The Impact of Parbaking on the Crumb Firming Mechanism of Fully

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The Impact of Parbaking on the Crumb Firming Mechanism of Fully Baked Tin Wheat Bread Mieke Armande Nivelle, Geertrui Bosmans, and JAN A DELCOUR J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b03053 • Publication Date (Web): 21 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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Journal of Agricultural and Food Chemistry

The Impact of Parbaking on the Crumb Firming Mechanism of Fully Baked Tin Wheat Bread

Mieke A. Nivelle*, Geertrui M. Bosmans, Jan A. Delcour

Laboratory of Food Chemistry and Biochemistry and Leuven Food Science and Nutrition Research Centre (LFoRCe), KU Leuven, Kasteelpark Arenberg 20, B-3001 Leuven, Belgium

*Corresponding author: Mieke Nivelle Phone: +32 (0) 16 37 42 38 Fax: +32 (0) 16 32 19 97 E-mail address: [email protected]

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Abstract

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The impact of parbaking on the quality and shelf-life of large tin bread baked from 270 g of

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wheat flour was investigated using a proton nuclear magnetic resonance method combined

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with techniques measuring at different length scales. With increasing partial baking time the

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resilience of fresh partially baked crumb increased because of its more extended amylose and

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gluten networks. During subsequent storage, crumb became more firm due to an increased

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extent of amylopectin retrogradation and moisture redistribution. Although only amylopectin

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retrogradation was reversed during final baking, a fresh fully baked (FB) bread with reversed

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crumb softness was obtained. Furthermore, the rate of crumb firming during final storage of

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FB bread was not higher than that of conventionally baked bread. This was attributed to the

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high crumb to crust ratio of large tin bread which caused the crumb moisture content to

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remain sufficiently high despite non-reversible moisture redistribution during intermediate

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storage.

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Keywords

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Parbaking, time-domain proton nuclear magnetic resonance, baking time, amylopectin

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retrogradation, water redistribution, crumb firming, crumb to crust ratio

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Introduction

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During wheat bread baking and cooling semi-crystalline amylose (AM) and thermoset gluten

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networks are formed. These networks are responsible for the initial crumb firmness and

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resilience and, thus, the desired crumb texture of freshly baked bread.1 Wheat bread is a staple

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food in the Western world. However, storage makes its crumb firm and its crust lose its

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crispiness and flavor,2 rendering the product unacceptable for consumers. During the first

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days of storage, crumb firming is strongly related to amylopectin (AP) retrogradation. The

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formed B-type AP crystals3 include more water in their crystal unit cell than the A-type

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crystals of native wheat starch.4 Besides increasing the strength of the semi-crystalline starch

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network, their formation therefore results in migration of water from the gluten to the starch

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network and thus contributes to dehydration of the gluten network. In addition, water also

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migrates from crumb to crust, resulting in further dehydration. After prolonged storage, the

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gluten is no longer fully hydrated and the resulting increase in stiffness contributes to crumb

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firmness.3 However, besides changes in the starch and gluten fractions and related water

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redistribution that determine crumb firming during storage, crumb firmness itself depends on

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bread loaf’s density, which in turn is inter alia related to the gluten properties.5

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Depending on the temperature-time profile imposed during bread baking, the extent of both

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AM leaching from the starch granules6-7 and gluten polymerization varies.7-8 These

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differences are reflected in fresh crumb texture, since initial crumb firmness and resilience are

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largely determined by the strength of the semi-crystalline AM network, for which AM

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leaching is a prerequisite,9 and that of the thermoset gluten network.1 Furthermore, AP

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retrogradation and the related increase in crumb firmness during storage occur to a larger

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extent with prolonged baking times and higher crumb center temperatures reached during

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baking.6-7, 10



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For a number of years already, partial baking, i.e. parbaking, has been applied in the bread

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making industry. The partial baking phase is executed in such way that complete crumb

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setting is realized, but without significant occurrence of Maillard reactions that would result

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in crust browning11-13 (when the surface temperature exceeds 120 °C)14. After intermediate

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storage of the partially baked (PB) bread, the final baking step is performed to melt the AP

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crystals formed during storage and to obtain a fresh fully baked (FB) bread with a typical

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brown crust and aroma.11-13 The quality of FB bread is often said to be lower than that of

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conventional bread baked in a single step (CB).15-17 FB bread is also believed to firm more

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rapidly than CB bread.18-20 However, the published studies15-20 mostly have involved frozen or

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refrigerated intermediate storage of either PB bread prepared from only 100 g of flour or PB

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French bread. Such bread types have a lower crumb to crust ratio than larger tin breads21-22

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and therefore are subject to pronounced crumb dehydration due to crumb to crust moisture

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migration during storage. To the best of our knowledge, the firming mechanisms of both PB

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and the resultant FB tin bread of a size relevant for households and stored at ambient

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temperature remain to be elucidated. Against this background, the aim of this study is i) to

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unravel the impact of shorter baking times on fresh bread quality and changes thereof during

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subsequent ambient storage and ii) to investigate whether bread prepared by parbaking and

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later final baking firms more rapidly than bread prepared by a conventional baking process.

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Time-domain proton nuclear magnetic resonance (TD 1H NMR) has already proved valuable

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for studying changes in bread constituents during storage at molecular and mesoscale, e.g. the

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extent of AP retrogradation and the redistribution of water.3, 23 Therefore, it is often used to

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provide information about starch (re)crystallization and crumb firming and, thus, is

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complementary to respectively differential scanning calorimetry (DSC), measuring at

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molecular scale, and textural analyses, measuring at macroscopic scale.


