Sorption equilibrium moisture and isosteric heat of adsorption of Chinese dried wheat noodles

Journal of Stored Products Research 67 (2016) 19e27

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Sorption equilibrium moisture and isosteric heat of adsorption of Chinese dried wheat noodles Yang Li a, b, Xin Wang b, Ping Jiang a, Xing-jun Li a, * a b

Academy of the State Administration of Grains, Beijing 100037, China College of Biological and Agricultural Engineering, Jilin University, Chuangchun 130000, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 July 2015 Received in revised form 15 January 2016 Accepted 31 January 2016 Available online xxx

Equilibrium moisture content (EMC) data for dried wheat noodles of ten Chinese varieties were collected by a gravimetric method at 11e96% equilibrium relative humidity (ERH) and 15
C, 20
C, 25
C, 30
C, and 35
C. Five models were fitted to the sorption data, namely the modified Chung Pfost equation (MCPE), modified Henderson equation (MHE), modified Guggenheim Anderson deBoer equation (MGAB), modified Oswin equation (MOE), and a polynomial equation. The best fitting equations were MGAB and the polynomial equation. At a constant ERH, the EMC decreased with increasing temperature, despite the minor effect of temperature on the sorption isotherms of dried noodles. Initially, the isosteric heats of adsorption for dried wheat noodles decrease rapidly with increasing sample moisture content (m.c.); however, after the moisture content is more than 15% of the dry basis (d.b.), they decrease slowly with increasing m.c. The heat of vaporization of Chinese dried wheat noodles approaches the latent heat of pure water at a moisture content of ~20% d.b., which is ~2500 kJ/kg. The isosteric heats of sorption of Chinese dried noodles predicted by MCPE and MHE models at lower temperatures were higher than those at higher temperatures. When the equilibrium relative humidity (ERH) is 60%, the safe-storage moisture content of Chinese dried wheat noodles are 11.74% and 11.57% d.b. at 25
C and 35
C, respectively. Among ten varieties of dried wheat noodles, the egg-flavoured noodle had the highest onset temperature (To), peak temperature (Tp), and conclusion temperature (Tc) of gelatinization, but the golden-silk egg noodles had the highest peak enthalpy of gelatinization. The gelatinization To, Tp, and Tc of golden-silk egg noodles were the lowest. Most of the ten varieties of dried wheat noodles demonstrated similar thermal properties and hygroscopic behaviour. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Equilibrium moisture content Chinese dried wheat noodles Sorption isotherm Isosteric heat Thermal properties

1. Introduction Noodles are a traditional food in China and other Asian countries, and have been flavoured by the Chinese people for over 2000 years. Noodles constitute almost 40% of wheat products in Asia (Janto et al., 1998; Hu et al., 2006). Dried noodle is a hygroscopic food product made of wheat flour and salt (Huang and Lai, 2010). The water vapour transmission between a hygroscopic product and the surrounding environment is a physical phenomenon which may have adverse effects on the quality of the products during storage, as most food products are vulnerable to spoilage in highmoisture conditions (Deman, 1999). Usually a food product is manufactured in one climatic region and is sold in diverse climatic

* Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (X.-j. Li). http://dx.doi.org/10.1016/j.jspr.2016.01.007 0022-474X/© 2016 Elsevier Ltd. All rights reserved.

regions where the water vapour pressures (relative humidities) differ from each other. In this circumstance water vapour transmission between food products and the surrounding environment occurs at different speeds (Bruin and Berg, 1981). Therefore noodle processors must have adequate knowledge of the quality of dried noodles during storage with regard to moisture ingress, and select an appropriate packing material for dried noodles in order to prevent adverse consequences due to moisture migration (Cooksey, 2004). However, there are few reports on the moisture sorption isotherms of Chinese dried wheat noodles. The quality of wheat noodles and instant noodles depends mainly on their physical, chemical, and microbiological stability (Menkov et al., 2005). This stability is a consequence of the relationship between the EMC of noodles and their corresponding water activity or equilibrium relative humidity (ERH) at a given temperature. Inazu et al. (2001) determined desorption isotherms of Japanese noodles (udon) at 20
C, 30
C, and 40
C, and described

20

Y. Li et al. / Journal of Stored Products Research 67 (2016) 19e27

Nomenclature aw water activity A,B,C,D,E,F,G model coefficients d.b. dry basis ERH equilibrium relative humidity EMC equilibrium moisture content m.c. moisture content mi experimental value mmi the average of experimental value mpi the predicated value MRE the mean relative percentage error (%) n the number of observations hs isosteric heat of sorption (KJ/Kg)

them by several isotherm models, such as the GAB, Oswin, and Smith equations. Navaratne (2013) determined the 20
C moisture sorption isotherm of wheat noodles by a gravimetric method at 32.5e92.3% relative humidity (r.h.), and, from this isotherm, determined the maximum allowable moisture content for safe keeping. It is imperative to evaluate the allowable moisture content for longer-term storage of Chinese dried wheat noodles. Knowledge of the energy requirement, state, and mode of moisture sorption is important in designing effective noodle drying and storage systems. The analysis of food product moisture sorption isotherms by a thermodynamic approach could supply information about energy requirements during dehydration, microstructure, surface physical phenomena, moisture properties, and sorption dynamics (Tsami, 1991; Wang and Bremman, 1991; Fasima et al., 1999; Thorpe, 2001). Isosteric heat of sorption, often referred to as differential heat of sorption (hw), is useful in estimating the state of water adsorbed by the solid particles (Fasina et al., 1999). Knowledge of hw is of great importance when designing equipment for dehydration processes. The level of material moisture content at which the net isosteric heat of sorption approaches the latent heat of vaporization of water is often regarded as an indication of the amount of ‘bound water’ in the product € € (Kiranoudis et al., 1993; Oztekin and Soysal, 2000; Li, 2012). Oztekin and Soysal (2000) adopt the EMC data of durum wheat, soft wheat and hard wheat from ASAE Standards (1994) and compared the sorption isosteric heats among these wheat types. However, few reports deals with the sorption isosteric heat of wheat noodles. The objectives of this study were to collect the EMC/ERH data of Chinese dried wheat noodles at 11e96% ERH and 15
Ce35
C, and then determine a suitable model for describing their isotherms, and calculate the maximum allowable moisture content for safe storage and the isosteric heat of sorption. The overall aim was to provide a guideline for the drying process and longer-term storage of Chinese dried wheat noodles.

