bolus moisture content of solid foods during mastication

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BOLUS MOISTURE CONTENT OF SOLID FOODS DURING MASTICATION LIDIA MOTOI 1,3 , MARCO P. MORGENSTERN 1,3 , DUNCAN I. HEDDERLEY 2 , ARRAN J. WILSON 1 and SINAG BALITA 1 1 The New Zealand Institute for Plant & Food Research Limited, Private Bag 4704, Christchurch Mail Centre, Christchurch 8140, New Zealand 2 The New Zealand Institute for Plant & Food Research Limited, Palmerston North, New Zealand KEYWORDS Bolus, mastication stages, moisture content, saliva 3 Corresponding authors. TEL: +64-3-3259481; FAX: +64-3-3252074; EMAIL: [email protected]; [email protected] Accepted for Publication July 8, 2013 doi:10.1111/jtxs.12036 ABSTRACT Saliva addition plays an important role in bolus formation. During chewing, food breaks down, exposing food particles to saliva. The aim of this study was to explore and understand how bolus moisture content changes during the oral pro- cessing of solid foods. Twelve subjects chewed commercially produced solid foods; the boluses were collected at different stages of the mastication process, including the swallowing point, and all of the boluses produced were analyzed for their moisture content. The chewing sessions were recorded on video, enabling the number of chews to be assessed for each subject and food type. Results showed that moisture content of boluses during mastication increased linearly at a rate depending on the subject and food types studied. It was found that for the food types studied, an increase in initial food moisture content increased the bolus moisture content at the swallowing point. PRACTICAL APPLICATIONS Sensory perception experienced during oral processing of foods is through sensing the bolus properties, which change continuously in between ingestion and swal- lowing. Understanding changes in bolus moisture content, the role of food struc- ture and effects on sensory perception can help in designing foods with predictable sensory attributes. This study demonstrated that while saliva additions vary between people and food types, they increase linearly during mastication. Changes in bolus moisture content during mastication could be used to guide the design of solid foods with target properties. INTRODUCTION During oral processing a solid food is transformed into a bolus (Bourne 2002) until the consistency of the bolus reaches conditions for safe swallowing (Prinz and Lucas 1995; de Wijk et al. 2003; Chen and Lolivret 2011). Mastica- tion involves complex oral processes (Engelen et al. 2007; Chen 2009; Chen and Engelen 2012), including breakdown of the food, jaw movements, moistening, lubrication and dissolution by saliva as well as heating and shearing of the food. Numerous studies (Kohyama et al. 2002, 2008; Peyron et al. 2002; Piancino et al. 2008) have investigated the oral processing parameters of solid foods, including the number of chews, chew frequency, chew time, oral residence time, jaw movements, muscle activity, friction and many other parameters (Riqueto Gambareli et al. 2007; Sazonov et al. 2008; Moritaka and Nakazawa 2010) in order to determine how changes in food, when processed in the mouth, relate to texture perception. It has been documented that oral food manipulation has an impact on the way food is per- ceived (Mioche 2004; Engelen et al. 2007; Lenfant et al. A journal to advance the fundamental understanding of food texture and sensory perception Journal of Texture Studies ISSN 1745-4603 468 Journal of Texture Studies 44 (2013) 468–479 © 2013 Wiley Periodicals, Inc.

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Page 1: Bolus Moisture Content of Solid Foods during Mastication

BOLUS MOISTURE CONTENT OF SOLID FOODS DURINGMASTICATIONLIDIA MOTOI1,3, MARCO P. MORGENSTERN1,3, DUNCAN I. HEDDERLEY2, ARRAN J. WILSON1 andSINAG BALITA1

1The New Zealand Institute for Plant & Food Research Limited, Private Bag 4704, Christchurch Mail Centre, Christchurch 8140, New Zealand2The New Zealand Institute for Plant & Food Research Limited, Palmerston North, New Zealand

KEYWORDSBolus, mastication stages, moisture content,saliva

3Corresponding authors. TEL: +64-3-3259481;FAX: +64-3-3252074; EMAIL:[email protected];[email protected]

Accepted for Publication July 8, 2013

doi:10.1111/jtxs.12036

ABSTRACT

Saliva addition plays an important role in bolus formation. During chewing, foodbreaks down, exposing food particles to saliva. The aim of this study was toexplore and understand how bolus moisture content changes during the oral pro-cessing of solid foods. Twelve subjects chewed commercially produced solid foods;the boluses were collected at different stages of the mastication process, includingthe swallowing point, and all of the boluses produced were analyzed for theirmoisture content. The chewing sessions were recorded on video, enabling thenumber of chews to be assessed for each subject and food type. Results showedthat moisture content of boluses during mastication increased linearly at a ratedepending on the subject and food types studied. It was found that for the foodtypes studied, an increase in initial food moisture content increased the bolusmoisture content at the swallowing point.