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Materials and methods

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Chemicals and materials

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Wheat flour (Crousti) [68.4% starch, 10.9% protein (N x 5.7) and 14.1% moisture content

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(MC)] was from Dossche Mills (Deinze, Belgium). Yeast was provided by AB Mauri

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(Merelbeke, Belgium). Reagents and solvents for determining the starch content were

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respectively from Acros Organics (Geel, Belgium) and VWR International (Oud-Heverlee,

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Belgium). Chemicals for extracting protein under non-reducing conditions were from VWR

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International. Dithiothreitol (DTT) from Acros Organics was used to extract protein under

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reducing conditions. All other chemicals were from Sigma-Aldrich (Bornem, Belgium).

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Composition analysis

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Starch content was calculated as 0.9 times the total monosaccharide content measured with

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gas-liquid chromatography based on Englyst and Cummings.24 Protein content was

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determined using an adaptation of the AOAC Official Method 990.0325 to an automated

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Dumas protein analysis system (EAS vario Max C/N, Elt, Gouda, The Netherlands) with 5.7

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as nitrogen to protein conversion factor. MCs of wheat flour, bread crumb and crust were

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determined according to AACC method 44-15.02.26

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Bread making and storage

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Bread was prepared according to a straight-dough method27 from wheat flour (1,000 parts;

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14.0% MC), deionized water (590 parts), sucrose (60 parts), compressed yeast (53 parts),

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sodium chloride (15 parts) and calcium propionate (2.5 parts). To obtain bread loaves with

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different crumb to crust ratios, dough was prepared from either 10, 100 or 270 g of flour (with

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all other ingredients in appropriate amounts to respect the above mentioned ratios). When

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preparing dough from 270 g of flour, ingredients were mixed for 330 s in a slightly greased

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spiral mixer (De Danieli, Legnaro, Italy) at 23 °C. The obtained dough was divided into 450 g



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pieces. For preparation of dough from 10 or 100 g of flour, ingredients were mixed at 23 °C

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for 240 s in a slightly greased 10 g or 100 g pin mixing bowl (National Manufacturing,

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Lincoln, NE, USA), respectively. All doughs were transferred to a slightly greased bowl and

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put in a fermentation cabinet (National Manufacturing) at 30 °C and 90% relative humidity.

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Fermentation lasted 90 min with intermediate punching at 52, 77 and 90 min using a dough

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sheeter (National Manufacturing). After proofing (36 min at 30 °C and 90% relative

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humidity) in a slightly greased baking tin, dough was conventionally baked in a rotary hearth

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oven (National Manufacturing) for 13 min at 232 °C, 24 min at 215 °C or 40 min at 210 °C

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depending on flour weight (10, 100 or 270 g of flour, respectively). Bread loaves prepared

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from 270 g of flour were also partially baked for 42.5 and 60% of their total baking time and

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are further referred to as PB42.5 and PB60. After intermediate storage for 6 days, PB bread

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loaves were baked for respectively 30 and 40% of total baking time at 210 °C, resulting in

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FB42.5-30 and FB60-40 breads. After each baking phase, bread loaves were cooled for 120 min at

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23 °C.

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Since the maximal crumb temperature is reached more slowly in the crumb center,

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temperature-dependent changes occurring during baking start later in the crumb center.

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Analyses were therefore performed on samples withdrawn from the crumb center. The crumb

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center temperature during conventional, partial and final baking and subsequent cooling was

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monitored using a Datapaq (Cambridge, UK) temperature logger (Multipaq 21) with type T

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thermocouples. A stainless steel thermal barrier (Datapaq) protected the logger during baking.

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For temperature monitoring during cooling, the baking tin was removed.

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Fresh cooled bread loaves were wrapped in plastic foil and stored at 23 °C in plastic bags

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which were sealed to avoid moisture loss. After different storage times, samples from at least

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two bread loaves were withdrawn from the crumb center and were further analyzed with DSC,

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TD 1H NMR and texture analysis. PB bread loaves were stored for 0 (i.e. after cooling), 3 and


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6 days, while storage of FB bread was for 0, 2, 6 and 7 days. CB bread loaves were stored for

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either 0, 3 and 6 days or 0, 2, 6 and 7 days for comparison reasons.

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Differential scanning calorimetry

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DSC was performed with a Q2000 DSC (TA Instruments, New Castle, DE, USA). Crumb

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samples from the bread center were freeze-dried and gently ground. At least triplicate samples

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were accurately weighed (2.5 – 4.0 mg) in aluminum pans (Perkin-Elmer, Waltham, MA,

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USA), and deionized water was added [1:3 (w/w) starch dry matter (dm):water]. Pans were

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hermetically sealed and equilibrated at 0 °C before being heated from 0 to 130 °C at 4

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°C/min. Temperatures and enthalpies [∆HAP (J/g sample dm)] associated with AP crystal

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melting were determined with TA Universal Analysis software.

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Proton nuclear magnetic resonance

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Measurements of proton distributions in bread crumb were performed with a Bruker Minispec

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mq 20 (Rheinstetten, Germany) TD NMR spectrometer (operating resonance frequency of 20

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MHz for 1H, magnetic field strength of 0.47 T). The probe head was kept at 25 ± 1 °C using

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an external water bath. Spin-spin or transverse (T2) relaxation times were studied. The T2

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relaxation curves for less and more mobile protons were obtained by performing single 90°

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pulse (free induction decay, FID) and Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences,

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respectively. The pulse lengths of the 90° and 180° pulses were respectively 2.86 and 5.42 µs.

