The influence of freeze drying conditions on microstructural changes of food products

Transcription

1 Available online at Procedia Food Science 1 (2011) th International Congre on Engineering and Food (ICEF11) The influence of freeze drying condition on microtructural change of food product Vailiki P. Oikonomopoulou a*, Magdalini K. Krokida a, Vaio T. Karathano b a Department of Chemical Engineering, National Technical Univerity of Athen, Zografou Campu, Athen, 15780, Greece vaiaoik@central.ntua.gr, mkrok@central.ntua.gr b.department of Nutrition, Harokopio Univerity, Kallithea, Athen, 17671, Greece vkarath@hua.gr Abtract The drying of food product can reult in ignificant change in the chemical compoition, morphology and phyical propertie of food and can lead to tabilized product with longer helf-life and eaier commercialization. Information on porou tructure of food i very important, characterizing the quality and texture of dehydrated food product. Therefore, tructural propertie, uch a poroity, bulk denity and true denity of freeze-dried food product were invetigated a affected by proce condition. Rice kernel were boiled for different time period and agricultural product, including potato, muhroom and trawberry, were cut into cube. The ample were frozen, tempered in liquid N 2 and freeze-dried, under variou vacuum condition, uing a laboratory freeze-dryer. True denity of the product wa meaured uing a helium tereo-pycnometer. Bulk denity wa obtained by meauring the dimenion of the ample with a Vernier caliper. Simple mathematical model were developed in order to correlate the tructural propertie with proce condition. The microtructure of food product wa alo analyzed by Scanning Electron Microcopy and image analyi. Bulk denity of freeze-dried material increaed with the applied preure during freeze-drying, while poroity decreaed. In addition, bulk denity of freeze-dried rice kernel decreaed with the increment of boiling time, while poroity increaed. The change in bulk denity and poroity were cloely upported by microtructural obervation, according to SEM image. The microtructural change of product, freeze-dried under variou vacuum condition, can be predicted uing the propoed model and image analyi Publihed by Elevier B.V. Open acce under CC BY-NC-ND licene. Selection and/or peer-review under reponibility of 11th International Congre on Engineering and Food (ICEF 11) Executive Committee. Keyword: agricultural product; freeze drying; rice; SEM; tructure * Correponding author. Tel.: ; fax: addre: vaiaoik@central.ntua.gr X 2011 Publihed by Elevier B.V. Open acce under CC BY-NC-ND licene. Selection and/or peer-review under reponibility of 11th International Congre on Engineering and Food (ICEF 11) Executive Committee. doi: /j.profoo

2 648 Vailiki P. Oikonomopoulou et al. / Procedia Food Science 1 (2011) Introduction Dehydration operation are widely ued for the preervation of food, ince the removal of water minimize the microbial poilage and prevent the phyical and chemical reaction of the food compound during torage [1, 2]. Dehydrated food product are eaily obtained and maintain the characteritic of natural product [3]. Drying procedure comprie of imultaneou heat and ma tranfer, which caue ignificant change in the phyical and chemical compoition a well a in the tructure of food product, depending on the tranport mechanim applied. Therefore, the microtructure and morphology of food and a a reult the quality of the final product, are related to the drying method and elected condition applied [2]. Among the drying method that are ued in food proceing indutrie, freeze-drying i conidered one of the mot advanced method for drying high value product enitive to heat, ince it prevent undeirable hrinkage and produce material with high poroity, unchanged nutrition quality, uperior tate, aroma, flavor and color retention, a well a better rehydration propertie [4], uperior to thoe dried with conventional technique. Freeze-drying i ued for the preervation of enitive material and the facilitation of tranport and i carried out in two tage; the product i firt frozen and then the ice i removed by ublimation directly from the olid to the vapor phae. During freeze-drying, ice ublimation caue ignificant change in the hape and volume of the food product. Depending on the proce condition, the ice crytal which ublimate create pore or gap with different characteritic, thu, it eem very intereting to invetigate the effect of freeze-drying proce condition on the tructural propertie of food product [4]. Structural propertie, like denity and poroity, characterize the texture and quality of dehydrated product by controlling the tate and appearance. Beide the poroity of food product, pore ize ditribution play a crucial role. Pore ize ditribution can be etimated by image analyi of two dimenional image [5]. Information on porou formation in food during proceing i needed for proce deign, influencing a wide variety of other propertie, uch a mechanical propertie, thermal conductivity, thermal diffuivity and ma diffuion [2, 6]. The effect of drying condition on tructural propertie of freeze-dried agricultural product ha been tudied by Krokida et al. (1998) [4]. Rahman et al. (2003) examined the variation of poroity of freezedried abalone depending on the change of freeze-drying temperature. Karathano et al. (1996) [7] invetigated the tructural collape of agricultural plant during freeze-drying. Regier et al. (2007) [5] determined the pore ize ditribution of two dimenional image of bread and extruded nack uing image analyi. The objective of the preent reearch wa to determine the effect of proce condition on the tructural propertie of freeze-dried food product. Freeze-drying wa performed under controlled drying condition, regulating preure during drying. Simple mathematical model were developed according to the experimental data, in order to predict the value of poroity and bulk denity correlated with proce condition. The microtructure of food product wa alo analyzed by Scanning Electron Microcopy and image analyi. 2.Material & Method 2.1.Freeze Drying Parboiled rice and freh agricultural product, including potato, muhroom and trawberry, were choen a raw material. The material were tored at room temperature and dark condition before the experimental procedure. Rice wa boiled in exce de-ionized water at 100 o C for different time period, ranging from 4 to 24 min. Agricultural product were cut into cube of approximately 20 mm length, 20 mm width and a thickne of 10 mm. The material were then frozen at -30 o C for 72 h, tempered for 1 h