2. Materials and methods 2.1. Dried noodle samples and experimental procedures The ten varieties of dried wheat noodles used in this work were collected from two major food plants in China in July 2014. Their moisture contents and nutritional sign are shown in Table 1. Their average moisture content (m.c.) was 10.81% d.b. For the sorption experiment, all noodle threads were cut to 4 cm. The equilibrium moisture contents of each species of dried

hv hw R2 Ps r.h. RSS SE T t Tc To Tp w.b. △H

latent heat of vaporization of free water (KJ/Kg) differential heat of wetting (KJ/Kg) coefficient of determination saturate vapour pressure (Pa) relative humidity residue sum of squares standard error absolute temperature (K) temperature (
C) conclusion temperature of gelatinization (
C) onset temperature of gelatinization (
C) peak temperature of gelatinization (
C) wet basis enthalpy of gelatinization

noodle at five constant temperatures (15
C, 20
C, 25
C, 30
C, and 35
C) over an equilibrium relative humidity of 11.3e96.0% were determined as described in our previous reports (Li et al., 2011; Li, 2012) using the static gravimetric method. Briefly, 27 widemouthed glass bottles (250 mL), each containing 65 mL saturated salt solution, were kept in a temperature-controlled cabinet to maintain nine groups of different ERH levels in the range 11.3e96.0%. The salt solutions included lithium chloride, potassium acetate, magnesium chloride, potassium carbonate, magnesium nitrate, cupric chloride, sodium chloride, potassium chloride, and potassium nitrate. The measurement at each relative humidity condition was conducted in triplicate, and a total of 135 bottles were used in an experiment for five sorption isotherms of each noodle variety. Each sample of dried wheat noodles (～5.0000 g) was placed into a small bucket (3 cm diameter and 4 cm length) made from copper wire gauze, and hung into the wide mouth glass bottle on a copper wire pothook under a rubber plug, at 2e3 cm above the saturated salt solutions. Beyond fifteen days after exposing the samples to the saturated vapour at 35
C, the copper wire buckets with samples were weighed every other day until the change in mass between two successive readings was less than 2 mg. When this constant stage was reached, the moisture content of the sample was defined as the EMC and was determined by the oven method (AOAC, 1980). The sample was dried to a constant weight at 103.0 ± 0.5
C for 4 h. When the samples were exposed at a lower temperature, they were left longer to equilibrate. However, the noodles exposed to the saturated potassium nitrate and potassium chloride solutions for 3e6 days at higher temperatures were susceptible to fungal growth, and were removed immediately if any mould was visually observed.

2.2. Analysis of the sorption data The experimental EMC/ERH data were used to make isotherm curves in Kaleidagraph for Mac 4.1.3 software, using ERH data on the x-axis and EMC data on the y-axis. Five equations were adopted to fit the EMC data of dried noodles (Table 2). The fitting was conducted using the non-linear regression procedure in SPSS 13.0 for Windows (SPSS Inc., 2006). Determination coefficient (R2), residue sum of squares (RSS), the standard error (SE), and mean relative percentage error (MRE) were used as the criteria to determine the best equation for the EMC/ERH data. The equations (1)e(4) were used for calculating R2, RSS, SE, and MRE, respectively.

Y. Li et al. / Journal of Stored Products Research 67 (2016) 19e27

21

Table 1 Ingredient composition of Chinese dried wheat noodles. No.

Noodle varieties

m.c. (% d.b)

Energy (KJ/100 g)

Protein (g/100 g)

Fat (g/100 g)

Carbohydrate (g/100 g)

Sodium (mg/100 g)

1 2 3 4 5 6 7 8 9 10

Carrot noodle Celery noodle Spinach noodle Tomato noodle Golden-silk egg Ultra-cool Country of origin Mushroom flavour Classical flavour Egg flavour

10.67 10.99 9.40 11.28 9.76 10.56 10.05 11.45 12.55 11.40

1445 1445 1445 1445 1490 1482 1482 1450 1450 1460

13.2 13.2 13.2 13.2 12.0 11.5 11.5 11.0 11.0 11.5

0 0 0 0 1.0 1.0 1.0 1.2 1.2 1.2

70.4 70.4 70.4 70.4 72.7 73.2 73.2 72.0 72.0 72.0

368 368 368 368 436 436 436 672 672 672

Table 2 EMC/ERH equations used in this study. Equation name (abbreviation)

Formulaa

Reference

Modified-Chung-Pfost (MCPE) Modified Guggenheim- Andersonede Boer (MGAB)

ERH ¼ exp½­A,expðC,EMCÞ=ðB þ tÞ        2 1 A A 2þ Ct 1 f 2þ Ct 1 4ð1C=tÞg2 EMC EMC ERH ¼ 2Bð1C=tÞ

