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  • 8/9/2019 Inulins-Investigating the Influence of Inulin as a Fat

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    Investigating the influence of inulin as a fat 

    substitute in comminuted products using rheology,

    calorimetric and microscopy techniques

    Derek F. Keenan a,* , Mark A.E. Auty b,2, Linda Doran b,2, Joseph P. Kerry c ,3,Ruth M. Hamilla,1

    aTeagasc,  Food   Research  Centre   Ashtown,  Dublin   15,  IrelandbTeagasc,

     

    Food 

    Research 

    Centre 

    Moorepark, 

    Fermoy, 

    Co. 

    Cork, 

    IrelandcFood

     

    Packaging 

    Group, 

    School 

    of  

    Food 

    and 

    Nutritional 

    Sciences, 

    University 

    College 

    Cork, 

    Co. 

    Cork, 

    Ireland

    f o od s t ru c tu r e x xx ( 2 01 4 ) x xx –x x x

    * Corresponding author.  Tel.: +353 1 8059500; fax: +353 1 8059550.E-mail addresses: [email protected] (D.F. Keenan), [email protected] (Mark A.E. Auty), [email protected] (J.P. Kerry).

    1 Tel.: +353 1 8059500; fax: +353 1 8059550.2 Tel.: +353 25 42222; fax: +353 25 42340.3 Tel.: +353 21 4903000; fax: +353 21 4903000.

    o

     Article history:

    Received 16 January 2014

    Received in revised form

    17 April 2014

    Accepted 16 June 2014

    Available online xxx

    Keywords:

    SausageFat replacement

    Inulin

    Design of experiment (DOE)

    Relaxation studies

    Cryo-scanning electron microscopy

    t

    Thepresentmanuscript studied the effects of fat substitutionwith twocommercial inulins

    on themagnetic resonance, rheological, calorimetric andmicroscopic properties of break-

    fast sausages. Sausage formulations were evaluated using mixture design (D-optimal). A

    total of 17 experimental treatments were employed, with each representing a different

    substitution level for fat. Sausage batters were formulated to contain lean pork shoulder,

    pork back fat/inulin, water, rusk andseasoning (44.3, 18.7, 27.5, 7 and2.5% w/w, respective-

    ly). The resultant products’ water mobility, deformation and thermal behaviors were

    analyzed for each treatment group using nuclear magnetic resonance (NMR), rheology,

    differential scanning calorimetry (DSC), while their ultra-structural properties were ana-lyzed using light, confocal andscanning electronmicroscopy for selected extremes. Signifi-

    cantmodelswere produced forwatermobilitywith inulin inclusions in sausages increasing 

    the relative protonpopulations of boundwater (T2b) values (  p < 0.0001)anddecreasing free

    water (T22)  population ( p < 0.0001). Inulin inclusions significantly altered the rheological

    characteristicswith increases inboth thegel strength (G00 G00

    0)  andunit interactionstrength

    ( An) ( p < 0.0001, respectively). Complementary temperature-dependent behavior was ob-

    servedusing rheology andDSCwhichshowed increasedelastic behavior (G0)  circa 40 8C that

    corresponded to the endothermic peaks for the onset of protein denaturation. Cryo-scan-

    ning electron and confocal laser microscopy techniques permitted visualization of the

    aggregation of inulin micro-crystals and distribution of fat within the cooked sausage

    matrix. Overall, the work presented has improved our understanding of the fundamental

    properties of sausage products and will enable a more scientific-based approach to future

    product development.#

      2014 Elsevier Ltd. All rights reserved.

    FOOSTR-15;  No.  of   Pages  13

    Please cite this article in press as: Keenan, D. F., et al. Investigating the influence of inulin as a fat substitute in comminuted products using rheology, calorimetric and microscopy techniques. Food Structure (2014), http://dx.doi.org/10.1016/j.foostr.2014.06.001

     Available  online  at  www.sciencedirect.com

    ScienceDirect 

    journal homepage: www.elsevier.com/locate/foostr

    http://dx.doi.org/10.1016/j.foostr.2014.06.0012213-3291/# 2014 Elsevier Ltd.  All rights reserved.