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For the FID signal, an acquisition window of 0.5 ms was used and 500 data points were

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acquired. For the CPMG sequence, the separation between the 90° and 180° pulses was 0.1

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ms and 2,500 data points were collected. For both measurements, the recycle delay was 3.0 s

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and 32 scans were accumulated to increase the signal-to-noise ratio.T2 relaxation curves were

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transformed to continuous distributions of T2 relaxation times with an inverse Laplace

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transformation using the CONTIN algorithm of Provencher28 (Bruker software). The



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calculations were performed on 500 data points, logarithmically spread between T2 relaxation

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times of 0.007 and 0.5 ms and of 0.208 and 500 ms for FID and CPMG measurements,

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respectively. The resulting proton populations are characterized by their area (proportional to

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the number of protons in it) and their mean T2 relaxation time (reflecting the mobility of the

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environment the protons are in). Because the inhomogeneity of the static magnetic field

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affects the output of the most mobile FID population (around 0.5 ms), it was not taken into

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account in further analyses and it is not shown in Figures 2 and 3.

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Per fresh bread loaf, three samples (ca. 0.3 g, accurately weighed) from the crumb center were

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placed in separate NMR tubes (external diameter 10 mm) and compressed to remove air

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bubbles (sample height of ca. 8 mm). The tubes were sealed to prevent moisture loss and were

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analyzed after different storage times. This way, the effect of AP retrogradation and related

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water redistribution on changes in proton distributions during crumb storage were

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investigated without interference of crumb to crust water migration. The effect of crumb to

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crust water migration on the proton distributions in bread crumb was investigated by

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analyzing samples withdrawn from crumb stored with crust for different times (see above).

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Size exclusion high-performance liquid chromatography

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Freeze-dried samples from fermented dough and from the center of CB, PB and FB bread

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loaves were extracted in triplicate under non-reducing conditions with 0.05 M sodium

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phosphate buffer (pH 6.8) containing 2.0% (w/v) sodium dodecyl sulfate (SDS) (medium A).

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Protein extraction under reducing conditions was carried out under nitrogen atmosphere using

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medium A but now containing 1.0% DTT. Samples (1.0 mg protein/ml extraction medium)

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were shaken (60 min, room temperature) and centrifuged (10000 g, 10 min). The resulting

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supernatants were filtered (Millex-HP, 0.45 µm, polyethersulfone; Millipore, Carrigtwohill,

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Ireland). Size exclusion high-performance liquid chromatography (SE-HPLC) was conducted

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using a LC-2010 system (Shimadzu, Kyoto, Japan) with automated injection. Extracts (20 µl) 8 ACS Paragon Plus Environment

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were loaded on a Biosep-SEC-S4000 column (pore size 500 Å, Phenomenex, Torrance, CA,

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USA). The elution solvent was medium A (flow rate 1.0 ml/min, 30 °C). Protein elution was

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monitored at 214 nm. The protein extractability in SDS-containing medium under non-

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reducing conditions (SDS-EP) was calculated from the area under the chromatogram of a

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sample and expressed as a percentage of the total area obtained when extracting the samples

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under reducing conditions.

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Crumb texture analysis

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Crumb firmness was measured with an Instron 3342 (Norwood, MA, USA). Four cylindrical

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samples (height 25 mm, diameter 30 mm) were cut from the crumb center. Samples were

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compressed by a cylindrical probe (diameter 75 mm) at a constant speed of 100 mm/min.

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Crumb firmness, i.e. the maximal force (N) required to compress samples by 30% under these

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conditions, was derived from the force-time curve. At the same time, crumb resilience was

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measured as the strain energy recovered, i.e. the recoverable work (%), during a compression-

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decompression cycle with 30% compression. The recoverable work was calculated as the ratio

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of the areas under the decompression and compression stress-strain curves.10, 29

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Statistical analysis

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Statistical analyses were performed with JMP Pro 12 (SAS Inst., Cary, NC, USA). One-way

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Analysis of Variance was combined with Tukey’s honest significant difference test to identify

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significant differences (α < 0.05) for several variables, based on at least three measurements.

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Results and discussion

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Partially baked bread

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After baking for either 42.5, 60 or 100% (i.e. CB bread) of the total baking time, a

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temperature of respectively 86, 96 and 100 °C was reached in the crumb center of the bread

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loaves prepared from 270 g of flour. Complete crumb setting was attained in all three cases 9 ACS Paragon Plus Environment


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(results not shown). Starch gelatinization was considered to be complete as DSC showed

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little if any residual ∆HAP in fresh bread crumb (Table 1). Partial baking impacted the

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mobility (T2 relaxation time) and area of proton populations in fresh bread crumb as detected

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with 1H NMR (Table 2). Based on previous work,30 these populations (Figures 2, 3 and 4)

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could be assigned as follows: population A contains rigid non-exchanging CH protons of AM

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crystals formed during cooling and of amorphous starch and gluten not in contact with water.

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Populations B and C represent the same environment and therefore both contain amorphous

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CH protons of gluten and starch in little contact with water. Protons in population D are CH

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protons of gluten and exchanging protons of water, starch, and gluten. Population E is

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attributed to mobile exchanging protons of starch, gluten and water containing soluble

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components (e.g. sucrose, soluble proteins) in the formed gel network. Population F consisted

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of protons from lipids present in flour and shortening used to grease baking tins. Populations

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A and E predominated NMR profiles. Changes in their areas and mobility, therefore, are

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focused on to study the impact of parbaking on fresh and stored bread loaves.