PRACTICAL APPLICATIONS

Sensory perception experienced during oral processing of foods is through sensingthe bolus properties, which change continuously in between ingestion and swal-lowing. Understanding changes in bolus moisture content, the role of food struc-ture and effects on sensory perception can help in designing foods withpredictable sensory attributes. This study demonstrated that while saliva additionsvary between people and food types, they increase linearly during mastication.Changes in bolus moisture content during mastication could be used to guide thedesign of solid foods with target properties.

INTRODUCTION

During oral processing a solid food is transformed into abolus (Bourne 2002) until the consistency of the bolusreaches conditions for safe swallowing (Prinz and Lucas1995; de Wijk et al. 2003; Chen and Lolivret 2011). Mastica-tion involves complex oral processes (Engelen et al. 2007;Chen 2009; Chen and Engelen 2012), including breakdownof the food, jaw movements, moistening, lubrication anddissolution by saliva as well as heating and shearing of thefood.

Numerous studies (Kohyama et al. 2002, 2008; Peyronet al. 2002; Piancino et al. 2008) have investigated the oralprocessing parameters of solid foods, including the numberof chews, chew frequency, chew time, oral residence time,jaw movements, muscle activity, friction and many otherparameters (Riqueto Gambareli et al. 2007; Sazonov et al.2008; Moritaka and Nakazawa 2010) in order to determinehow changes in food, when processed in the mouth, relateto texture perception. It has been documented that oralfood manipulation has an impact on the way food is per-ceived (Mioche 2004; Engelen et al. 2007; Lenfant et al.

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A journal to advance the fundamental understanding of food texture and sensory perception

Journal of Texture Studies ISSN 1745-4603

468 Journal of Texture Studies 44 (2013) 468–479 © 2013 Wiley Periodicals, Inc.

Page 2: Bolus Moisture Content of Solid Foods during Mastication

2009; Chen and Lolivret 2011; Drago et al. 2011; Foster et al.2011; Lillford 2011), but how this takes place has not beenfully demonstrated yet. Apart from the oral processingparameters mentioned, the addition of saliva during masti-cation is a key factor in the formation of a bolus and itsproperties.

Saliva plays an important role in the sensory perceptionof foods (Engelen et al. 2007; Stokes and Davies 2007). Anumber of researchers have studied the role of saliva inrelation to perception of flavor, taste and texture of foods.Early studies (Pangborn and Lundgren 1977; Guinard et al.1997) showed promising results regarding the relationshipbetween the amount of saliva secreted and the texture andtaste of solid foods. The researchers studied the influence offood physical form on saliva flow. They concluded thatgreater amounts of saliva were secreted in response topowder than pieces of the same food. That is, a food with agreater surface area required more saliva to lubricate it.Their work showed potential to use salivary flow as an ana-lytical measurement to evaluate sensory perception. Theyconcluded that further investigations ought to be made tobetter understand the relationship between saliva flow, mas-tication, textural perception and swallowing.

Saliva is mainly water (>99%) so it is expected that therole of saliva in the perception of texture is largely due tothe effects of water. This hypothesis that changing the watercontent in the bolus by adding water to solid foods willchange the perception of the food has been investigated byvarious researchers (Pereira et al. 2006, 2007; Shiozawa andSaeki 2009). The authors investigated the muscle activityneeded to chew food with and without additional liquid.Results showed that adding water to solid food had animpact on the oral physiology parameters and perceivedtexture of all foods studied, except carrot and cheesesamples. The number of chews and muscle activity were sig-nificantly decreased by the amount of water added, but theywere not affected by the fluid type (water or water withα-amylase).

Texture perception occurs during the whole chewingsequence from ingestion to swallowing and is affected bythe changes in bolus properties as a result of particle sizereduction and saliva addition (Lenfant et al. 2009; Loretet al. 2009). Therefore, for a full understanding of textureperception, it is important to determine the saliva flow rateduring chewing. Few studies provide data on saliva flowrates during oral processing and most work focuses onproperties at the point of swallow to understand the triggersfor swallowing. Loret et al. linked the bolus moisturecontent just before swallowing (Loret et al. 2009, 2011) to asensory property (fluidity). They used wheat flakes with dif-ferent textural properties and different moisture contentsand showed that just before swallowing, moisture contentand perceived fluidity were similar for all samples, suggest-

ing that fluidity is the primary trigger for a bolus ready toswallow. For the wheat flake samples, fluidity was directlyrelated to the moisture content.

The way saliva addition changes during mastication andthe way food breaks down influences the bolus properties,and these properties depend on the initial food structureand moisture content. The breakdown behavior and bolusformation are of key importance for both texture and tasteperception of food. It is known that moisture contentchanges during mastication due to saliva participation inthe process. It is interesting to understand how the salivaryrates are affected by the initial moisture content of the food.The objective of this study was to investigate how the mois-ture content of the bolus changes during the mastication ofsolid foods due to saliva secretion (experiment 1), and howmoisture content at the swallowing point is affected by theinitial moisture content of a solid food (experiment 2).