    http://dx.doi.org/10.1016/j.foostr.2014.06.001mailto:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.foostr.2014.06.001http://www.sciencedirect.com/science/journal/aip/22133291http://www.elsevier.com/locate/foostrhttp://dx.doi.org/10.1016/j.foostr.2014.06.001http://dx.doi.org/10.1016/j.foostr.2014.06.001http://www.elsevier.com/locate/foostrhttp://www.sciencedirect.com/science/journal/aip/22133291http://dx.doi.org/10.1016/j.foostr.2014.06.001mailto:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.foostr.2014.06.001

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    products  represented   two  commercial  forms  of   inulin))  were

    developed  using   Design   Expert  software  (v.  7.6.1,   Stat-Ease

    Inc.,  Minneapolis,  MN,  USA).  Pork  shoulder  (95%  lean)  and  pork

    back   fat  (Granby  Meats,  Dublin,  Ireland)  were  minced  (model

    PT-82/22  Mainca  Barcelona,  Spain)  twice  (5  mm  plate  size)  and

    bowl  chopped  with  powdered  inulin,  ice  water,  seasoning   and

    rusk  for  2  min.  Sausage  batter  was  piped  into  a  cellulose  casing 

    and  blast  frozen  (air  speed  3.75   m/s)  and  stored  (20   8C)  for  allsubsequent  analyses  as  outlined  in  a  previous  study  (Keenan

    et  al.,  2014). Sausages  (five  per  treatment,  vacuum-packed)

    were  cooked  usingwaterbath  immersion  (85  8C)  offive  vacuum-

    packed 

    sausages 

    per 

    treatment was 

    until they 

    had 

    achieved a

    core   temperature of 73  8C.  Core  temperature  profiles  were

    recorded during   the process  using an  Ellab  E-Val TM  TM9608

    data  module  (Ellab  [UK] Ltd.,  Norfolk,  England)  connected  to  a

    laptop.  A  standard  Ellab  SSA-12080-G700-TS  temperature  probe

    was  inserted  through an  Ellab  GKM-13009-C020 packing   gland

    (20 

    mm) 

    into 

    the 

    largest sample 

    in 

    the vacuum 

    bag.

    2.2. 

    Time 

    domain 

    nuclear 

    magnetic 

    resonance 

    (TD-NMR)

    Nuclear 

    magnetic 

    resonance 

    (NMR) 

    relaxation 

    measurements

    were  carried  out  as  previously  described  (McDonnell  et   al.,

    2013),  on  a  Maran  Ultra  instrument  (Oxford  Instruments,

    Abington,  Oxfordshire,  UK)  with  a  resonance  frequency  for

    protons   of 23.2  MHz.  Transverse   relaxation  (T2)  times were

    measured using   Carr-Purcell-Meiboom-Gill  (CPMG)  pulse   se-

    quence 

    with 

    the 

    resultant relaxation 

    decays 

    analyzed 

    by 

    tri-

    exponential   unsupervised   fitting   in  the  RI  Win-DXP  software

    (V.  1.2.3 Oxford  Instruments,   Abington,   Oxfordshire,   UK).

    2.3. 

    Rheology

    Rheological measurements were  performed  on a  Physica  MCR301  rheometer (Anton  Paar   GmbH, Graz, Austria) fitted  with

    parallel plate (50  mm;  smooth)  geometry running   Rheoplus

    software  package  (version  3.21, Anton   Paar   GmbH,   Germany).

    Sausage  batterswere pressed  fromtheir casingsand  placed onto

    the  center of the   base plate. The  upper  plate was  moved  into

    position,  i.e. the  distancebetween the  twoplates  (gap)was  set to

    1  mm.  Excess material  was  trimmed  from  the  plate edges and

    samples  were  allowed  to  rest   for  5  min  to  achievea  constant test

    temperature 

    (25 

    8C 

    regulated 

    by the 

    rheometer’s 

    Pelltier 

    plate

    and  temperature hood),  and  for  relaxation of residual  stresses.

    Viscoelastic  properties were  assessed  by performing a  prelimi-

    nary   amplitude sweep to identify the   linear  viscoelastic  (LVE)

    region of   the  samples and  the  strain  (0.1%)  that should be usedfor  the  resultant frequency sweep. A  frequency  sweep from  0.1

    to  10  Hz  was  performedand  the  results  for  storagemodulus  (G0),

    loss modulus  (G00),  and  complex modulus (G*)  were  recorded.