Pfost et al., 1976 Jayas and Mazza, 1993

Modified Henderson (MHE)

ERH ¼ 1  exp½Aðt þ BÞEMCC 

Modified Oswin (MOE)

ERH ¼

Polynomial

EMC ¼ A,ERH þ B,ERH2 þ C,ERH þ D,ERH2 t þ E,ERH,t þ F,t þ G

Thompson et al., 1986 Chen and Morey, 1989

1 1þ½ðAþBtÞ=EMCC 3

Li and Jiang, 2015

ERH represents equilibrium relative humidity(%), EMC is equilibrium moisture content (% dry basis), t is temperature (
C). A, B, C, D, E, F, and G are the coefficients of equations. a

R2 ¼ 1 

n  X 2 mi  mpi

,

i¼1

n X

ðmi  mmi Þ2

(1)

i¼1

n  X 2 RSS ¼ mi  mpi

(5)

hv ¼ 2501:33  2:363  t

(6)

  6800  exp  t þ 273:15 ð273:15 þ tÞ5 6  1025

(3)

Ps ¼

(4)

  dPs Ps 6800 5 ¼  t þ 273:15 dT ðt þ 273:15Þ

(8)

 vr:h: A  r:h: ¼  expðC  MÞ vT M ðt þ BÞ2

(9)

i¼1

 n  m  mpi  100 X  i  MRE% ¼ n i¼1  mi 

 hs ps dT vr:h:  ¼1þ  vT M hv r:h: dPs

(2)

i¼1

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n uX  2 . ðn  1Þ mi  mpi SE ¼ t

1987, 1991; Thorpe, 2001). The differential heat of sorption (hs) for dried noodles was calculated by the equations below.

where mi is the experimental value, mpi is the predicted value, mmi is the average of experimental values, and n is the number of observations. The fit of an equation to the EMC/ERH data of dried wheat noodles was considered good enough when MRE was less than 10% (Aguerre et al., 1989). The differences between the predicted EMC by a given equation and the measured EMC were plotted against measured EMC values to give residual plots. The residual plots were assessed for patterns or randomness.

2.3. Determination of the isosteric heat of sorption The isosteric heat of sorption for dried wheat noodles was assayed essentially according to our previous reports (Li et al., 2011; Li, 2012). The total energy required to remove a unit mass of water from dried wheat noodles, the differential heat of sorption (hs), was partitioned into two components, which were the latent heat of vaporization of free water (hv) and the differential heat of wetting (hw). It is customary to calculate the ratio, hs/hv because isosteric refers to a material with the same chemical composition (Hunter,

 vr:h: ¼ n vT M

 1þ

1 AþBt M

(  )  BC A þ Bt C1 C o M M 2

 i h vr:h: ¼ AM C exp  Aðt þ BÞM C  vT M

(7)

(10)

(11)

Equation (5) was used to calculate the ratio hs =hv , provided that    could be evaluated by equations (8) and (9), dPs =dT and vr:h: vT  M

respectively. The hv of free water in equation (6) was dependent on temperature. The saturated vapour pressure (Ps) was calculated by    equation (7). The vr:h: vT  depends on the sorption isotherm equation M

used, and the modified Chung Pfost equation (MCPE) in equation (9), the modified Oswin equation (MOE) in equation (10), and the modified Henderson equation (MHE) in equation (11) were

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Y. Li et al. / Journal of Stored Products Research 67 (2016) 19e27

adopted in this study, respectively.

Table 3 The experimental data of average EMC for dried wheat noodles.

2.4. Determination of the gelatinization properties of dried wheat noodles The thermal properties of dried wheat noodle powder were determined with a differential scanning calorimeter (DSC) 200F3 (Netzsch, Germany). The dried wheat noodle samples were milled with an 800 mm screen. The sample (5.0e5.2 mg) was weighed in an aluminium crucible and distilled water was added to give a water/sample ratio of 2:1. The aluminium crucible was sealed and equilibrated at 4
C overnight. The DSC temperature was raised from 20 to 110
C with a heating rate of 10
C/min. Each sample was run in triplicate. The data, such as onset temperature (To), peak temperature (Tp), conclusion temperature (Tc) and enthalpy of noodle gelatinization, were subjected to analysis of variance and the means were separated by Duncan's multiple range test at P  0.05 or P  0.01 using the software SPSS 13.0 for Windows (SPSS Inc., 2006).

Saturated salt solution

Lithium chloride Potassium acetate Magnesium chloride Potassium carbonate Magnesium nitrate Cupric chloride Sodium chloride Potassium chloride Potassium nitrate

EMC (% d.b.) 15
C

20
C

25
C

30
C

35
C

5.37 7.79 9.31 10.82 11.85 13.05 15.77 21.49 31.59

4.51 6.98 8.65 10.46 11.46 12.84 15.32 20.32 28.64

4.31 6.65 8.42 10.36 11.17 12.62 15.18 19.69 27.59

3.97 6.17 8.00 9.73 10.91 12.39 14.87 18.99 25.92

3.67 5.72 7.53 9.28 10.35 12.12 14.76 18.11 23.89

3. Results

At a constant ERH, EMC decreased with an increase in temperature (Table 3). Fig. 1B shows the effect of noodle varieties on the 20
C sorption isotherms. Among the ten varieties of dried wheat noodle, the classically flavoured noodle had the highest EMC value, and the EMC values of golden-silk egg noodle were lowest. The EMC data of most of the dried noodles were similar.