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Despite differences in maximal crumb temperature, the initial crumb MCs in fresh bread

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loaves were similar (Table 1). Crumb MCs can be related to the areas of population E3, which

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were also similar for all fresh bread loaves (Table 2). The initial crust MC, however,

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decreased with longer baking times (Table 1). In fresh PB42.5 and PB60 bread the areas of

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population A were similar and significantly (P < 0.05) lower than that of fresh CB bread

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(Table 2). Possibly, the extent of AM leaching during baking increased with increasing

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baking time and, thus, crumb temperature.6-7 As a result, probably more AM could crystallize

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during cooling, resulting in an increased area of population A in fresh CB bread. The mobility

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of population E was similar for fresh PB42.5 and PB60 bread but significantly lower for longer

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baking times (Table 2). Possibly, the increased extent of AM crystallization during cooling

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because of more AM leaching during prolonged baking caused the starch network to be more 10 ACS Paragon Plus Environment

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extended and, therefore, its protons to be less mobile. Furthermore, with increasing baking

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time and crumb center temperature, SDS-EP values decreased (Table 1) and, thus, the extent

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of protein polymerization through disulfide (SS) bond formation increased. At temperatures

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lower than 90 °C, mostly glutenin polymerizes8 through both sulfhydryl (SH)-SS exchange

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reactions31-32 and oxidative cross-linking.33 At temperatures exceeding 90 °C, also gliadin

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becomes covalently incorporated into the gluten network through SH-SS exchange reactions.8

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Such high temperatures allow for conformational changes which expose regions that are

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initially unavailable for polymerization reactions.34 After baking for 42.5% of total baking

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time, the level of extractable glutenin (eluting between 5 min and 7 min 45 s) and extractable

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albumin and globulin (eluting between 9 min 30 s and 11 min) had almost decreased to the

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plateau level of protein extractability noted for CB bread (Figure 1). In PB42.5 bread, the

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maximal center temperature reached was 86 °C. As also noted by Lagrain et al.,8 some gliadin

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(eluting between 7 min 45 s and 9 min 30 s) was already incorporated into the gluten network

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at this temperature. Further incorporation of gliadin required prolonged baking, and its

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extractability decreased with baking time until a plateau level was reached (Figure 1 – CB

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bread). Both a more extended starch and gluten (in terms of gliadin cross-linking) network

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could contribute to a decrease in mobility of population E with baking time and temperature.

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The presence of more extended biopolymer networks was also reflected in the initial crumb

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resilience, which was lower for PB42.5 than for PB60 and CB bread loaves. Firmness readings,

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however, were similar for all fresh bread loaves (Table 1).

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Storage impacted crumb MC of all bread loaves to a minor extent. Only that of CB bread

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decreased significantly in time (Table 1). This is in line with the change in areas of population

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E observed in crumb samples stored with crust (Table 2), which has been observed

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previously.3 However, the crust MC of all bread loaves significantly increased during storage

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because of crumb to crust moisture migration (Table 1). Since the crust MC of fresh CB bread 11 ACS Paragon Plus Environment


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was lower and, thus, a higher moisture gradient between its crumb and crust existed, crumb to

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crust moisture migration occurred to a larger extent in CB than in PB42.5 and PB60 bread

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loaves. That only a minor portion of crumb water was required to significantly increase crust

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MC during storage was related to the high crumb to crust ratio of large bread loaves prepared

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from 270 g of flour. Since crumb MC changed only little during storage, changes in proton

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distributions observed in crumb samples stored with crust could exclusively be attributed to

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changes in starch and protein networks.10 AP retrogradation, observed as an increase in ∆HAP

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measured with DSC during storage (Table 1), was also detected with 1H NMR as an increase

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in area of population A (Table 2) and a decrease in the areas of populations B and C (Figure

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2). AP retrogradation causes amorphous CH protons of starch in populations B and C to

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become more rigid and, thus, to shift to population A.3 With higher maximal crumb

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temperatures reached during baking, a stronger increase in ∆HAP as a result of storage was

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observed (Table 1). This is in line with literature stating that longer baking times and higher

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crumb temperatures during baking are associated with higher degrees of AP retrogradation.6-7,

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10

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Bosmans et al.35 suggested this based on the relation between crumb MC and the extent of

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retrogradation described by Zeleznak and Hoseney.36 After storage, crumb MC was indeed

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lower in CB than in PB42.5 bread, although the differences were small (Table 1). Zhou et al.37

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further reported that AP retrogradation is affected by AM, provided the water content of

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starch gels is sufficiently high (70-80%). These authors hypothesized that the AM network,

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which in our study presumably was more developed in CB than in PB bread, immobilizes a

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considerable amount of water, thereby reduces the local MC, and in this way facilitates AP

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retrogradation. However, it would seem more plausible to us that a better developed AM

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network results in less interference of AM molecules with AP retrogradation, which can then

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proceed to a larger extent. In contrast to what was detected with DSC, an increased degree of

A possible explanation for this is that a lower crumb MC promotes AP retrogradation.