    These data  were  modeled  using the following power  law

    equations  as  suggested   by Friedrich  and  Heymann (1998):

    G0¼  G

    0

    0vn0 (1)

    G00¼  G

    00

    0vn00 (2)

     

     Anvn (3)

    Thermal gelation  properties  were assessed  as  previously

    described (Cofrades,Serrano,Ayo, Carballo,&  Jiménez-Colmenero,

    2008) with  slight  modifications. Before testing, samples  were

    rested (5  min) to achieve a  constant test  temperature (5  8C)  and

    relaxationof residual  stresses. Thermal  gelationwas  inducedby

    heating   samples  from5   8C  to  85  8C  at  1  8C  min1 (Brunton, Lyng,

    Zhang, &   Jacquier, 2006).  Temperature was  controlled by the

    aforementioned  Pelltier  plate  and  temperature  hood.  Samples

    were sheared at a fixedfrequency of1.0  Hz with a  strain of0.02%.

    Sampleperimeterwas coated  with a  thin  layerofpetroleumjellyto  prevent dehydration  during testing. Changes  in  the  storage

    modulus  (G0)  were  monitored  throughout the  gelling   process.

    2.4.  Differential  scanning  calorimetry  (DSC)

    Thermal  transition  properties  were  measured  using   a  TA

    Instruments  DSC  (Model  No.  DSC  2010,   TA  Instruments  Inc.,

    New  Castle,  DE,  USA)  equipped  with  nitrogen   cooling.  Indium

    (melting   point  156.6   8C)  and  baseline  (empty  pan)  calibrations

    were 

    carried 

    out 

    prior 

    to 

    testing. 

    Homogenized 

    (Robot 

    Coupé

    Blixer  41 mono,  Bourgogne,  France)  sausages  samples

    (15–20   mg) were  weighed  into   aluminum  pans  and  hermeti-

    cally  sealed.  Samples  were   equilibrated  at  10  8C  and  thenheated

     

    from 

    10 

    to 

    90 

    8C 

    at 

    heating  

    rate 

    of  

    10 

    8C 

    min1

    against a  reference  (empty)  pan  (McArdle,  Kerry, Mullen,  Allen,

    &  Hamill,  2011). Onset  temperature,  protein  denaturation  (T o),

    the   peak  temperature  (T p)  and  the   denaturation  enthalpy  (DH)

    were  recorded.  Samples  were  analyzed  in  triplicate.

    Fig.  1  –  Representative  distribution  of   T2  relaxation  times

    for 

     breakfast  

    sausages: 

    (a) 

    comparison 

     between 

    raw 

    (  

     )

    and 

    cooked 

    (–) 

    in 

    control 

    sausages 

    (run 

    10) 

    and 

    (b)

    comparison 

     between 

    cooked 

    control 

    (–) 

    sausages 

    and

    selected  fat-substituted  counterparts  [-  -  -  100%

    substitution 

    HP 

    (run 

    10)]; 

    67% 

    substitution 

    HP:GR  

    1:1

    (run  15)].

    f o od s t ru c tu r e x xx ( 2 01 4 ) x xx – xx x 3

    FOOSTR-15;  No.  of   Pages  13

    Please cite this article in press as: Keenan, D. F., et al. Investigating the influence of inulin as a fat substitute in comminuted products using rheology, calorimetric and microscopy techniques. Food Structure (2014), http://dx.doi.org/10.1016/j.foostr.2014.06.001

    http://dx.doi.org/10.1016/j.foostr.2014.06.001http://dx.doi.org/10.1016/j.foostr.2014.06.001

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    2.5. 

    Microscopy

    Sausage 

    samples 

    (1 

    cm3) 

    were 

    flash 

    frozen 

    in 

    liquid 

    nitrogen

    and  stored  at  80  8C.  Sections  were  cut  (20  mm) using   a  Leica

    CM1950   cryostat  (Leica  Biosystems,  Nussloch,  Germany)  after

    equilibration  to  specimen  chamber  temperature  (25   8C).

    Light   microscopy  sections  were  stained  with  fast  green   and

    iodine  (ratio  10:1)   stains  and  examined  using   a  Leica  DMLB

    light 

    microscope 

    (Leica 

    Microsystems 

    AG, 

    Wetzlar, 

    Germany).

