANICAL PROPERTIES OF INDIVIDUAL BRITTLE PARTICLES FROM THEIR BULK COMPRESSIBILITY1

Two puffed cereal particles were compressed both individually and in cells of different diameters and heights. The degree of jaggedness of their force-displacement curves were expressed in terms of their apparent fractal dimension determined from the Richardson plot and by the box counting method (Kolmogorov's dimension). Plots of the degree of jaggedness versus cell diameter and height produced approximately planar relationships in a three dimensional space. Their extrapolation to the individual particle characteristic dimensions gave jaggedness values in agreement with those determined experimentally on the individual particles. This demonstrates that it is possible, at least in certain products, to assess the “brittleness” of individual particles from their bulk compressibility pattern. The same procedure could also be used to estimate the particle “stiffness” expressed in terms of the force at a preselected displacement level (e.g., 15 or 25%).     

CHARACTERIZATION OF THE POWER SPECTRUM OF FORCE-DEFORMATION RELATIONSHIPS OF CRUNCHY FOODS1

The mean magnitude of the power spectra of the compressive force-deformation relationships of breadsticks, pretzels and zwiebacks, exposed to five levels of relative humidity, was determined at various cutoff frequencies. In all three foods the mean magnitude had a linear correlation with the Fractal dimension of the normalized original force-deformation relationship, regardless of the cutoff frequency. This implies that the jaggedness of the force-deformation curve, which is most probably a manifestation of the material crunchiness or crumbliness, can be quantified by an additional single numerical index, the mean magnitude of the power spectrum. The correlation's independence of the cutoff frequency is an indication that the loss of crunchiness, as a result of moisture sorption, can be expressed in the shape of the whole spectrum and not only in its high frequency region.     

Impact of Almond Form and Moisture Content on Texture Attributes and Acceptability

The objectives of this study were to measure sensory texture attributes of 5 types of almonds (blanched slivered, natural sliced, whole blanched, whole dry roasted, and natural whole) conditioned at 4 different moisture levels, to measure liking of a subset of these products, and to compare the sensory texture measurements with consumer liking ratings. Thirteen panelists trained to evaluate almond texture rated the texture attributes of the 20 almond samples. A panel of 113 almond consumers rated their liking of a subset of 8 of these almonds. Compared with the whole almonds, sliced and slivered almonds had less hardness, less crunchiness, less cohesiveness, less tooth packing, and required fewer chews and swallows to consume. Compared with slivered almonds, sliced almonds were more powdery, had more surface roughness, more loose particles, and were crisper. Compared with slivered almonds, sliced almonds were less hard, broke into fewer pieces, had less moistness and cohesiveness of mass, less fatty film, and required fewer chews and fewer swallows to consume. Dry roasted almonds were generally harder, more crisp, more crunchy, and produced more loose particles than natural almonds, which were, in turn, more hard, crisp, and crunchy than blanched almonds. As moisture content increased, moistness of mass and cohesiveness of mass increased. Crispness, number of pieces, hardness, crunchiness, persistence of crunch, and particulate mass decreased with increasing moisture content. Consumer texture liking ratings were highly positively correlated with the attributes crispness, crunchiness, and persistence of crunch.     

Sensory, Acoustical, and Force-Deformation Measurements of Potato Chip Crispness

The auditory (sound only) and oral (normal biting and chewing) crispness of five different potato chip products, each adjusted to two moisture contents, were measured. Instrumental acoustical and force-deformation measurements were made on these same samples and compared with the sensory crispness measurements. Oral crispness judgments appeared to be made on the basis of both auditory and oral tactile sensations, since auditory crispness was not highly correlated to oral crispness. A combination of acoustical and force-deformation measurements provided an excellent measure of oral chip crispness.     

Consumer perception of crispness and crunchiness in fruits and vegetables

Crispy and crunchy textures are desirable qualities and contribute to our enjoyment of foods, and this is particularly true in the case of fruits and vegetables. However, most of the literature on crispness and crunchiness focuses on dry foods, and little work has been carried out on wet foods such as fruits and vegetables. The study presented here was designed to address two main issues: firstly, to gain a better understanding of consumer terminology relevant to the textural characteristics of fruits and vegetables, with particular emphasis on crispy and crunchy terms; secondly, to investigate the relationship between the vocabularies used by trained panels and consumers. Products were selected to give a wide range of textural characteristics. Free choice profiling was carried out with a small number of naı̈ve consumers, as well as with trained panellists. Both sets of data were analysed using generalised Procrustes analysis (GPA), which allowed comparison of the two sets of vocabularies generated. GPA maps showed that sensory perceptions expressed by a trained panel correlated well with consumer terminology. The study also reinforced the idea that crispness and crunchiness are complex concepts that combine a wide range of perceptions such as fracture characteristics, sound, density and geometry. Results showed a better agreement on the meaning and use of crunchy terms by consumers and panellists alike. On the other hand, crispness was less well understood, showing a much wider conceptual space.     