3.1. The experimental EMC/ERH data of dried wheat noodles

3.2. Fitting of sorption equations to experimental sorption data

The average equilibrium moisture contents of sorption at nine relative humidities ranging from 11.3 to 96.0% and five temperatures (15, 20, 25, 30, and 35
C) were obtained for ten varieties of dried wheat noodles and are given in Fig. 1A. The curves of sorption for all ten noodle varieties were sigmoidal in shape. At a constant temperature, the EMC increased with an increase in ERH, especially rapidly when ERH equals to 70%.

The results of nonlinear regression analyses of fitting the sorption equations to the experimental data of noodle sorption isotherms are shown in Tables 4 and 5. The statistical parameters such as R2, RSS, SE, and MRE, used in comparing the equations are also given in Tables 4 and 5 The residual plot was also evaluated for goodness-of-fit for each equation. The four commonly used equations, MCPE, MHE, MOE, and modified Guggenheim Anderson deBoer equation (MGAB), and our developed polynomial equation (Table 2), better fit the experimental data of noodle sorption isotherms in the range of 11.3e96.0% ERH. Further comparisons of the sorption equations with the form of M ¼ f ðr:h:; tÞ or r:h: ¼ f ðM; tÞ for ten sets of dried wheat noodle isotherm data are given in Table 6. The average values of R2 and error parameters (RSS, SE, and MRE) for the ten sets of isotherm data were calculated. For the form M ¼ f ðr:h:; tÞ, the accuracy of the equations was ranked in the following order: polynomial, MGAB, MOE, MCPE, and MHE; but for r:h: ¼ f ðM; tÞ, the order was: MCPE, MGAB, MHE, and MOE. The polynomial model of the form of M ¼ f ðr:h:; tÞ, and the MCPE, MGAB models of the form of r:h: ¼ f ðM; tÞ were considered to better describe the equilibrium moisture data of the ten dried wheat noodles varieties in the range of 11.3e96.0% ERH. The better fitted coefficients of equations for average sorption isotherms of dried wheat noodles are summarized in Table 7. These calculated coefficients can be used for describing the process of wheat noodle drying, and improving physical control of moisture in noodle storage (Table 8).

35 15 °C 20 °C 25 °C 30 °C 35 °C

EMC (% d.b.)

30 25 20 15 10 5

A

0 0

20

40

60

80

100

ERH (%)

35 30

Carrot noodle Celery noodle Spinach noodle Tomato noodle Golden-silk egg Ultra-cool Country of origin Mushroom flavour Classical flavour Egg flavour

EMC (% d.b.)

25 20 15 10 5

B

0 0

20

40

60

80

100

ERH (%) Fig. 1. Influence of temperature (
C) and varieties on the sorption isotherms of Chinese dried wheat noodles.

3.3. Prediction of moisture sorption isotherms and safe storage m.c. by the best fitting equation Figs. 2 and 3 show the predicted sorption isotherms of dried wheat noodles by different models. Temperature had a relatively bigger effect on the sorption isotherms of dried noodles predicted by the MGAB and polynomial models when the ERH was below 60%, but above this value the effect of temperature was minor. The MHE model was able to show the effect of temperature on the sorption isotherms of dried noodles when the ERH was above 60%. The predicted isotherms by the MCPE model distinguished the effect of temperature.

Y. Li et al. / Journal of Stored Products Research 67 (2016) 19e27

23

Table 4 The estimation of the coefficients of four commonly used equations in form of M ¼ f ðr:h:; tÞ fitted to the sorption isotherm data of ten varieties of dried wheat noodles. Sample no.

Equation

1

MCPE MHE MOE MGAB MCPE MHE MOE MGAB MCPE MHE MOE MGAB MCPE MHE MOE MGAB MCPE MHE MOE MGAB MCPE MHE MOE MGAB MCPE MHE MOE MGAB MCPE MHE MOE MGAB MCPE MHE MOE MGAB MCPE MHE MOE MGAB

2

3

4

5

6

7

8

9

10

R2

Equation coefficients A

B

C

937.173 3.47E-05 10.193 6.255 732.168 4.95E-05 10.219 6.183 601.11 7.83E-05 9.201 5.476 413.276 9.41E-05 11.461 6.633 469.362 1.19E-04 9.352 5.524 926.737 4.26E-05 10.861 6.269 548.798 7.68E-05 11.079 6.318 643.832 5.11E-05 10.578 6.159 582.734 5.96E-05 11.217 6.398 442.803 9.48E-05 10.119 5.788

188.178 350.858 9.59E-03 0.8094 149.109 268.438 4.84E-03 0.8152 146.46 302.704 7.48E-03 0.8434 79.303 129.474 1.76E-02 0.8202 109.674 188.273 4.41E-03 0.8371 191.129 288.452 6.48E-03 0.8303 103.089 147.051 1.26E-02 0.8201 143.582 323.342 9.66E-03 0.8424 133.429 279.925 7.31E-03 0.8509 114.188 282.725 7.24E-03 0.8669

0.172 1.678 2.964 671.291 0.169 1.639 2.911 617.481 0.165 1.464 2.659 524.447 0.153 1.599 2.862 504.584 0.167 1.479 2.676 424.112 0.159 1.631 2.886 1091.911 0.163 1.652 2.929 745.051 0.151 1.527 2.737 775.531 0.139 1.488 2.679 819.153 0.139 1.357 2.495 567.503

Error parameters

0.9779 0.9678 0.9847 0.9848 0.9766 0.9672 0.9837 0.9845 0.9696 0.9666 0.9821 0.9866 0.9731 0.9656 0.9792 0.9793 0.9748 0.9705 0.9851 0.9833 0.9657 0.9547 0.9847 0.9819 0.9776 0.9676 0.9906 0.9862 0.9647 0.9574 0.9841 0.9839 0.9586 0.9531 0.9816 0.9815 0.9556 0.9571 0.9809 0.9842