    Confocal  scanning   laser  microscopy  (CSLM)  was  used  in

    conjunction  with  differential  staining   to  visualize  the   distri-

    bution  of   the   fat  component  within   the  sausages.  Sectionswere

     

    stained 

    with 

    Fast 

    Green 

    (FCF) 

    and 

    Nile 

    Red 

    stains 

    and

    examined  under  a  Leica  SP5  confocal  microscope  (Leica

    Microsystems  GmBH,  Mannheim,  Germany).

    For  Cryogenic  Scanning   Electron  Microscopy  (Cryo-SEM),

    cooked  samples  were   frozen  in  liquid  nitrogen   slush  (210   8C)

    and  transferred  to  an  Alto  2500   cryo   preparation  chamber

    (Gatan 

    Ltd., 

    Oxfordshire, 

    UK) 

    at 

    185 

    8C. 

    Samples 

    were

    fractured  used   a  cooled  knife  and  then   warmed  to  95   8C

    Table 

    – 

    Regression 

    coefficients 

    for 

    significant  

    quality 

    and 

    structural 

    parameters 

    of  

    sausages.

    Dependent   variables  Independent  variables  R2  p  model   p  lack  of   fit

    X1   X2   X3   X1*X2   X1*X3   X2*X3   X1*X2*X3

    DH p1 Y 1   1.05 0.22 0.17 0.85 0.0001 0.25

    DH p2 Y 2   1.36 2.97 3.77 1.96 2.38 1.89 0.90 0.0001 0.78

    G0

    0  Y 3   8.73 10.66 11.67 0.89 0.0001 0.84

    G0

    0G

    00

    0  Y 4   8.47 10.40 11.42 0.89 0.0001 0.84 An Y 5   8.80 10.66 11.74 0.84 0.0001 0.86

    n0 Y 6   0.18 0.37 0.32 0.40 0.0286 0.76

    G0(30) Y 7   45.00 222.28 303.96 0.75 0.0001 0.87

    G0(72) Y 8   47.10 114.40 128.10 0.35 0.0468 0.60

    T2b (P) Y 9   15.40 27.71 30.03 6.86 11.39 30.63** 0.90 0.0001 0.79

    T2b (T) Y 10   17.52 14.09 12.10 0.61 0.0013 0.98

    T21 (P) Y 11   69.48 65.01 64.75 5.45 14.77 27.13** 0.70 0.0100 0.56

    T21 (T) Y 12   40.30 26.87 21.94 0.96 0.0001 0.99

    T22 (P) Y 13   15.51 7.08 5.12 0.96 0.0001 0.56

    T22 (T) Y 14   171.68 140.96 136.22 0.85 0.0001 0.08

    *, 

    **, 

    ***Significant at  p < 0.05,  p < 0.01 and  p < 0.001 respectively.

    Fig. 

    – 

    Contour 

    plots 

    of  

    T2 

    relaxation 

    data 

    of  

    (a) 

    relative 

    proton 

    population 

    for 

     bound 

    water 

    (T2b 

    – 

    %); 

    (b) 

    time 

    constant  

    for

     bound 

    water 

    population 

    (ms); 

    (c) 

    relative 

    proton 

    population 

    for 

    intra-cellular 

    water 

    (T21 

    – 

    %); 

    (d) 

    time 

    constant  

    for 

    intra-

    cellular  water  population  (ms);  (e)  relative  proton  population  for  extra-cellular  water  (T22  –  %);  (f)  time  constant   for  extra-

    cellular 

    water 

    population 

    (ms); 

    for 

    fat  

    substituted 

    sausages 

    (A, 

    pork 

    fat; 

    B, 

    OraftiW GR; 

    C, 

    OraftiW HP; 

    where 

    18.7%

    of   total  mixture).

    f o od s t ru c tu r e x x x ( 2 01 4 ) x xx – xx x4

    FOOSTR-15;  No.  of   Pages  13

    Please cite this article in press as: Keenan, D. F., et al. Investigating the influence of inulin as a fat substitute in comminuted products using rheology, calorimetric and microscopy techniques. Food Structure (2014), http://dx.doi.org/10.1016/j.foostr.2014.06.001

    http://dx.doi.org/10.1016/j.foostr.2014.06.001http://dx.doi.org/10.1016/j.foostr.2014.06.001

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    for  5  min to  remove  surface  ice.  Fracture  surfaces  were  sputter

    coated  with   platinum  for  120   s  at  130   8C  and  the   sample

    transferred  to  the   cold  stage   in   the  SEM  instrument.  Images

    were  acquired  at  125   8C  and  2  kV  accelerating   voltage  in  a

    Carl   Zeiss   Supra  40VP  field  emission  scanning   electron

    microscope  (Carl  Zeiss  Ltd.,  Hertfortshire,  UK).