Mechanical Characterization of Individual Instant Coffee Agglomerates

Agglomerated instant coffee particles were compressed individually with a TA-XT2 Texture analyzer. Up to about 57% equilibrium relative humidity (ERH), the force-displacement curves were very jagged. The slopes of plots of highest local force peaks vs deformation were used as a measure of agglomerate stiffness. The degree of jaggedness of the original force-displacement curves, a brittleness measure, was quantified in terms of an apparent fractal dimension using two algorithms. Stiffness and brittleness were little affected by moisture at ERH levels in the range 11 to 52%. At ERH 57% both decreased sharply, indicating plasticization of the soluble material.     

DIRECTION REVERSALS IN THE MECHANICAL SIGNATURE OF CELLULAR SNACKS: A MEASURE OF BRITTLENESS?

The “mechanical signatures” of brittle cellular foods are known to be irregular and irreproducible. Nevertheless, their degree of jaggedness is remarkably similar and can be a measure of such foods' brittleness and perceived crunchiness/crispiness. An irregular curve's “jaggedness” is manifested by frequent direction reversals. Hence, their number can serve as a jaggedness's index. Counting the number of direction reversals in a digitized force‐displacement curve can be done with a simple algorithm, implemented in any general‐purpose software. The number of direction reversals is strongly correlated with the signature's apparent fractal dimension. It is also a reproducible enough parameter to monitor the loss of brittleness in snacks as a result of moisture sorption. This measure of jaggedness, however, is inapplicable to smooth force‐displacement curves because in such records, the small random force fluctuations produced by the digitizing process can create a false impression of “jaggedness.” Although a correction for this artifact can be introduced into the program, its utility is doubtful.     

Relationship between sensory analysis,penetrometry and visible–NIR spectroscopy of apples belonging to different cultivars

Nineteen French apple cultivars were characterised by sensory profiling, penetrometry and visible/near infrared (vis/NIR) spectroscopy. The main purpose was to investigate the relationships between sensory attributes, including apple texture and flavour, and destructive penetrometric measurements and non-destructive vis/NIR spectroscopy. Sensory crunchiness, touch resistance and crispness correlated well with total puncture force, flesh rupture breakdown force, and the work associated with that force. Juiciness and mealiness, two very important quality indices, were strongly correlated with the slope of force-deformation curves. A relationship was also found between different vis/NIR wavelengths and sensory attributes for apples (roughness, crunchiness, mealiness, sour and sweet taste). Subsequent studies will investigate the possible benefit of using vis/NIR spectroscopy to estimate certain sensory attributes measured by trained panellists.     

ACOUSTIC WAVE ANALYSIS FOR FOOD CRISPNESS EVALUATION

Sound signals for five snack food products at two moisture levels were recorded digitally and product crispness was evaluated by a trained sensory panel. Sound signal features were extracted by analyzing signal-value and power-value dependencies. Principal component regression and neural network techniques were used to determine the usefulness of the sound signal features as predictors of sensory crispness. In a validation test, a trained neural network model predicted sensory crispness from sound signal features to an R²-value of 0.89. The results show the effectiveness of the techniques employed to extract and use sound signal features for crispness evaluation.     

DETERMINATION OF THE APPARENT FRACTAL DIMENSION OF THE FORCE-DISPLACEMENT CURVES OF BRITTLE SNACKS BY FOUR DIFFERENT ALGORITHMS1

The apparent fractal dimension of digitized experimental force-displacement curves of cheese balls and Zwiebacks at various levels of water activity were determined with the Richardson, Minkowski, Kolmogorov and Korcak algorithms. All four, but especially the first three, produced a consistent dimension despite the fact that the curves themselves were not truly fractal. These calculated dimensions were effective jaggedness measures and could be used interchangeably to monitor moisture sorption effects on the shape of force-displacement relationships. The apparent dimension versus water activity plot had a sigmoid form which was characterized by a shifted Fermi equation. The center point of the jaggedness loss as well as its span was revealed from the magnitude of the equation's constants or directly from the plot.     