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AP retrogradation with increasing baking time could not be detected with NMR. Because

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NMR population A not only contains AP crystal protons but also protons from gluten and AM

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crystals, we postulate that small differences in the extent of retrogradation are not reflected in

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the area of population A. Independent of baking time, population D shifted to slightly lower

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T2 relaxation times during storage (Figures 2b and 3b). Bosmans et al.3 described a similar

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decrease in mobility of population D for bread loaves prepared from 100 g of flour, but in

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their case this population eventually merged with population C as a result of bread storage.

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This has been attributed to dehydration of the gluten network as a consequence of both water

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immobilization in the formed AP B-type crystals and water migration from crumb to crust. In

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the larger bread loaves described here, populations C and D did not merge as a result of

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storage (Figures 2b and 3b). Possibly, the crumb MC, which remained high during storage of

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all bread loaves (Table 1), compensated for the migration of water from gluten to starch that

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otherwise would lead to gluten network dehydration. While the area of population E, which is

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positively related to crumb MC, remained similar, the T2 relaxation time of this population

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decreased significantly during storage of all bread types (Table 2). The reduced mobility of

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population E can be attributed to strengthening of the starch and (to a lesser extent) protein

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networks during storage due to respectively AP retrogradation and (slight) gluten dehydration.

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T2 relaxation time readings in crumb from bread loaves baked for longer times were even

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lower (Table 2). This can be explained by the fact that prolonged baking times induce more

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pronounced AP retrogradation (see above). Bosmans et al.3 stated that changes in T2

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relaxation times of population E are negatively related to changes in crumb firmness during

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bread storage. Indeed, more pronounced AP retrogradation and crumb to crust moisture

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migration during storage of bread loaves baked for longer times, resulted in a stronger

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increase of crumb firmness during storage (Table 1). With shorter baking times, the crumb

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density (g/ml) tended to be higher (results not shown) and therefore may enhance crumb



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firmness.5 Nevertheless, crumb firmness of stored bread loaves increased with partial baking

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time. Therefore the impact of crumb density is further not taken into account. Crumb

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resilience was higher in fresh CB than in PB42.5 bread because of more extended gluten and

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starch networks present (see above), but also remained higher during storage (Table 1). These

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results allow hypothesizing that a crystallizing starch network together with water

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redistribution (from gluten to starch and from crumb to crust), resulting in gluten dehydration,

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dominate the undesired changes in crumb softness and resilience during storage as also

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postulated by Bosmans et al.,3 while extended starch and gluten (more gliadin incorporation)

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networks which are flexible (no rigid AP crystals and well hydrated) contribute to the desired

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initial crumb resilience. As will be discussed below, the changes in starch network

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organization and moisture distribution during this storage phase are not fully reversed during

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the final baking step.3 In view of the shelf-life of refreshed bread, the duration of the first

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baking phase should therefore be well considered.

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Fully baked bread

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To meet the requirements that i) complete gelatinization and crumb setting must be reached

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during the first baking phase, and ii) AP crystals formed during the intermediate storage

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period melt during the second baking phase, a crumb temperature of respectively 85 to 90 °C

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in the first and 60 °C in the second baking phase should be reached.38 In tin bread loaves

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prepared from 270 g of flour, this corresponded to respectively 42.5 and 30% of the total

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baking time. During final baking of stored PB42.5 bread to obtain fresh FB42.5-30 bread, the

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crust MC decreased while the crumb MC remained rather constant (Table 1 – day 6 and Table

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3 – day 0). Similar observations were made by Leuschner, O’Callaghan and Arendt.12, 39 Final

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baking caused all AP crystals formed during intermediate storage of PB42.5 to melt, since little

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if any residual ∆HAP was detected in fresh FB42.5-30 bread loaves (Table 3). This melting of AP

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crystals and the related increased starch network mobility was reflected in a decreased area of 14 ACS Paragon Plus Environment

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population A, containing rigid CH protons of inter alia starch, and an increased mobility of

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population E, containing exchanging protons in the gel network, compared to what was

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observed for stored PB42.5 bread (Figure 3, Table 2 – day 6 and Table 4 – day 0). However,

316

neither the area of population A nor the mobility of population E were restored to their initial

317

values in fresh PB42.5 bread (Figure 3, Table 2 – day 0 and Table 4 – day 0). These results

318

indicate that heating reversed neither the moisture redistribution between gluten and starch

319

nor that between crust and crumb which occurred during intermediate storage. The latter can

320

be explained by the absence of a driving force. The former was also observed by Bosmans et

321

al.3 when reheating a stored starch gel. They postulated that despite melting of retrograded AP

322

crystals, the starch network organization could not be fully reversed and that water,

323

incorporated in the starch network during storage, remained associated with it.3 Moreover,

324

further protein polymerization, resulting in a more rigid protein network, could additionally

325

prevent the mobility of population E from being restored to its initial value in PB42.5 bread.

326

The latter was investigated in more detail by studying the impact of the final baking time and

327

the corresponding crumb temperature on additional protein polymerization. To that end, PB60

328

bread was also fully baked to obtain FB60-40 bread (Figure 1). The sum of the partial and the

329

final baking times corresponds to the total baking time of CB bread (i.e. 40 min). During this

330

baking phase, the area of the peak corresponding to gliadins in the HPLC profiles (eluting

331

between 7 min 45 s and 9 min 30 s) was further reduced (Figure 1). However, the maximal

332

crumb temperature reached during final baking was 76 °C and, thus, well below the

333

temperature required for gliadin cross-linking (90 °C).8 It has been suggested that, when a

334

sufficiently high crumb temperature is reached during partial baking, reactive groups are

335

exposed34 and remain available for polymerization reactions in the subsequent final baking

336

phase. Additional cross-linking of gliadin was not observed during final baking of PB42.5

337

bread for 30% of total baking time, showing that a certain minimal temperature should still be



338

exceeded (Figure 1). Nevertheless, the protein network may have further developed through

339

non-covalent interactions during the final baking phase.