    2.6.   Analysis  of   data

    Mixture  design  experiments  were  designed  and  analyzed

    using   Design   Expert  (v.  7.6.1,   Stat-Ease  Inc.,  Minneapolis,  MN,

    USA) 

    as 

    previously 

    described 

    (Keenan 

    et 

    al., 

    2014). All

    parameters  of   NMR  spectrometry,  rheometry  and  calorimetry

    were  assessed  and  modeled  using   linear,  quadratic,  or

    Scheffe’s  special  cubic  models.  Models  were  subjected  to

    analysis  of   variance  (ANOVA)  to  determine  the   significance

    (  p  

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    concentrations  of   13–50%   due  to  these   limited  surfactant

    properties  (Kim  et   al.,  2001).

    T21 was   the  second  peak  identified  and  occurred  between

    23–41   ms. T21  values  represent  water  trapped  by  the   dense

    myofibrillar  network,  i.e.   ‘intra-myofibrillar  water’  (Bertram

    et  al.,  2001).The  T21 population  represented  the   majority  of   the

    water  present  in  the  sausage  matrix  for  all  samples  and  this

    overall  trend  is  consistent   with   the  findings   of   other  authors(Møller,  Gunvig,  &   Bertram,  2010). T21 values  were  fitted  to  a

    quadratic  model  which  was  found  to  be  significant  (  p  <   0.01)

    with  a  good  fit  to  the   experimental  data  (R2 =  0.70).   The   model

    showed 

    that 

    linear 

    terms 

    were 

    significant 

     p 

    0.0140) 

    and 

    that

    fat  formulations  resulted   in  higher   T21 values  than  their   inulin

    substituted  counterparts  (Fig.  2c).  Additional  interactions  for

    the   B  and  C  components  (GR  and  HP)  were   observed

    (  p 

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    implying   a  relatively  weak  structure,  which  is  indicative  of   a

    gel-like  material  consisting   of   a  loosely  ordered  structure  with

    a  viscoelastic  consistency  (Delgado-Pando,  Cofrades,  Ruiz-

    Capillas,  Triki,  &   Jiménez-Colmenero,  2012). Power  law-fitted

    parameters  were  derived  from  G0,  G00 and  G* moduli  as

    previously  described  (Campo  &  Tovar,  2008). G00   and  G00

    0

    (Eqs.  (1)  and  (2)) are  the  initial  storage  and  loss  moduli,

    respectively,  and  are  a  measure  of   the   resistance  of   a  testmaterial  to  elastic   (storage)  and  viscous  (loss)  deformation

    (Zhou  &  Mulvaney,  1998) at  an  angular  frequency  of 

    0.5   rad  s1.  The   parameters  were  fitted  with  a  linear  model,

    which 

    was 

    transformed 

    using  

    natural 

    log  

    recommended 

    by

    the  Box–Cox  method.  The   initial  elastic  behavior  of   sausages,

    G0

    0,  increased  in  sausages   which  contained  more  inulin,

    particularly  in   full  substitutions  with   Orafti  HP,  compared  to

    intermediate  formulations  and  fat-only  controls  (Fig.   4a).  The

    predicted  model  for  G00 was   found  to  be  significant  (  p  

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    and  exhibited  a  lower  frequency  dependence.  The   reason  for  a

    weakening   of   the   protein  conformation  is  unclear  as  inulin

    appeared  to  add  structural  stability  to  the   sausage  matrix  (as

    evidenced  by  the   other  rheological  parameters).  NMR  data

    showed  more  bound  water  which  could  be  attributed  to  the

    inulin  molecules.  Typically,  sausage  emulsions  are  formed  by

    the   salt-soluble  myofibrillar  proteins  emulsifying   the   fat  and

    immobilization  of   water  (Feiner,   2006, chap.  12).  Less   availablewater  may have  affected  the   emulsion  structure,  with  less

    protein  being   solubilized  and  hence   reduced  emulsification  of 

    the   fat  component  and  consequently,  increased  its  frequency

    dependence.