MATERIALS AND METHODS

Subject Selection

Screening of 34 volunteer subjects allowed the selection of12 healthy subjects for the study. The subjects (sevenwomen and five men) ranged from 20 to 29 years of age.All of the subjects had a good general health, good naturaldentition with no major dental work, and no dentures orprosthetic teeth. None of the subjects were taking any medi-cation that could affect muscle function or saliva flow.The study was given ethical approval by the Upper South ARegional Ethics Committee (URA/11/EXP/028) and all thesubjects gave their written consent.

Test Foods

Griffins “Super Wine” biscuits (Griffins Foods Ltd,Auckland, New Zealand) and Dutch cake (Ontbijtkoek)were used in experiment 1, and Baker’s Delight fresh whitebread and Dutch cake (Ontbijtkoek) were used in experi-ment 2. Protein, fat and moisture contents for each foodstudied were measured using, respectively, AACC method46-30 (protein by combustion), AACC method 02-01A (fatacidity) and AACC method 44-15A (air oven moisturecontent) while the total carbohydrates and the sugarcontent were according to the nutritional values determinedby the producers (Table 1).

Sample Preparation

In experiment 1, biscuit samples were prepared by cuttingthe round biscuits into squares of 2.3 (±) 0.3 g weightsusing a Micro Bandsaw MBS/E (Proxxon GmbH, Föhren,Germany), while the Dutch cake samples were prepared by

L. MOTOI ET AL. BOLUS MOISTURE CONTENT OF SOLID FOODS

469Journal of Texture Studies 44 (2013) 468–479 © 2013 Wiley Periodicals, Inc.

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slicing them using an electric bread slicer on a clearance setat 18 mm. Slices were punched using a stainless steel borerto produce samples of 2.3 (±) 0.3 g weights of the food.

Samples for experiment 2 were prepared from wholefresh white bread and Dutch cake following (for both) thesame procedure as for the Dutch cake in the first experi-ment. The moisture content of the foods was altered bydrying or by adding water to the basic product to obtainsamples at three different levels of moisture content, thustogether with the moisture level of the original sample (L2)a total of four moisture levels was obtained. Bread/Dutchcake pieces (60 pieces per tray) were dried in a prover(M190F, Contherm Scientific, Lower Hutt, New Zealand)that was preset at 40C. Drying was performed for 1.5 and3.5 h for Dutch cake, and 1 and 2.5 h for bread in order toproduce two moisture content levels (L3 and L4). Thesamples were cooled prior to packaging in labeled alumi-num foil bags, and stored in a temperature-controlled roomat room temperature. A high moisture level (L1) wasobtained by adding 0.5 mL water to the bite size samplesand leaving them to equilibrate for a minimum of 2 h in anair-tight container. The moisture content of each level wasmeasured in three replicates for each food every day of theexperiment.

We used biscuit in experiment 1 because we had usedthem in our preliminary studies to assess changes in mois-ture content during mastication. However, it proved verydifficult to produce biscuit with four initial moisture con-tents so we selected white fresh bread for experiment 2.

Experimental Procedure

Two experiments were carried out; the same 12 subjectsparticipated in both experiments. Participants were asked toattend a total of six sessions, which included two trainingsessions. They were asked to produce 12–15 boluses persession. Each chewing sequence lasted a maximum of5 min. The total time per session was 75 min per subject.The training sessions lasted no more than 30 min per par-ticipant. The experimental procedure is summarized inFig. 1.

Video Data Capture

A video recording method was chosen to determine thetime and number of chews needed to reach the swallowing

point. Optical tracking of soft tissue markers has beenshown to give reasonable agreement with underlying jawmovement (Jemt and Hedegard 1982; Haggman-Henriksonet al. 1998; Green et al. 2007; Chen et al. 2011), and hasbeen widely used in chewing research (Anderson et al. 2002;Gerstner et al. 2005; Wintergerst et al. 2008). While electro-myograms can provide similar data (Piancino et al. 2008;Çakır et al. 2012), they require more invasive monitoringwith the attachment of wired sensors to the skin. Smallyellow dots were placed on each subject’s chin and nose astracking points. The yellow dots in each frame were trackedusing Kinovea (http://www.kinovea.org) and the two-dimensional position of chin and nose in pixels in eachframe was recorded. The relative movement of the chincompared with the nose in the vertical direction versus time(Fig. 2) was used to measure the number of chews and thetime to reach the swallowing point. Each subject was askedto chew naturally while they were video recorded using aCanon compact digital camera (IXUS 115HS, Canon Inc.,Tokyo, Japan) at resolution 1,920 × 1,080 pixels at 24 framesper second. The subjects were asked to keep their headsstraight forward during the chewing in order not to alter thetracking references of the dots. This method allows the sub-jects to chew without them having to consider their chewfrequency or number of chews. All sessions for both experi-ments were recorded on video.