340

Although the moisture distribution was not reversed and the protein network may have further

341

developed during final baking, AP crystal melting restored the crumb firmness and resilience

342

of fresh FB42.5-30 bread loaves (Table 3 – day 0) to the initial value observed for PB42.5 bread

343

(Table 1 – day 0). These results further support the above formulated hypothesis that a

344

crystallizing starch network together with water redistribution, resulting in dehydration of the

345

gluten network, dominate the undesired changes in crumb softness and resilience during

346

storage as also postulated by Bosmans et al.,3 while extended and flexible starch and gluten

347

(more gliadin incorporation) networks contribute to the desired initial crumb resilience. The

348

fact that melting of AP crystals formed during intermediate storage without complete

349

reversion of water redistribution was sufficient to obtain similar values for initial crumb

350

firmness and resilience, shows that crumb MC of the bread loaves used in this study was still

351

high enough to keep the gluten network hydrated and flexible. The latter can be explained by

352

their high crumb to crust ratio.

353

We next compared the properties of FB with those of CB bread. FB42.5-30 bread and its

354

production process meet the above requirements for a parbaking and later final baking process

355

and is further referred to as standard fully baked (FB’) bread. In industrial practice, a CB

356

bread based on 270 g of flour is typically baked for about 25 min, which corresponds to PB60

357

bread in this study. PB60 bread is therefore further referred to as a standard conventionally

358

baked (CB’) bread. Although all AP crystals were melted during final baking (Table 3), a

359

higher area of population A and a lower mobility of population E were detected for FB’ than

360

for CB’ bread (Table 4). As pointed out above, these findings indicated the presence of an

361

amorphous cross-linked starch network that holds water after final baking, since the moisture

362

redistribution during intermediate storage is not heat-reversible. Nevertheless, crumb firmness 16 ACS Paragon Plus Environment

Page 16 of 37

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363

was restored to its initial value before intermediate storage (Table 1 – day 0 and Table 3 – day

364

0), resulting in similar values of crumb firmness in refreshed FB’ bread and fresh CB’ bread.

365

However, a lower initial crumb resilience was detected for FB’ than for CB’ bread (Table 3).

366

This was attributed to less covalent gliadin incorporation into the gluten network in FB’

367

bread, as can be observed in Figure 1 (FB42.5-30 and PB60).

368

During storage, AP retrogradation occurred to a larger extent in CB’ than in FB’ bread as

369

detected with DSC (Table 3). In this case, this was also measured as a stronger increase in

370

area of NMR population A in CB’ bread (Table 4). Moreover, the degree of AP retrogradation

371

during storage of FB’ bread was comparable to that during its preceding intermediate storage

372

phase, i.e. storage of PB42.5 bread (Tables 3 and 1, respectively). Ghiasi et al.18 also reported a

373

similar rate of AP retrogradation in bread before and after refreshing. It is therefore postulated

374

that the rate and extent of retrogradation not only during intermediate storage of PB bread, but

375

also during final storage of FB bread are impacted by the partial baking time and, thus, the

376

reached crumb center temperature. Obviously, this statement is only valid when all AP

377

crystals are melted during final baking. If not, retrogradation slowly progresses from an

378

advanced state of recrystallization (results not shown). In the present case, crumb to crust

379

moisture migration during storage was more pronounced in FB’ than in CB’ bread, since the

380

area of population E, which inter alia is related to crumb MC, decreased to a larger extent in

381

FB’ than in CB’ bread (Table 4). Nevertheless, crumb MC of both bread types changed only

382

to minor extent because of their high crumb to crust ratio (Table 3). The degree of crumb to

383

crust moisture migration seemed to be counterbalanced by the degree of AP retrogradation,

384

since crumb firmness after seven days of storage was similar for FB’ and CB’ bread (Table

385

3). Furthermore, despite differences in fresh FB’ and CB’ bread loaves, their resilience was

386

comparable after storage. These observations confirm that crumb firmness and resilience in

387

stored bread are dominated by starch crystallization and water redistribution (see above).3 17 ACS Paragon Plus Environment


388

Crumb firming thus did not occur more rapidly in FB’ than in CB’ bread, in contrast to what

389

was reported for bread prepared from (only) 100 g of wheat flour with ambient intermediate

390

storage18 and for bread prepared from 100 g of gluten-free20 or wheat flour19 with refrigerated

391

intermediate storage. It is therefore further investigated if the impact of parbaking on stored

392

bread quality evaluated as crumb texture could depend on bread loaf size.