    3.2.2.   Temperature-dependent   behavior

    Temperature  sweeps   showed  that   the   thermal  behavior  of 

    samples  in  terms  of   their  storage,  loss  moduli  and  phase  angle

    (G0,  G00 and  d)  as  a  function  of   temperature  (5–85   8C)  (Fig.   5a–c).

    Differences 

    in 

    thermal 

    rheological 

    properties 

    were 

    observed

    between   the   different  formulations  due  to  their   differing 

    compositions  (presence  or  absence  of   fat/inulins)  as  expected

    and  are  supported  by  the  previous  texture   and  rheologicalexperiments.

     

    As 

    was 

    the 

    case 

    for 

    frequency 

    sweeps, 

    storage

    modulus  (elastic  behavior)  exceeded  the   loss  modulus

    (viscous  behavior)  over  the   temperature  range,  indicating 

    gel  network  (elastic)  formation  containing   a  substantial

    emulsion  structure  (viscous).  These   observations  were  in  line

    with  previous  studies   on  the   rheological  thermal  gelling 

    properties  of   low  fat  pork  liver  patés formulated  with   other

    polysaccharides  (Delgado-Pando  et   al.,  2012). Fig.  5a  and  b

    shows  a  slight  decrease  in  both  G0 and  G00 moduli  for  samples

    containing   higher   fat  content,  such  as  the   full  fat  control  (and

    to  a  lesser   extent   the   intermediate  formulations)  at  30–35   8C.  If 

    we  consider  the   absolute  values  of   G0 at  30  8C  in  the   mixturedesign,  it  shows   that   data  was  significantly  (  p  <   0.0001)   fitted

    to  a  linear  model  with   a  good  fit  to  the  experimental  data

    (R2 =  0.75)  and  showed  that  the   linear  components  were   the

    most 

    significant 

    terms. 

    The 

    modeled 

    surface 

    shows 

    that 

    G0

    values  were  lower  in  higher  fat  containing   formulations  than

    their  inulin  containing   counterparts  (Fig.  4e).  This  observation

    is  consistent  with  the   melting   of   pork  back  fat  ( Jiménez-

    Colmenero  et   al.,  2012) and  is  in  agreement  with  the   DSC  data

    in  the   present   study.  This  was  followed  by  a  dramatic  increase

    in 

    G0 and 

    G00 values 

    for 

    all 

    samples 

    between 

    40 

    and 

    80 

    8C 

    during 

    which  the   main  rheological  changes  occurred  (Fig.  5a  and  b).

    This  can  be  attributed  to  conformational  changes  in  the   meat

    proteins  that   occur  in  this   temperature  range  leading   to  thecharacteristic

     

    stiff  

    elastic 

    matrix 

    of  

    meat 

    gels 

    The 

    absolute

    values  of   G0 at  72   8C  (finishing   temperature  of   the   sausages  in

    the   present   study)  were  successfully  fitted   (  p   <   0.0468)   to  a

    linear  model  (R2 =  0.35).   Similarly,  samples  containing   inulin

    Fig. 

    – 

    (a) 

    Representative 

    thermal 

     behavior 

    of  

    control 

    ( ~ ) 

    sausages 

    and 

    selected 

    fat-substituted 

    counterparts 

    [& 

    – 

    100%

    substitution 

    HP 

    (run 

    10)]; 

    – 

    67% 

    substitution 

    HP:GR  

    1:1 

    (run 

    15)]; 

    [+ 

    – 

    50% 

    substitution 

    HP:GR  

    1:1 

    (run 

    16)]; 

    [* 

    – 

    33%

    substitution  HP:GR   1:1  (run  5)];  and  contour  plots  of   thermal   behavior:  endothermic  peaks  (EP)  (b)  1;  and  (c)  2;  in

    fat-substituted 

    sausages 

     by 

    differential 

    scanning 

    calorimetry 

    (DSC) 

    (A, 

    pork 

    fat; 

    B, 

    Orafti1 GR; 

    C, 

    Orafti1 HP; 

    where

    A  +  B  +  C  =  18.7%  of   total  mixture).

    f o od s t ru c tu r e x x x ( 2 01 4 ) x xx – xx x8

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    had  higher   storage   moduli  than  those   containing   fat  (Fig.   4f).