Training Sessions

During two training sessions we familiarized the subjectswith the procedures and products and determined the timeneeded to have the bolus ready to be swallowed (SwT) foreach subject and each food type. We asked the subjects tochew each food product until the necessary swallowing con-sistency was reached, in order to determine the time neededto swallow. The SwT(s) determined for each food productand each subject were used to scale the times for each masti-cation stage analyzed up to the swallowing point and beyond(experiment 1). In experiment 2, each food product waschewed to the swallowing point (experiment 2). The subjectsnaturally chewed each food type and swallowed it while theywere recorded on video. They were asked to mark the swal-lowing time by lifting their hands. The video recordings wereprocessed and for each subject, each food type and eachinitial food moisture content; the time needed to reach theswallowing point was determined (in duplicate).

TABLE 1. COMPOSITION OF THE SOLID FOODS USED

Food Protein (%) Fat (%) Moisture content (%) Carbohydrate*(%) Sugar* (%)

Griffin’s Super Wine biscuits 5.6 17.2 2.4 74.6 25.7Dutch (breakfast) cake 3.1 2.9 24.2 69.0 37.0Fresh white bread 9.7 1.9 44.0 44.4 3.5

* From nutrition information on the product label.

BOLUS MOISTURE CONTENT OF SOLID FOODS L. MOTOI ET AL.

470 Journal of Texture Studies 44 (2013) 468–479 © 2013 Wiley Periodicals, Inc.

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Experiment 1

The first experiment determined changes in bolus moisturecontent during mastication. Subjects were asked to chewand expectorate boluses at different stages of the mastica-tion time. They expectorated the boluses and did not rinsethe mouth so the remaining particles dispersed in themouth were not collected. These stages were determinedaccording to the times needed to form a bolus of each foodtype ready for swallowing during the first two sessions. Thetime needed to reach the swallowing point was considered

to have a mastication stage of 100%. Three data points wereselected before the swallowing point by calculating thetimes that corresponded to 25, 50 and 75% of the total mas-tication time. One data point was selected after the swallow-ing point by asking the subjects to hold the food in themouth and continue to chew for a time 25% beyond theswallow point (125% mastication stage). The subjects pro-duced three replicates for each stage of the mastication andeach food type. A total of 360 boluses corresponding to dif-ferent mastication stages for the two food types studiedwere analyzed for moisture content.

FIG. 1. EXPERIMENT PROCEDURE DESIGN

L. MOTOI ET AL. BOLUS MOISTURE CONTENT OF SOLID FOODS

471Journal of Texture Studies 44 (2013) 468–479 © 2013 Wiley Periodicals, Inc.

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Experiment 2

The second experiment explored changes in moisturecontent of boluses at swallowing point of foods with differ-ent initial moisture contents. The products tested were freshwhite bread with moisture contents ranging from 20 to 60%(wet basis) and Dutch cake with four initial moisture con-tents ranging from 9 to 38% (wet basis). Subjects were askedto chew the foods provided for the previously determinedtimes SwT for each moisture level, and expectorate theboluses just before the swallowing point, while theirchewing sessions were recorded on video. The subjects pro-duced three replicates for each food type and each moisturelevel. A total of 288 boluses at swallowing point for differentinitial food moisture contents were analyzed for their mois-ture content.

Moisture Content Measurement

Moisture contents of foods used in both experiments weredetermined by measuring the weight loss of a sample driedat 105C in an air oven overnight for a minimum of 16 h(AACC method 45-30). The boluses were weighed immedi-ately after they were expectorated and then placed in theoven. The moisture content in percentage on a wet weightbasis was calculated as MCwet = (m0 − m1)*100/m0, where m0

is the mass in grams of the expectorated bolus and m1 is themass in grams of the bolus after oven drying. The moisturecontent on a dry basis, which represents the ratio betweenthe moisture mass (saliva and water) present in the bolusand the dry matter of the bolus after drying in the oven, wascalculated using the formula MCdb = MCwet/(1 − MCwet).

We assumed that by using this method for determiningmoisture content we measure only the food water evapora-tion. To determine whether fat evaporation affected themoisture content measurement we selected the foodproduct with the highest fat content (Table 1), biscuits, and

we measured its fat content using the crude fat determina-tion test (AACC 30-10). We determined the fat content ofthe expectorated bolus before it was placed in the oven andafter drying. We found that expectorated boluses of biscuitshad a fat content of 18.1% (dry basis) before drying and18.6% (dry basis) after drying. Thus, fat evaporation wasminimal and the mass loss during oven drying was assumedto be due to water evaporation only.

Number of Chews and Chewing Frequency

The number of chews was determined from data processedfrom the video recorded. The chewing frequency (in Hz)was calculated as ratio of the number of chews and the time(SwT).