SENSORY, INSTRUMENTAL AND ACOUSTIC CHARACTERISTICS OF EXTRUDED SNACK FOOD PRODUCTS

Corn based extrudates produced at three moisture contents and stored at four water activities were evaluated by sensory evaluation and instrumental tests in order to assess how processing and storage affect the sensory characteristics of the products. Acoustic signatures were collected by recording the sounds produced as the samples were bitten through with the back molars. These signatures were analyzed by fast Fourier transformation and the Kolmogorov dimension of fractal analysis. Products with a low moisture content and water activity were crisper than those at a higher moisture content and water activity. Fourier transformations of the data showed peaks between 1 and 2 kHz and between 6 and 7 kHz. Fractal analysis of the normalized raw time-amplitude curves showed significant positive correlations between fractal dimensions and the sensory characteristics of crispness, crumbliness and pitch.     

CELLULARITY, MECHANICAL FAILURE, AND TEXTURAL PERCEPTION OF CORN MEAL EXTRUDATES

Relationships among cellular structure, fracturability, and sensory properties in porous, brittle extrudates were investigated. Corn-based extrudates intentionally processed to exhibit a range of physical structures were characterized in terms of cell size distribution, bulk density, mechanical strength, fracturability, and sensory attributes. These measurements show both mechanical strength, defined by average compressive stress during extended deformation, and fracturability, quantified by fractal and Fourier analyses of stress-strain functions, increasing with either decreasing mean cell size or increasing bulk density. Fracturability parameters or structural characteristics are furthermore correlated with sensory scores for crunchiness, crispness, hardness and perceived density. These results indicate that cellularity strongly influences the pattern of mechanical failure and that failure characteristics, such as fractal dimension or power spectrum of stress-strain functions, are reflective of sensory texture.     

FORCE-DEFORMATION CONDITIONS ASSOCIATED WITH THE EVALUATION OF BRITTLENESS AND CRISPNESS IN SELECTED FOODS1

The brittleness and crispness of low moisture foods and the crispness of selected high moisture foods were evaluated both instrumentally by Instron fracture tests and using a sensory panel. Brittleness was evaluated sensorially during the “first bite” whereas crispness was identified at a later stage of mastication. The maximum force at fracture of low moisture foods correlated inversely with panelists’ evaluations of brittleness either in the mouth or using the fingers. Panelists' evaluations of crispness in the mouth correlated reasonably well with the initial linear portion of the Instron force-deformation curves when the sample were supported near their ends and also in their middle regions. The shapes of the force-deformation curves for high moisture foods were very much influenced by the way in which samples were supported in fracture tests, the method of fracture and the Instron cross-head speed.     

A RESEARCH NOTE COMPRESSIVE STRESS-STRAIN RELATIONSHIPS of AGGLOMERATED INSTANT COFFEE1

Shallow beds (3–9 mm) of two commercial agglomerated spray dried instant coffees were compressed using a Universal testing machine. the stress-stress relationships had the sigmoid shape characteristic of cellular solids and could be described by a four parameter model originally developed for solid foams. the magnitude of the model parameters was less sensitive to the relative humidity level at storage in the range of 11–65% than to the bed's thickness. the jaggedness of the stress-strain relationship is a manifestation of the particles brittleness and crumbliness and it is considerably reduced when they are plasticized, as a result of moisture uptake, for example. the degree of jaggedness, determined in the normalized (dimensionless) stress-strain relationships, could be qualified and compared in terms of an apparent (“natural”) fractal dimension. It was calculated from the linear region of the corresponding Richardson plots using the “blanket” algorithm. This apparent fractal dimension, however, was only a relative measure of jaggedness, since both the scales and resolution of the mechanical signatures were arbitrarily selected. the jaggedness of the individual agglomerates signatures could not be directly inferred from those of their assemblies, primarily because the effects of averaging and the bed's cushioning could not be separated.     

THE RELATIONSHIPS OF PITCH,LOUDNESS AND EATING TECHNIQUE TO JUDGMENTS OF THE CRISPNESS AND CRUNCHINESS OF FOOD SOUNDS2

Twenty subjects judged a variety of tape recorded bite sounds and chew sounds for both crispness and crunchiness. Crispness scores were generally higher for the bite sounds than for the chew sounds. Crunchiness judgments showed no overall difference between bites and chews. However, the interaction between the eating technique and the individual food sounds was highly significant for both crispness and crunchiness judgments, and in several cases the result for a specific food differed from the general conclusions. From the results of the first test, two subsets of the sounds were selected based on significant t-tests: one set in which the sounds were either more crisp than crunchy or more crunchy than crisp, and a second set in which the bite and chew sounds for the same product differed in crispness. These subsets were tested twice by a second group of twenty subjects to determine whether the differences might be related to loudness or pitch. The crisper sounds were typically both higher in pitch and louder than the crunchier sounds although exceptions occurred. In the majority of bite versus chew pairs, the crisper sounds were also louder- and/or higher-pitched than the less crisp sounds.