Residual plot

RSS

SE

MRE%

38.1655 55.7333 26.1124 26.2799 42.1597 58.9164 29.2774 27.9079 57.6759 63.2711 33.8795 25.3847 59.3068 76.0599 45.9452 45.8021 46.2624 54.169 27.4354 30.6651 70.2151 92.8147 31.3394 36.9176 43.0598 62.1904 18.0783 26.5899 80.5351 97.2828 36.2677 36.8878 111.6288 126.5024 49.6571 49.7985 120.3092 116.0711 51.7139 42.6955

0.9087 1.3269 0.6217 0.6257 1.0038 1.4028 0.6971 0.6645 1.3732 1.5065 0.8067 0.6044 1.4121 1.8109 1.0939 1.0905 1.1015 1.2897 0.6532 0.7301 1.6718 2.2099 0.7462 0.8789 1.0252 1.4807 0.4304 0.6331 1.9175 2.3163 0.8635 0.8783 2.6578 3.0119 1.1823 1.1857 2.8645 2.7636 1.2313 1.0166

5.9987 8.7299 6.1111 6.6047 6.3237 8.8512 6.8642 7.0555 8.9104 10.5088 7.8123 7.3217 7.0926 9.2294 7.4605 7.1972 7.5045 9.1993 8.8837 8.3544 7.9573 10.96 5.3584 6.9941 6.7627 9.4164 5.4264 6.2085 8.8389 11.3379 6.6114 7.2786 9.7559 12.2857 6.9789 7.6975 11.9876 13.7912 8.5903 8.2682

Patterned Random Patterned Random Patterned Patterned Random Random Patterned Random Patterned Random Random Random Patterned Random Patterned Random Patterned Random Patterned Random Patterned Random Patterned Random Patterned Random Patterned Random Patterned Random Patterned Random Patterned Random Patterned Random Patterned Random

Table 5 The estimation of the coefficients of the polynomial equations fitted to the sorption isotherm data of ten varieties of dried wheat noodles. Sample

Equation coefficients

No.

A

B

C

D

E

F

G

1 2 3 4 5 6 7 8 9 10

101.281 104.817 107.963 114.393 108.082 117.284 104.486 127.501 140.802 143.307

132.741 135.065 134.818 149.319 134.036 145.561 123.611 165.946 181.824 183.945

63.005 62.649 59.968 72.208 59.245 63.169 52.769 75.032 81.316 81.147

1.76E-02 9.22E-02 1.55E-01 7.11E-03 3.51E-01 2.40E-01 4.60E-01 4.27E-02 6.21E-03 7.79E-03

5.72E-02 1.62E-01 2.19E-01 1.10E-02 4.03E-01 3.08E-01 4.92E-01 5.80E-02 8.16E-02 9.41E-02

6.54E-02 8.92E-02 9.79E-02 7.00E-02 1.36E-01 1.07E-01 1.49E-01 8.55E-02 9.04E-02 9.81E-02

3.52E-02 1.66E-01 1.29E-01 7.37E-01 2.15E-02 1.03Eþ00 2.30Eþ00 5.05E-01 7.82E-01 1.49Eþ00

a

R2

Error parametersa RSS

SE

MRE%

0.9931 0.9931 0.9923 0.9883 0.9906 0.9911 0.9953 0.9907 0.9877 0.9895

12.0752 12.3469 14.6203 25.8904 17.2361 18.3475 9.0929 21.1789 33.1279 28.3621

0.3178 0.3249 0.3847 0.6813 0.4536 0.4828 0.2393 0.5574 0.8718 0.7464

4.2754 4.5541 5.4575 6.3201 6.1684 4.4098 3.3773 5.9184 6.7616 7.2319

All the residual plot is random.

Fig. 3 shows the predicted sorption isotherms of noodle varieties at 20
C and 30
C by MGAB. Among the ten varieties of dried wheat noodle, the classically flavoured noodle had the highest EMC values, and the EMC values of golden-silk egg noodle were lowest. The EMC data of most of the dried noodles were similar. Table 9 shows the predicted safe storage moisture content of dried noodle under different combinations of temperature and r.h. When the ERH was 60%, the predicted safe storage moisture contents of dried noodles by MGAB were 11.74% and 11.57% d.b. at 25
C

and 35
C, respectively. When the ERH was 65%, the predicted safe storage moisture contents of dried noodles by MGAB were 12.88% and 12.74% d.b. at 25
C and 35
C, respectively. 3.4. Isosteric heat of dried noodle sorption The isosteric heat of sorption (hs) was calculated from equations 5e11. Fig. 4 shows the influence of moisture content on the isosteric heats of sorption of dried noodles as determined from the sorption

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Y. Li et al. / Journal of Stored Products Research 67 (2016) 19e27

decreased smoothly with increasing moisture content. At moisture contents below 15%, the isosteric heats of noodle sorption at lower temperatures were similar to those at higher temperatures, but when the moisture contents were above 15%, the isosteric heats of noodle sorption at lower temperatures were higher than those at higher temperatures. When the MHE model was employed to calculate the isosteric heats of sorption of dried noodle, equation (11) was adopted. The isosteric heats of sorption for dried noodles from 15 to 35
C almost decreased linearly with an increase in noodle moisture content. As for the MCPE model, the isosteric heats of sorption of noodles predicted by MHE at lower temperatures were higher than those at higher temperatures.