Saliva Addition

Saliva added (SA) per gram dry food was estimated usingthe formula SA = MCdb bolus − MCdb food, where MCdb bolus rep-resents the moisture content of the collected bolus on a drybasis, and MCdb food represents the dry basis food moisturecontent before chewing. In each instance, less than thewhole bolus was recovered from the mouth, but we madethe assumption that boluses expectorated were representa-tive for the sample analyzed. In our study, the average bolusrecovery ranged between 60 and 79%. We calculated thebolus recovery by subtracting the initial dry weight of thefood from the dry weight of the bolus. Another assumptionwe made was that we neglected the influence of the solidscontained in the saliva, knowing that saliva is mainly water(>99%).

Saliva Flow Rate

The saliva flow rate was calculated by dividing the SA,expressed in grams per grams of dry food, by the number of

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FIG. 2. ILLUSTRATION OF A TYPICAL CHEWING RECORDING BY A CANON HD CAMERAPlot of chin position relative to nose in the vertical direction in pixels per frame recorded at 24 frames per second. Black squares indicate points ofmaximum mouth opening. A chewing cycle is defined as the time between two adjacent mouth openings.

BOLUS MOISTURE CONTENT OF SOLID FOODS L. MOTOI ET AL.

472 Journal of Texture Studies 44 (2013) 468–479 © 2013 Wiley Periodicals, Inc.

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cycles corresponding to the mastication stage. For example,saliva flow rate at 50% is calculated as the ratio between SAat 50% mastication and number of chews to reach 50% ofmastication.

Statistical Analyses

Analysis of variance (ANOVA) was used to summarize dif-ferences between foods and mastication stages, using a split-plot design with subjects as replicates, foods as main plotsand mastication stages as the subplots. The data from the0% mastication (i.e., no chewing) were excluded from thecalculation of significance levels because there was nosubject-to-subject variation in the data and including itmight have led to overestimating significance levels andunderestimating least significant differences. A similar split-plot ANOVA was used to summarize differences betweenfoods and added moisture levels, while allowing for thevariability between average levels among subjects. Becausethe added moisture levels differed between the two foods,no main effect for added moisture was fitted. To summarizethese relationships, linear mixed-effects models were fittedto estimate the slope of the effect of initial moisture andwhether there were differences between foods, either inintercept or slope, compared with the variation of thoseeffects among subjects.

RESULTS AND DISCUSSION

Bolus Moisture Content/Saliva Additionduring Mastication (Experiment 1)

The differences in bolus moisture content, number of chewsand chew frequency between the solid food samples at dif-ferent mastication stages are summarized in Table 2. Therandom variability between the mean of the three replicatesfrom different subjects for the same food and same mastica-tion stage was four to eight times higher than the variabilityamong the replicates (Table 2 – variance ratio for theresidual in the Subject × Food × Mastication stage is typi-cally 4–8, except for the chewing frequency and saliva flow).This means that given the same task on the same day, asubject produced similar results, but different subjects pro-duced quite different results. Similarly, the variance ratio forthe subject effect was also large (14–23 for chews, about 7for moisture content, 5–8 for saliva flow), confirming thatthere were large differences among individuals for the vari-ables measured. Similar interindividual differences amongsubjects have been observed by scientists studying the par-ticle size of food boluses (Flynn et al. 2011; Peyron et al.2011; Moongngarm et al. 2012). Subject and product differ-ences in saliva incorporated in boluses of a dairy modelproduct were reported by Drago et al. (2011). They TA

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L. MOTOI ET AL. BOLUS MOISTURE CONTENT OF SOLID FOODS

473Journal of Texture Studies 44 (2013) 468–479 © 2013 Wiley Periodicals, Inc.

Page 7: Bolus Moisture Content of Solid Foods during Mastication

observed that the subject had a significant effect on bolusmoisture content (62–85%) and saliva incorporated (Dragoet al. 2011). These could explain the differences in perceivedtexture that may be affected by the mastication process. Theeffect of food type on number of chews needed to reachthe necessary bolus consistency was reported previously(Gavião et al. 2004; Engelen et al. 2005). They found a sig-nificant difference among the number of chews for variousfood products. The mean value of number of chews to theswallowing point for breakfast cake determined in ourexperiment was 19.3. These values are similar to those pre-viously published for breakfast cake by Engelen et al.(2005).

Biscuits required more chew cycles than Dutch cake(P < 0.026) (Table 2), but chewing frequency was notsignificantly different between the two food products(P = 0.662). The average chewing frequency value for allmastication stages and all subjects was 1.57 Hz (range 1.02–1.94) for biscuit and 1.56 Hz (range 1.00–2.04) for Dutchcake. The chewing frequency values determined in ourstudy were in agreement with frequency values (1.45–1.68 Hz) of similar or different food types published byother authors (Foster et al. 2006; Peyron et al. 2011; Po et al.2011).