393

Crumb to crust ratio

394

As pointed out above, the minor changes in crumb MC during storage of tin bread based on

395

270 g of wheat flour (Tables 1 and 3) were due to its high crumb to crust ratio. It is suggested

396

that when the crumb to crust ratio is low, as is the case for small bread loaves22 or French

397

bread,21 crumb MC decreases and crumb firming rate increases significantly. It would thus

398

seem plausible that parbaking of such bread types does result in a higher firming rate of FB

399

than of CB bread. To confirm this hypothesis, changes in water mobility during storage of

400

bread loaves with different crumb to crust ratio were investigated. Figure 4 shows the CPMG

401

proton distributions of fresh and stored (either with or without crust) CB bread crumb

402

prepared from different amounts of flour (i.e. 10, 100 or 270 g). As the loaf volume increased,

403

so did the crumb to crust ratio. The decrease in area of population E during storage of bread

404

with crust was more pronounced when crumb to crust ratio decreased (Figure 4). Indeed,

405

during storage of respectively the largest bread loaves and bread loaves prepared from 100 g

406

of flour, crumb MC decreased by approximately 1% and 8% (Table 5). Furthermore, a larger

407

decrease in crumb MC resulted in a stronger dehydration of the gluten network, since merging

408

of populations C and D was more pronounced in smaller bread loaves stored either with or

409

without crust (Figure 4). Moreover, the average mobility of this merged population decreased

410

with decreasing loaf volume (Figures 4b and 4c). With decreasing loaf volume and, thus,

411

crumb to crust ratio, the mobility of population E decreased to a larger extent when crumb

412

was stored with crust (Figure 4). Because this mobility is negatively correlated with crumb 18 ACS Paragon Plus Environment

Page 18 of 37

Page 19 of 37


413

firmness during storage,3 it is assumed that the extent of crumb firming increased with

414

decreasing crumb to crust ratio. As hypothesized earlier, crumb firming is dominated by both

415

AP retrogradation and water redistribution.3 Since AP retrogradation occurred to a similar

416

extent during storage of bread loaves with different volumes (Table 5), differences in the

417

extent of water redistribution are largely responsible for changes in the extent of crumb

418

firming when the bread loaf size was altered. It can therefore be concluded that a low crumb

419

to crust ratio causes a pronounced decrease in crumb MC during storage, resulting in strong

420

dehydration of crumb biopolymer networks and, hence, a large increase in crumb firmness.

421

Since this moisture redistribution is not heat-reversible as mentioned above, it is thus very

422

well possible that PB small or French bread loaves, which have a low crumb to crust ratio,

423

firm more rapidly after final baking than CB bread as described in literature.18-20

424

In conclusion, different temperature-time baking profiles impacted the extent of AM leaching

425

and protein polymerization. When both baking time and the corresponding crumb temperature

426

increase, a more extended starch network and a more developed gluten network in terms of

427

gliadin cross-linking were formed, resulting in lower proton mobility in the gel network and a

428

higher initial crumb resilience. During storage of PB bread, the extent of crumb firming

429

increased with longer baking times of the previous baking phase due to i) more pronounced

430

AP retrogradation (and related moisture redistribution from gluten to starch) and ii) more

431

pronounced moisture redistribution from crumb to crust. These phenomena resulted from

432

respectively i) a more developed AM network in which AM molecules would interact more

433

with each other and interfere less with AP retrogradation and ii) a lower fresh crust MC

434

resulting in a higher driving force for crumb to crust moisture migration. However, despite the

435

occurrence of crumb to crust moisture migration, crumb MC remained high in all bread types

436

as a consequence of the high crumb to crust ratio. During final baking, all retrograded AP

437

crystals melted, resulting in refreshed FB bread. However, moisture redistribution from crumb 19 ACS Paragon Plus Environment


438

to crust and from gluten to starch during intermediate storage of PB bread loaves could not be

439

reversed. Moreover, additional gluten network organization may occur. As a result,

440

biopolymer organization and water distribution differed between fresh PB and refreshed FB

441

bread. Because of the high crumb MC, however, these differences were not reflected in crumb

442

texture, since softness and resilience of refreshed FB bread were restored to their initial values

443

in fresh PB bread. It can therefore be concluded that starch crystallization together with water

444

redistribution, resulting in gluten network dehydration, dominate changes in crumb softness

445

and resilience during storage, while extended starch and gluten (more gliadin incorporation)

446

networks which are flexible (no rigid AP crystals and well hydrated) dominate initial crumb

447

resilience. During storage of refreshed FB bread, the extent of AP retrogradation is

448

determined by the partial baking time and occurs to a similar extent as during intermediate

449

storage of PB bread. The degree to which crumb MC is impacted is largely determined by the

450

crumb to crust ratio of the bread. When this ratio is high, such as for tin bread with a large

451

loaf volume, the decrease in crumb MC due to crumb to crust moisture migration during both

452

intermediate and final storage is limited, resulting in biopolymer networks in bread crumb

453

which remain well hydrated. Consequently, parbaking of large tin bread loaves does not result

454

in higher crumb firming rates after final baking than those of CB bread. However, when the

455

crumb to crust ratio is low, such as for small bread loaves or French bread, the decrease in

456

crumb MC during storage becomes more pronounced and results in an increased rate and

457

extent of crumb firming. These results stress the contribution of moisture (re)distribution to

458

the crumb firming mechanism and, thus, the shelf-life of bread. This is especially important in

459

the case of parbaking, since moisture redistribution is not heat-reversible during final baking.

460

Abbreviations

461

PBx, partially baked for x% of total baking time; FBx-y(’), (standard) fully baked for x% and

462

y% of total baking time during respectively partial and final baking; CB(’), (standard) 20 ACS Paragon Plus Environment

Page 20 of 37

Page 21 of 37


463

conventionally baked; AM, amylose; AP, amylopectin; TD 1H NMR, time-domain proton

464

nuclear magnetic resonance; DSC, differential scanning calorimetry; MC, moisture content;

465

SE-HPLC, size exclusion high-performance liquid chromatography, SDS, sodium dodecyl

466

sulfate; DTT, dithiothreitol; SDS-EP, protein extractability in SDS-containing media; dm, dry

467

matter; T2 relaxation time, transverse relaxation time; FID, free induction decay; CPMG,

468

Carr-Purcell-Meiboom-Gill; SS, disulfide; SH, sulfhydryl; au, arbitrary units; ∆HAP, melting

469

enthalpy of (retrograded) amylopectin.