Table 6 Summary of the results of fitting equations to the ten data sets of dried wheat noodle sorption. Model function

M ¼ f ðr:h:; tÞ

r:h: ¼ f ðM; tÞ

a

Statistical parametersa

Equation

MCPE MHE MOE MGAB Polynomial MCPE MHE MOE MGAB

Order

R2

RSS

SE

MRE%

0.9694 0.9628 0.9837 0.9836 0.9912 0.9845 0.9777 0.9842 0.9827

66.9318 80.3011 34.9706 34.8929 19.2278 5.02E-02 7.21E-02 5.01E-02 5.59E-02

1.5936 1.9119 0.8326 0.8308 0.5060 1.19E-03 1.73E-03 1.19E-03 1.33E-03

8.1132 10.4309 7.0097 7.2980 5.4475 5.7554 7.5230 8.4811 7.2621

4 5 3 2 1 1 3 4 2

Each statistical parameter is the mean of ten data sets.

Table 7 The better fitting coefficients of MGAB, MCPE, MHE, MOE equations for average sorption isotherms of ten varieties of dried wheat noodles. Model function

Equation

Equation coefficients A

B

C

D

E

F

G

M ¼ f ðr:h:; tÞ

Polynomial MGAB MOE MCPE MHE MCPE MGAB MHE MOE

116.991 6.079 10.435 586.267 6.960E-05 601.171 7.015 5.225E-05 11.622

148.685 0.8352 1.613E-03 127.508 231.261 93.715 0.7777 120.309 4.520E-02

67.053 652.241 2.773 0.157 1.547 0.187 413.024 1.901 2.818

1.25E-01

1.89E-01

9.87E-02

2.22E-02

r:h: ¼ f ðM; tÞ

Table 8 The predicted data of average EMC for ten varieties of dried wheat noodles by polynomial model. Saturated salt solution

Lithium chloride Potassium acetate Magnesium chloride Potassium carbonate Magnesium nitrate Cupric chloride Sodium chloride Potassium chloride Potassium nitrate

EMC

(% d.b.)

15
C

20
C

25
C

30
C

35
C

5.03 8.41 9.64 10.16 11.29 13.46 15.97 22.30 30.49

4.24 7.74 9.13 9.82 10.63 13.04 15.65 20.89 28.75

3.85 7.41 8.85 9.61 10.31 12.64 15.43 20.22 27.63

3.45 6.79 8.55 9.41 10.01 12.37 15.21 19.63 24.95

3.06 6.32 8.26 9.21 9.70 12.09 14.99 19.11 22.93

isotherms. When the MCPE model was employed to calculate the isosteric heats of dried noodle sorption, the isosteric heats of sorption for dried noodles decreased parabolically and rapidly with an increase in noodle moisture content up to a moisture content of 20%. However, above 20%, the isosteric heats decreased smoothly with increasing moisture content. At moisture contents below 22.5%, the isosteric heats of noodle sorption at lower temperatures were higher than those at higher temperatures. At a moisture content of 20%, the isosteric heats of noodle sorption were close to that of pure water. When the MOE model was employed to calculate the isosteric heats of dried noodle sorption, equation (10) was adopted. At a moisture content below 8%, the isosteric heats for dried noodle sorption from 20 to 35
C increased rapidly with an increase in noodle moisture content, but above 8% m.c. they decreased rapidly until a moisture content of 20% was reached, thereafter they

3.5. Changes in thermal properties of dried noodles Table 10 shows the thermal properties of the powder ground from ten varieties of dried noodles. Among the ten varieties of dried wheat noodles, the egg-flavoured noodles had the highest To, Tp, and Tc of gelatinization, but the golden-silk egg noodles had the highest peak enthalpy of gelatinization. The gelatinization To, Tp, and Tc of golden-silk egg noodles were lowest. Most of these ten varieties of dried noodles had similar thermal properties. 4. Discussion Inazu et al. (2001) found that the desorption isotherms of Japanese noodles (udon) at 20
C, 30
C, and 40
C were satisfactorily fitted by the GAB, Oswin, and Smith equations. The binding energy of moisture calculated by Clausius-Clapeyron equation was negligibly small above a moisture content of 16.3% d.b. In this study, we conclude that a polynomial model with the form of M ¼ f ðr:h:; tÞ, and the MCPE and MGAB equations with the form r:h: ¼ f ðM; tÞ better describe the equilibrium moisture data of ten Chinese dried wheat noodles varieties in the range of 11.3e96.0% ERH. Due to their similar thermal properties, most of these ten noodles varieties had similar moisture sorption isotherms. The water activity of dried noodle has a strong influence on its quality properties like cracks and microbiological stability (Shibata et al., 1976; Hu et al., 2006). Dried noodles belong to low water activity food with 0.5～0.6 of aw (Zhu et al., 1998). Basic methods for decreasing noodle aw are drying, and adding salt or sugar to bind water molecules (Huang and Lai, 2010). In this study, when the ERH was 50%, the safe storage moisture contents for Chinese dried wheat noodles at 25
C and 35
C were 9.91% and 9.71% d.b., respectively, but when the ERH was 60%, the values were 11.74%

Y. Li et al. / Journal of Stored Products Research 67 (2016) 19e27

20

25 20

15

15

10

10

5

MGAB

MCPE

0

0 30

15 °C 20 °C 25 °C 30 °C 35 °C

15 °C 20 °C 25 °C 30 °C 35 °C

25

EMC (% d.b.)

5

20

30 25 20

15

15

10

10

5

MHE

Polynomial

0 0

20

40

60

80

0

20

40

60

80

EMC (% d.b.)