In our study, chewing frequency tended to be slightlylower at the 25% mastication stage than at other mastica-tion stages, but the differences were small (Fig. 3). A similarreduction in chewing frequency for the first four to fivecycles, and no significant change after, was observed byBhatka et al. (2004) when evaluating how chewing gumbolus size affects human chewing cycles.

Our results showed, as was expected, that the amount ofSA increased significantly with increasing numbers of chews(P < 0.001). In terms of the amount of SA as chewing pro-ceeds, at the beginning of mastication (25% of the waythrough), more saliva was added to the Dutch cake than tothe biscuit, but from 25% of the way through mastication tojust before swallowing, more saliva was added to the bis-cuits. We assume that more saliva was added to Dutch cakeearly in the chewing process because its ingredient composi-

tion stimulates saliva or because the taste is favored by thesubject, which also stimulates saliva production (Lillford2011). Once mastication progresses, the texture of the foodproduct (e.g., extent of aeration and particle sizes) governsthe amount of SA.

Saliva flow rate during mastication did not differ betweenthe two foods (P = 0.809). Average saliva flow rate wassimilar for biscuit and Dutch cake, except for the 25% mas-tication stage, where it was high for Dutch cake and low forbiscuits, even though the initial food moisture content washigher for Dutch cake than for biscuits (Fig. 4). This couldbe explained by the differences in fat content between thetwo foods (Table 1). For the biscuit, which has a higher fatcontent, the salivary flow rate was lower in the beginning ofmastication, possibly due to the lubricating effect of the fat.

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

0 50 100 150

Chew

ing

freq

uenc

y [c

ycle

s/s]

Mastication stage [%]

Biscuits Dutch cake

FIG. 3. CHEWING FREQUENCY DURING MASTICATIONThe values are an average of three replicates from 12 subjects. Barsindicate least significant differences at 95% confidence level. Graphrepresents the dynamic of chewing frequency for biscuits (diamondsymbols) and Dutch cake (square symbols) during mastication.

25%

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0.0

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0.4

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a ad

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A

0

10

20

30

40

50

60

70

0% 50% 100% 150%

Saliv

a flo

w r

ate

[mg/

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Mastication stage

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B

FIG. 4. CHANGES IN SALIVA ADDED (A) ANDSALIVA FLOW RATE PER CHEWING CYCLE(B) DURING MASTICATIONThe symbols (diamond for biscuits and squarefor Dutch cake) represent the mean valuesof saliva flow rate cumulative for eachmastication stage. For example, saliva flowrate at 50% is calculated as ratio betweensaliva added up to 50% mastication andnumber of chews to reach 50% ofmastication. The symbols represent the meanvalues, and the error bars on the lines are± 1 SE.

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Page 8: Bolus Moisture Content of Solid Foods during Mastication

This is in agreement with previous tests of the chewing ofsolid foods (cake, melba toast and toast) with and withoutbutter (Engelen et al. 2005). The authors of that study con-cluded that buttering food enhanced lubrication and bolusformation.

The bolus final moisture content (on wet or dry percent-age weight basis) varied between the two foods; it wasalways higher for Dutch cake than for biscuits (Table 2). Themoisture content of a bolus on a wet weight basis was sig-nificantly affected by the food type and mastication stage(P < 0.001). Results from the ANOVA indicated that thebolus final moisture content on a dry weight basis had sig-nificant food (P < 0.001) and mastication stage (P < 0.001)main effects, but there was no interaction evident betweenthese two parameters (Fig. 5 and Table 2). This indicatesthat both foods had a similar rate of SA during mastication(from the original dry weight basis).

Saliva addition per gram dry food increased significantlyas mastication progressed (P < 0.001), but on average wassimilar for the two foods (P = 0.177). There was a smallinteraction between food type and mastication stage. Moresaliva was added to both foods at the start than later inmastication. When food is put in the mouth there isalready saliva present, which explains the faster rates in thebeginning.

Correlations with mastication stage for each subject andeach food are at R2 = 0.90–1.0 for moisture content ona dry weight basis and saliva addition. Using a random-coefficients regression to fit linear slopes to each person–food combination (mean slopes shown in Fig. 5), bothmoisture content on a dry weight basis and saliva additionwere linear over time. It can be concluded that the moisturecontent on a dry basis increased linearly with progressingmastication, and this change was similar for both food typesstudied, but with not very different slopes. It follows that, asa first approximation, saliva is added at a constant rate byeach individual and thus moisture content of the bolus canbe estimated every time during mastication by a linear

interpolation. Because individual saliva flow rates are verydifferent, the moisture content at different stages is also verydifferent, and therefore rheological bolus properties will bevery different.