470

Acknowledgements

471

The authors are grateful to Drs. Phil Latham and Joke Putseys (DSM Food Specialties , Delft,

472

The Netherlands) and Drs. Maarten van Oort and Emmie Dornez (Mauri Research; Made, The

473

Netherlands) for fruitful discussions during planning and execution of the work. Jan A.

474

Delcour is W.K. Kellogg Chair in Cereal Science and Nutrition at KU Leuven.



475

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amylose and amylopectin during retrogradation. Carbohyd. Polym. 2011, 86 (4), 1671-1674.

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quality of part baked breads as related to storage and rebaking conditions. J. Food Sci. 1999,

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574 25 ACS Paragon Plus Environment


575

Funding sources

576

The authors acknowledge DSM Food Specialties (Delft, The Netherlands) and Mauri

577

Research (Made, The Netherlands) for financial support. This work is part of the Methusalem

578

program “Food for the Future” at KU Leuven.

579


Page 26 of 37

Page 27 of 37


580

Figure captions

581

Figure 1

582

Size exclusion high-performance liquid chromatography (SE-HPLC) profiles of protein

583

extracts in sodium dodecyl sulfate (SDS) containing medium of fermented dough, of crumb

584

from fresh bread partially baked for 42.5 (PB42.5), 60 (PB60) or 100% (CB) of total baking

585

time and of crumb from fresh bread fully baked for 42.5 or 60 and 30 or 40% (FB42.5-30 and

586

FB60-40) of total baking time during respectively partial and final baking. Absorbance is given

587

in arbitrary units (au).

588

Figure 2

589

Free induction decay (FID) (a) and Carr-Purcell-Meiboom-Gill (CPMG) (b) proton

590

distributions of crumb withdrawn from fresh and stored (6 days at 23 °C) conventionally

591

baked (CB) bread (100% of total baking time). Amplitude is given in arbitrary units (au).

592

Figure 3

593

Free induction decay (FID) (a) and Carr-Purcell-Meiboom-Gill (CPMG) (b) proton

594

distributions of crumb withdrawn from fresh and stored (6 days at 23 °C) partially baked (PB)

595

bread (42.5% of total baking time) and of refreshed fully baked (FB) bread (42.5 and 30% of

596

total baking time during respectively partial and final baking with an intermediate storage

597

time of 6 days). Amplitude is given in arbitrary units (au).

598

Figure 4

599

Carr-Purcell-Meiboom-Gill (CPMG) proton distributions of crumb withdrawn from fresh and

600

stored (7 days at 23 °C) bread and of crumb from bread stored (7 days at 23 °C) without crust

601

(nc, no crumb to crust moisture migration) prepared from 270 (a), 100 (b), or 10 (c) g of

602

wheat flour.

603



Page 28 of 37

Tables Table 1 Moisture content (MC) of crumb and crust, firmness, resilience, melting enthalpy of (retrograded) amylopectin (AP, ∆HAP) and protein extractability in sodium dodecyl sulfate containing medium (SDS-EP) of crumb from bread baked for 42.5 (PB42.5), 60 (PB60), or 100% (CB) of total baking time and stored for 0, 3 or 6 days at 23 °C.

Baking process

Storage time (days)

MC (%) Crumb

0 44.9 (0.5)aA PB42.5 3 45.2 (0.1)aA 6 44.9 (0.1)aA 0 45.1 (0.3)aA PB60 3 44.7 (0.1)aB 6 44.2 (0.7)aAB 0 44.6 (0.9)aA CB 3 44.5 (0.06)abB 6 43.2 (0.3)bB Standard deviations are indicated between brackets.

Crumb firmness (N)

Crust 22.3 (0.4)aA 29.0 (0.3)bA 29.0 (0.4)bA 20.9 (0.6)aB 27.8 (1.9)bA 28.2 (1.2)bAB 17.7 (0.4)aC 23.8 (0.2)bB 26.4 (0.6)cB

0.8 (0.1)aA 2.2 (0.1)bA 2.7 (0.2)cA 1.0 (0.2)aA 2.4 (0.2)bA 3.3 (0.3)cB 0.9 (0.2)aA 2.9 (0.2)bB 4.4 (0.4)cC

Crumb resilience (%)

∆HAP [J/g crumb (dm)]

47.7 (1.9)aA 34.0 (0.1)bA 28.6 (3.0)cA 54.1 (3.2)aB 36.9 (1.8)bA 32.5 (1.9)cA 54.8 (1.0)aB 34.2 (3.6)bA 36.2 (3.2)bB

0.29 (0.22)aA 2.29 (0.18)bA 2.62 (0.18)bA 0.19 (0.06)aA 2.41 (0.07)bA 3.14 (0.28)cB 0.21 (0.09)aA 2.26 (0.22)bA 3.25 (0.33)cB

SDS-EP (%) 36.3 (1.1)A 22.9 (0.7)B 15.9 (0.4)C

Within one column, values with the same small letter at different storage times of one bread baking process and with the same capital letter at the same storage time for different bread baking processes are not significantly different from each other (P

The Impact of Parbaking on the Crumb Firming Mechanism of Fully

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