EMC (% d.b.)

25

30

15 °C 20 °C 25 °C 30 °C 35 °C

15 °C 20 °C 25 °C 30 °C 35 °C

EMC (% d.b.)

30

25

5 0

ERH (%)

ERH (%)

Fig. 2. The predicted moisture sorption isotherms for ten varieties of dried wheat noodles by the MGAB, MCPE, MHE, and polynomial models.

35 20°C

30

Carrot noodle Celery noodle Spinach noodle Tomato noodle Golden-silk egg Ultra-cool Country of origin Mushroom flavour Classical flavour Egg flavour

EMC (% d.b.)

25 20 15 10 5

A

0 0

20

40

60

80

100

ERH (%)

30 30°C

Carrot noodle Celery noodle Spinach noodle Tomato noodle Golden-silk egg Ultra-cool Country of origin Mushroom flavour Classical flavour Egg flavour

25

EMC (% d.b.)

20 15 10 5

B

0 0

20

40

60

80

100

ERH (%) Fig. 3. The predicted moisture sorption isotherms for ten varieties of dried wheat noodles by the MGAB model at 20
C and 30
C.

Table 9 The predicted safe storage moisture content of dried noodle by MGAB model under different combination of temperature and r.h. ERH %

Safe storage moisture content (% d.b.) 15
C

20
C

25
C

30
C

35
C

40
C

50 60 65 70

10.12 11.92 13.05 14.40

10.02 11.83 12.97 14.33

9.91 11.74 12.88 14.25

9.81 11.65 12.80 14.17

9.71 11.57 12.72 14.09

9.61 11.48 12.64 14.02

and 11.57% d.b., respectively. The initial moisture contents of ten varieties of dried wheat noodles averaged (10.81 ± 0.93)% d.b, which represents the safe moisture content for dried wheat noodles transported to all regions of China. In the range of 33e87% ERH, the EMC of Chinese dried wheat noodles at 20
C was 0.4e1.08% d.b. higher than at 30
C. These results were similar to the reports by Shibata et al. (1976) on dried Japanese noodles (udon). The safe storage m.c. of udon was 12.79%, with EMCs at 20
C in the range of 40e80% r.h. which were 0.6e0.8% d.b. higher than at 30
C. Japanese udon is obtained by using a high proportion of partial waxy wheat flour, which provides a soft, elastic texture because of its low amylose content, high peak paste viscosity, and high gelatinization temperature and enthalpy. For Chinese dried white salted noodles, consumers prefer a firmer texture to Japanese udon (Huang and Lai, 2010; Hu et al., 2006; Epstein, 2002). The present study shows that the Chinese dried noodles had a gelatinization temperature of 65.9e68.1
C and an enthalpy of gelatinization of 4.413e6.278 J/g. Our study shows that the maximum allowable moisture content of Chinese dried wheat noodle for safe keeping is 12% d.b. when the environmental temperature ranges from 15
C to 40
C and ERH is 60%. In practice, in the Asian region, the dried noodle producer usually hangs raw fresh noodles under the sun or in a drying chamber with temperature and humidity control to reduce noodle

Y. Li et al. / Journal of Stored Products Research 67 (2016) 19e27

Isosteric heat of sorption (KJ/Kg)

26

15 °C 20 °C 25 °C 30 °C 35 °C

3200 3000 2800 2600

MCPE

Isosteric heat of sorption (KJ/Kg)

2400 15 °C 20 °C 25 °C 30 °C 35 °C

2800 2700 2600 2500

MOE

Isosteric heat of sorption (KJ/Kg)

2400 15 °C 20 °C 25 °C 30 °C 35 °C

2800 2700 2600 2500

MHE 2400 0

5

10

15

20

25

30

35

Moisture content (% dry basis) Fig. 4. Comparison of the sorption isosteric heats of dried wheat noodles predicted by different models.

sensitivity to high atmospheric water vapour, and this hydrophobic phenomenon of wheat noodles may be due to the gluten present in the noodle strings. However, we found that ten varieties of dried wheat noodles developed mould between 3 and 6 days after the samples were exposed to saturated potassium chloride or potassium nitrate solutions at 30e35
C, i.e. the humidity level was 85e96% ERH. Having results on isosteric heats is convenient for computational purposes related to the drying and storage of Chinese wheat noodles. In this study, the heat of sorption for dried wheat noodles analysed by the MCPE model showed a rapid increase at moisture contents below 20%, and their values were higher than the latent heat of vaporization of water. When the moisture content was over 20%, there was no significant difference between the isosteric heat of dried noodle sorption and the latent heat of vaporization of water. Similar trends were reported for the isosteric heats of melon seeds and cassava (Aviara and Ajibola, 2002), starch powder (AlMuhtaseb et al., 2004), brussels sprouts (Irzyniec and Klimczak, € rul, 2006). In the study of Oztekin 2003) and tea (Arslan and Tog and Soysal (2000), the heat of sorption of wheat grains approached that of pure water at a moisture content of about 20%. Our previous reports (Li et al., 2011; Li, 2012) showed this m.c. was around 17.6% for wheat grains, close to those of alfalfa pellets (16%), gari (15%), winged bean seed (15%) and tea (15%) (Aviara and rul, 2006). In the Ajibola, 2002; Fasina et al., 1999; Arslan and Tog present study, when the heat of sorption of dried wheat noodles approached that of pure water, their moisture content was 20%, similar to that of wheat grains. As suggested, a decrease in the isosteric heats with higher amounts of sorbed water can be quantitatively explained by considering that, initially, sorption occurs on the most active available sites producing high interaction energy. As these sites become occupied, sorption occurs on the less active ones, resulting in lower heats of sorption (Wang and Bremman, rul, 2006). In low moisture contents, the 1991; Arslan and Tog values of the isosteric heats were higher than latent heat of vaporization of water, indicating that the energy of binding between the water molecules and the sorption sites was higher than the energy which holds the molecules of pure water together in the liquid phase (Al-Muhtaseb et al., 2004). We also found that the curves of the change in the heat of sorption of dried noodles with different moisture contents were clearly distinguishable at different temperatures when the MCPE and MHE models were employed for analysis. The isosteric heats of

Table 10 The thermal properties of the powder of ten varieties of Chinese dried wheat noodles. No.