Effect of Initial Food Moisture Content onSaliva Addition during Chewing(Experiment 2)

Differences between foods in saliva addition duringchewing with different initial food moisture contents, whileallowing for the variability between average levels amongsubjects and subject × foods, are summarized in Table 3.Chewing time, number of chews, bolus final moisturecontent on wet and dry weight bases, SA and saliva flow ratewere analyzed for food type and initial food moisturecontent effects as well for their interactions. There were sig-nificant differences in bolus final moisture content on a dryweight basis between the two foods (P < 0.001) as well asamong the initial moisture levels (P < 0.001). Saliva flowrate per cycle did not vary between food types (P = 0.844),but was significantly affected by the initial food moisturecontent (P < 0.001).

To understand how the number of chews related to initialmoisture content, a linear effects model was fitted (Fig. 6),testing the effect of food type, the slope of the initial mois-ture content effect and whether that differed between foodtypes against the variability of those effects from subject tosubject. The results (Table 4) confirmed significant differ-ences between the two food types (P < 0.001), a significantdifference among the initial food moisture levels (P < 0.001)and a significant but smaller difference in the slopesbetween the two food types (P = 0.012). With increases ininitial food moisture content, fewer cycles were needed toreach the bolus consistency for swallowing. Similar resultswere revealed by Pereira et al. (2006), who found thatnumber of chews was negatively correlated with food mois-ture content for melba toast, breakfast cake, carrot and

0%

50%

100%

0% 50% 100% 150%

Bolu

s moi

stur

e co

nten

t [%

dry

w

eigh

t bas

is]

Mastication stage

Biscuits Dutch cake

A

0.0

0.5

1.0

0% 50% 100% 150%

Saliv

a ad

ded

[g/g

dry

food

]

Mastication stage

Biscuits Dutch cake

B

FIG. 5. MOISTURE CONTENT ON DRYWEIGHT BASIS AND SALIVA ADDITIONDYNAMICS DURING MASTICATIONThe graphs (A) and (B) represent the fittedlinear model results for bolus moisturecontent on dry weight basis and saliva addedin grams per gram dry food duringmastication for biscuit (diamond symbols) andDutch cake (square symbols). The symbolsrepresent the mean values, and the error barson the lines are ± 1 SE.

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peanut. We found for all subjects and all food types studieda correlation between the chewing times and number ofcycles (R2 = 0.91), indicating that the chewing frequency didnot change. This compares well with results (R2 = 0.83)obtained in a previous study (Loret et al. 2011).

The linear mixed-effects models fitted for initial mois-ture content of foods on a dry weight basis and saliva addi-tion (Fig. 7) show a significant difference between the twofoods, and their moisture levels, but did not show differ-ences in the slopes between the two food types. For theDutch cake, in terms of initial moisture content and SA,there was an indication that there was a nonlinear elementto the data. The nonlinearity was only minor (increasingcorrelation between fitted values and data from 0.97 to 0.99for final moisture content, and from 0.95 to 0.97 for addedsaliva; at this level of fit even small differences can show assignificant, because the residual is so small). The resultsrevealed that boluses of Dutch cake at swallowing point didnot have as high a moisture content or as much addedsaliva as biscuit boluses at the swallowing point, even if theinitial moisture contents of the foods were similar. Thiscould be explained by the different ingredients in the twofoods. For instance, the protein content was lower for theDutch cake (Table 1). Less saliva was necessary to enablethe bolus to reach swallowing consistency. This is in agree-ment with previous results presented by Gavião et al.(2004). Those authors determined the salivary flow ratesfor solid foods (Melba toast, breakfast cake and cheese) andobserved no differences in the salivary flow rates perminute among the foods studied, but they did observedifferences in the amount of saliva per gram food, whichTA

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FIG. 6. EFFECT OF INITIAL FOOD MOISTURE CONTENT ON THENUMBER OF CHEWS REQUIRED FOR SWALLOWINGThe symbols represent the mean values, and the error bars on thelines are ± 1 SE.

BOLUS MOISTURE CONTENT OF SOLID FOODS L. MOTOI ET AL.

476 Journal of Texture Studies 44 (2013) 468–479 © 2013 Wiley Periodicals, Inc.

Page 10: Bolus Moisture Content of Solid Foods during Mastication

differed among the products. Direct comparison cannot bemade because they use different food sizes and methods tocalculate SA to the food.