    The  presence   of   inulin  increased  the   storage  and  loss  moduli

    due  to  the  formation  of   inulin’s  own  three-dimensional

    network  which  differs  to  that   of   pork  back   fat.  These   data

    corresponds  to  the   thermal  properties  observed  in  the   second

    endothermic  peak  of   DSC  data  (Fig.  6a).  However,  the   effect

    was  not  consistent   for  increasing   levels  of   inulin,  for  example,

    run  15   (containing   1/3  of   each  fat/inulin  component)  hadsubstantially  higher   G0 and  G00 values  than  those   containing 

    full  substitution  with  inulin.  Statistical  analysis  did  not

    support  any  possible  interactive  effect  between   the   two  inulin

    types 

    to 

    account 

    for 

    this 

    phenomenon. 

    It 

    could 

    be 

    attributed 

    to

    the  heterogeneous   nature  of   the   sausage  batter  or  due  to  the

    mechanisms  governing   the   formation  of   the   inulin  gel,

    i.e.  nucleation   and  crystallization,  which  are  difficult  to control

    and  rely  on the  mutual  arrangement of   the  crystals   that  can

    lead  to  gels   with  different   rheological   properties (Glibowski,

    2010; Stasiak  &  Dolatowski,   2008). Furthermore, increases  in  G0

    and  G00 values   were   less  pronounced  in  control   formulations

    compared  to their  inulin   containing   counterparts.

    Further  investigation of   the   absolute values  of   G0,  G00 and d

    and   specific  temperature  points in  the mixture  designrevealed some  interesting   observations.  Significant  differ-

    ences  were  observed  between different  formulations   for G0

    values  from5  to   72   8C.  However,  this was   only the  case   from 5

    to 

    50 

    8C 

    for G00 values. This 

    could 

    indicate 

    that 

    conditions 

    that

    conferred  elasticity  rather  than  viscosity  were   favored  after

    50   8C.  Phase  angle data  showed no significant  differences

    Fig. 

    – 

    Light  

    micrographs 

    of  

    cryostat  

    sections 

    of  

    (a) 

    control 

    (full 

    fat) 

    sausages 

    at  

    4T;  (b)  10T;  and  (c)  20T  magnifications  and

    (d)  fully  fat-substituted  sausages  at   4T;  (e)  10T;  and  (f)  20T  magnifications.

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    between formulations  between 5   and   30   8C,  implying   a

    similar   underlying structure   in  all formulations.  This

    changed  with  increasing   heat from 40 to  80   8C,  with  inulin-

    formulated   samples  showing   increases  in   viscoelastic  prop-

    erties  (data not shown).  These changes  imply substantial

    structural  changes  occurring   during the  application of   heat

    (as  expected)  and this changed significantly for different

    levels of   fat  substitution  (which   is   in   agreement  in   with  DSC,rheological  and   textural  parameters shown in  this study). A

    decrease in  G0 and G00 values  was  observed  between  80   and

    85   8C  for sausage samples  containing   inulin,  which  were

    more 

    notable in 

    samples 

    containing  

    the 

    inulin 

    form 

    Orafti

    HP. This  may be   attributable to  the   greater  degree  of 

    polymerization   (DP)   of   Orafti   HP (>23)   compared   to  Orafti

    GR  (>10).  Inulin  with   high  DP  is   thought  to  be   thermally

    unstable  and   temperatures  above80 8C  havebeen reported  to

    inhibit the  formation of   gels  (Bot et   al., 2004;  Glibowski  &

    Wasko, 2008).

    3.3. 

    Differential 

    scanning 

    calorimetry 

    (DSC)

    Fig. 

    6a 

    shows 

    typical 

    DSC 

    heat 

    curves 

    for 

    fat 

    and 

    inulin-

    enriched  sausages,  with  two  endothermic  peaks  obtained  for

    all  samples.  The  first  endothermic   peak  had  an  onset

    temperature  between   24  and  31   8C  and  corresponds  to  the

    melting   point  of   fat.  This   finding   is  in  agreement  with  data

    presented  by  other   authors  (Morin  et  al.,  2004). A  linear  model

    was 

    fitted 

    to 

    the 

    reaction 

    enthalpy 

    (data, 

    which 

    was

    transformed  using   an  inverse  square-root  power  law  recom-

    mended  by  the   Box–Cox  method).  The  model  was  significant

    (  p 

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    from  a  system  (Hinrichs,  Prinsen,   &  Frijlink,  2001). This  helps