Noodle varieties

Toa (
C)

1 2 3 4 5 6 7 8 9 10

Carrot noodle Celery noodle Spinach noodle Tomato noodle Golden-silk egg Ultra-cool Country of origin Mushroom flavour Classical flavour Egg flavour

61.0 61.5 62.0 60.8 60.8 62.1 61.3 61.1 61.5 62.3

a b

Tp (
C) ± ± ± ± ± ± ± ± ± ±

0.4bb 0.2 ab 0.9 ab 0.2c 0.2c 0.0a 0.2b 0.6 ab 0.3 ab 0.7a

66.3 66.6 66.9 66.4 65.9 67.1 66.7 67.0 67.4 68.1

△H (J/g)

Tc (
C) ± ± ± ± ± ± ± ± ± ±

0.1c 0.2bc 0.5bc 0.1bc 0.0d 0.3b 0.2bc 0.4b 0.3 ab 0.5a

71.7 72.0 72.1 72.1 71.5 72.5 72.3 73.1 73.7 74.8

± ± ± ± ± ± ± ± ± ±

0.1e 0.4de 0.8cde 0.0d 0.4e 0.4cd 0.2d 0.2c 0.2b 0.7a

5.093 5.431 4.413 5.519 6.278 5.446 5.463 4.953 5.682 4.962

± ± ± ± ± ± ± ± ± ±

0.496b 0.174b 1.131b 0.587 ab 0.184a 0.485b 0.083b 1.014b 0.049b 0.757b

To, onset temperature of gelatinization; Tp, peak temp.; Tc, conclusion temp.; △H, enthalpy of gelatinization. Data are given as the mean ± SD for triplicate. Values followed by the same small letter are not significantly different at p  0.05 according to Duncan's multiple ranges test.

moisture to 10e12% d.b. Navaratne (2013) showed that the critical moisture content of wheat noodles, 12% d.b., was not exceeded even at 85e90% r.h. level, and concluded that wheat noodle had low

sorption of Chinese dried noodles predicted by MCPE and MHE models at lower temperatures were higher than those at higher temperatures. This may be due to the reciprocal or exponential

Y. Li et al. / Journal of Stored Products Research 67 (2016) 19e27

relationship between the heat of sorption and temperature when respectively analysed by MCPE and MHE models, but relationship between the heat of sorption and temperature was not clear when calculated by MOE model. 5. Conclusion The curves of moisture sorption for ten dried wheat noodle varieties were sigmoidal in shape. At a constant temperature, EMC increased with an increase in ERH, with an especially rapid increased when the ERH was over 70%. At a constant ERH, EMC decreased with an increase in temperature, despite the temperature having only a minor effect on the sorption isotherms of dried wheat noodle. Among the tested ten varieties of dried noodles, the EMC data for most of the dried noodles were similar, although the classically flavoured noodles had the highest EMC values, and the EMC values of golden-silk egg noodles were lowest. Most of these ten varieties of dried noodles had similar thermal properties. The golden-silk egg noodles had the highest peak enthalpy of gelatinization and the lowest To, Tp and Tc. The maximum allowable moisture content of dried wheat noodle for safe keeping is 12% d.b when the environmental temperature ranges from 15
C to 40
C and ERH is 60%. The heat of sorption of dried wheat noodles approached that of pure water at the moisture content of 20% d.b. The isosteric heats of sorption of Chinese dried noodles predicted by MCPE and MHE models at lower temperatures were higher than those at higher temperatures. Acknowledgements The authors acknowledge the Special Fund for Grain Scientific Research in the Public Interest from the State Administration of Grains, China (201313001-03-01). We appreciate Professor C. Athanassiou and two anonymous referees reviewing our paper and giving invaluable suggestion and helps. References Aguerre, R.J., Suarez, C., Viollaz, P.E., 1989. New BET type multilayer sorption isotherms: Part II. Modelling water sorption in foods. Lebensm. Technol. 22, 192e195. Al-Muhtaseb, A.H., McMinn, W.A.M., Magee, T.R.A., 2004. Water sorption isotherms of starch powder. Part 2: thermodynamic characteristic. J. Food Eng. 62, 135e142. AOAC, 1980. Official methods of analysis, thirteenth ed. Assoc. Off. Anal. Chem, 1, (Washington DC). rul, H., 2006. The fitting of various models to water sorption isoArslan, N., Tog therms of tea stored in a chamber under controlled temperature and humidity. J. Stored Prod. Res. 42 (2), 112e135. Aviara, N.A., Ajibola, O., 2002. Thermodynamics of moisture sorption in melon seed and cassava. J. Food Eng. 55 (2), 107e113. Bruin, S., Berg, C.V.D., 1981. Water activity and its estimation in food system. In: Rockland, L.B., Stewart, G.F. (Eds.), Theoretical Aspect in Water Activity and Influence on Food Quality. Academic Press, New York, pp. 1e45.

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