In order to be swallowed comfortably, a food bolusshould have a certain consistency. It does not necessarilyneed a specific water content. At the swallowing point,each bolus has its own consistency that corresponds tospecific rheological properties that seem to be determinedby a combination of factors, among which is watercontent. At swallowing point, boluses of wheat flake cerealswith different initial moisture levels reach more or less thesame water content (same fluidity) (Loret et al. 2011). Thedata presented in Table 3 illustrate that bolus formationand the amount of SA to it depends not only on the initialmoisture content of the food, but also on the food type.Our results indicated a significant difference between theamount of moisture added to particular foods to form abolus that can be swallowed (P < 0.001), and a significanteffect of initial food moisture content on saliva addition

(P < 0.001). We did not correlate bolus moisture contentat swallowing point with the sensory perception of thefood types studied, so we do not know if differences inthe moisture content of these foods at swallowing pointare perceived differently by consumers. This experimentfocused on the material properties of foods and how theyaffect saliva addition during mastication. Food moisturecontent is just one parameter, but it provides an indicationof factors that affect bolus properties. As the initial mois-ture content of the food increased, the amount of addedsaliva decreased (Fig. 7), although not on a 1:1 basis. Forboth food types studied, we found that every 1% increasein initial moisture content on average decreased theamount of SA by about 0.25% (between 0.16 and 0.45%for individuals) (Fig. 7A). The relationship is similar forfresh white bread and Dutch cake, even though their initialmoisture contents differ. This is an interesting observationbut more research is needed with a larger range of foodtypes to generalize to other foods.

TABLE 4. MIXED MODEL RESULTS FOR SLOPE OF INITIAL FOOD MOISTURE CONTENT EFFECT

VarianceBolus Moisture Content[% dry weight basis]

Saliva added g/gfood

Chewing frequency[cycles/s] Number of chews

Random effects component s.e. component s.e. component s.e. component s.e.Subject 0.0021 0.0011 0.042 0.0208 0.0437 0.0222 39.46 20.52Subject x Food 0.0004 0.0003 0.0094 0.0059 0.0142 0.0067 14.76 6.88Subject x Food moisture content 0.0195 0.0112 0.3255 0.1825 0.0880 0.0719 214.61 123.95Subject x Food x Food moisture content 0 bound 0 bound 0.0634 0.0600 69.48 58.12Residual 0.0009 0.0002 0.0124 0.0022 0.0040 0.0008 11.19 1.03

Fixed effects F statistic P F statistic P F statistic P F statistic PFood (1 df) 77.48 <0.001 68.4 <0.001 271.32 <0.001 73.04 <0.001Food moisture content (1 df) 31.02 <0.001 36.39 <0.001 8.85 0.012 156.95 <0.001Food x Food moisture content (1 df) 2.87 0.095 0.11 0.744 0.5 0.493 8.9 0.012

Estimates Bread Cake Bread Cake Bread Cake Bread CakeIntercept 0.458 0.401 1.170 0.790 1.417 1.495 47.88 36.86(SE) (0.022) (0.018) (0.091) (0.079) (0.068) (0.064) (3.04) (2.96)Slope 0.307 0.219 1.180 1.119 0.392 0.284 53.98 68.26(SE) (0.040) (0.043) (0.167) (0.178) (0.139) (0.142) (4.94) (5.05)

0%

10%

20%

30%

40%

50%

60%

70%

0% 20% 40% 60%

Bol

us m

oist

ure

cont

ent a

t sw

allo

win

g po

int [

wet

bas

is]

Food moisture content [wet basis]

Bread Dutch cake

A

00.10.20.30.40.50.60.70.80.9

1

0% 20% 40% 60%

Add

ed s

aliv

a (g

/g d

ry fo

od)

Food moisture content [wet basis]

Bread Dutch cake

B

FIG. 7. EFFECT OF INITIAL FOOD MOISTURECONTENT ON BOLUS MOISTURE CONTENTAND SALIVA ADDITION FOR SWALLOWINGGraph (A) represents the changes in bolusmoisture content at swallowing point forbiscuits (diamond symbols) and Dutch cake(square symbols) when initial food moisturecontent increases, and graph (B) representsthe saliva added at swallowing point as afunction of initial food moisture content forthe same foods. The symbols represent themean values, and the error bars on the linesare ± 1 SE.

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CONCLUSIONS

The changes in bolus moisture content during masticationfor biscuits and Dutch cake were analyzed. The results indi-cated that moisture content and saliva addition varied withfood type and by subject in a linear pattern during mastica-tion for the food types studied.

The effect of initial food moisture content on the bolusmoisture content at swallowing point was analyzed for freshwhite bread and Dutch cake. The results indicated thatbolus moisture content at swallowing increased, but that thetotal saliva addition to boluses decreased with increasinginitial moisture content of foods.

The amount of moisture in a bolus affects its rheologicalproperties. Therefore, understanding how food moisturecontent changes during mastication allows us to betterunderstand bolus rheological properties during masticationand to relate this to dynamic sensory perception.

Further knowledge of the dynamics of moisture contentduring mastication could guide the design of foods withpredicted sensory attributes.

ACKNOWLEDGMENTS

This work was carried out as part of the New ZealandMinistry of Business, Innovation and Employment (MBIE)program Food Structure Platform, Contract No. C02X0807.Financial support from the NZ MBIE is gratefully acknowl-edged. The authors would also like to thank Mr. AugustineRomieu, student at Montpellier SupAgro, France, for pre-liminary work performed during his internship.

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