    to  maintain  the   proteins   native  confirmation  and  prevents

    denaturation  (Barclay  et   al.,  2010). Other  reported  findings  of 

    increased  DH  values  soy  protein  dispersions  containing   both

    sucrose   and inulin (using water as  a  solvent)   compared   to

    controls18   were postulated  as   a  thermal   stabilization   effect

    on   native soy  proteins by  inulin. Other  contributory factors

    which   are   responsible   for the   increased thermal  propertiesobserved in  the second thermal peak may have been  due to

    the   water binding effect of    inulin. This  could limit  the

    available   water to other matrix components,  e.g. rusk,

    thereby 

    limiting  

    hydration of   

    starch 

    and profoundly

    modifying the  thermal   properties of   starch   gelatinization

    ( Juszczak   et al.,  2012).

    3.4. 

    Microscopy

    Lightmicroscopy conducted on 

    iodine 

    and 

    fast green stained

    cryostat   sections  of   control  (full  fat) and fully  fat-substituted

    sausages  at different  magnifications (4,  10  and   20)  are

    presented  in   Fig.  7a–f.   Both  treatments  consisted  of   acontinuous

     

    protein 

    phase (green) 

    containing fat 

    globules

    and  adipose   cells (unstained). Some  larger  muscle fragments

    were   also   visible.  Large  aggregates  of   partially gelatinized

    starch   granules  (stained purple in  iodine) were  dispersed

    within  the   protein matrix. These  are most likely  rusk

    particles. No  obvious structural  differences  were  observed

    between the 

    control 

    and 

    inulin-containing samples. The

    presenceof   fat  and   its   distributionwithin the  sausage  matrix

    was   subsequently visualized using   confocal scanning laser

    microscopy (CSLM)  and differential  staining   with  fast   green

    and   Nile   red (Fig.  8a–c). Significant  differences  between the

    sausage  treatments were  observed. Fig. 8a  shows fat  (stained

    green)  was   abundantly  present in  full-fat control  and was

    coated  by  the   protein/water  continuous  phase forming   the

    typical   sausage  emulsion-like  structure. A  large  population

    of   adipose tissue  is   also  visible in  the   bottom  right of   theimage. Fat  substitution in  different  sausage  formulations  led

    to   a  concomitant reduction of   green  stain,  which can  be

    clearly seen  in   Fig.  8b  and c  representing 66%  fat reduction

    (run 

    15) 

    and 

    full fat 

    substitution, respectively. 

    The residual

    green  stain  in   the latter represents  the   native fat content   of 

    the   pork  shoulder  muscle.  The   presence of   the inulin and its

    distribution within the  sausage matrix was  visualized  in   a

    fully  fat-substituted formulation  (Fig.  9c and  d) using  

    cryogenic  scanning electron microscopy (cryo-SEM)  and

    compared against 

    control (Fig. 9a and 

    b) 

    formulation (full

    fat).  Gelatinized starch granules  from   the added  rusk  could

    be   clearly  identified in  the sausage matrix  (labeled  SG   –

    Fig.  9a). Sausage   treatment containing   inulin  had   crystal-line

     

    regions, often 

    in 

    the form of rounded spherulitic

    structures. These  structures  were notpresent in  the control

    sample,  which contained more conventional  fat morphol-

    ogy   (labeled F  –   Fig. 9b),   and   it   is   strongly suggested that

    they are  inulin  crystals (labeled I   –  Fig. 9d).   Previous   studies

    (Cooper   &  Carter,  1986; Cooper &  Steele, 1991)have   reported

    that inulin 

    particles normally 

    crystallize 

    from 

    water as

    ovoids   of 1–10 mm  diameter,  much like  the observations  of 

    Fig. 

    – 

    Cryo-scanning 

    electron 

    microscopy 

    (cryo-SEM) 

    of  

    (a 

    and 

     b) 

    control; 

    and 

    (c 

    and 

    d) 

    full 

    fat-substituted 

    sausages 

    (where

    GS,  gelatinized  starch;  F,  fat;  and  I,  inulin).

    f o od s t ru c tu r e x xx ( 2 01 4 ) x xx – xx x 11

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