3 Vailiki P. Oikonomopoulou et al. / Procedia Food Science 1 (2011) in liquid N 2 and dehydrated for 24 h in a laboratory freeze-dryer (Leybold-Heraeu GT 2A). Freezedrying wa performed under variou vacuum condition, ranging from 0.04 to 1.25 mbar for rice ample and 0.06 to 1.50 mbar for agricultural product. The vacuum wa reduced by leaking air through one of the preure releae valve. Two replicate for each drying condition were performed. 2.2.Meaurement of bulk denity, true denity and poroity The ma of the dried material wa meaured uing an electronic balance with an accuracy of 10-4 g. The true volume of the ample, ground to powder to remove mot of the internal pore, wa etimated uing a tereopycnometer (Quantachrome multipycnometer MVP-1) with an accuracy of cm 3, utilizing helium ga. Three replicate of each ample were ued. The true denity wa expreed by the equation: m t (1) V where t (gcm -3 ) i the true denity, m (g) the ma of dry olid and V (cm 3 ) the volume of dry olid. The total (bulk) volume wa obtained by meauring the actual geometric characteritic of freeze-dried material, uing a digital Vernier caliper with an accuracy of 0.001cm. The reult were the average of fifteen replicate for rice kernel and four replicate for agricultural product. Each rice kernel i conidered to conit of a cylindrical part and two hemiphere. The total volume of rice kernel wa etimated uing the equation: V t 2 3 d ( h d ) 4 d where V t (cm 3 ) i the total volume of each rice kernel, d (cm) the diameter and h (cm) the height of the cylindrical part of the rice kernel. The total volume of freeze-dried cube of agricultural product wa etimated uing the equation: Vt L W H (3) where V t (cm 3 ) i the total volume of each dried ample, L (cm) the length, W (cm) the width and H (cm) the thickne of each ample. The bulk denity wa determined uing the equation: m b (4) V t where b (gcm -3 ) i the bulk denity and m (g) the ma of dry olid. The poroity wa etimated uing the equation: b 1 (5) t (2)

4 650 Vailiki P. Oikonomopoulou et al. / Procedia Food Science 1 (2011) Mathematical modeling Several mathematical model were developed in order to predict the value of poroity and bulk denity correlated with freeze-drying condition for agricultural product and freeze-drying condition and boiling time for rice kernel. The implet and mot appropriate power model were elected according to the Eq. (6) for rice kernel and Eq. (7) for agricultural product: P m t o no k o ( ) exp( ) P (6) t o o P 1 ( ) m 1 k (7) P o where k o, k 1, m o, m 1, n o are parameter dependent on the material, i the poroity, P (mbar) i the preure in the freeze-dryer, t (min) i the boiling time, P o (mbar) and t o (min) are the correponding value at reference condition. Specifically, P o i the preure at -20 o C equal to 0.80 mbar and t o i the average value of boiling time equal to 14 min. 2.4.Scanning Electron Microcopy Scanning Electron Microcopy (SEM) wa ued to viualize the microtructure of freeze-dried rice kernel and agricultural product. Freeze-dried material were coated with gold uing a SC7620 Mini Sputter Coater (Quorum Technologie). The pecimen were then photographed uing a Scanning Electron Microcope (Quanta 200 FEI (2004)) operated at 20 kv for rice kernel and 25kV for agricultural product for variou magnification. 2.5.Data Analyi The obtained data from SEM image were proceed with image analyi in order to etimate the pore ize ditribution. Regreion analyi wa ued to etimate the model parameter. The influence of proce condition on the tructural characteritic of freeze-dried food product wa alo analyzed uing analyi of variance (ANOVA). The analye were performed uing Statitica oftware (Statitica Releae 7, Statoft Inc, Tula, OK, USA). 3.Reult & Dicuin 3.1.Bulk Denity, True Denity and Poroity True denity of freeze-dried food product wa determined uing Eq. 1 and it wa conidered contant, equal to the denity of the olid material. The value of true denity and the tandard deviation, of each dried product are hown in Table 1. Table 1. True denity of freeze dried material Material Rice Potato Strawberry Muhroom True denity (g/cm 3 ) 1.504± ± ± ±0.056 Bulk denity of freeze-dried food product wa found to be a trong function of freeze-drying condition. A far a rice kernel are concerned, bulk denity wa ignificantly affected by boiling time. The correponding reult are preented in Fig. 1 and 2, for rice kernel and agricultural product,

5 Vailiki P. Oikonomopoulou et al. / Procedia Food Science 1 (2011) repectively. Regreion analyi howed that bulk denity of freeze-dried material decreaed ignificantly (p < 0.001) with the decrement of the applied preure. In addition, rice kernel boiled for longer time period howed ignificantly (p < 0.001) lower value of bulk denity compared to thoe boiled for a hort time period. Among the agricultural product, potatoe preented the highet bulk denity, while muhroom howed the lowet one. Bulk denity (g/cm 3 ) P=0.04 mbar P=0.13 mbar P=1.25 mbar Calculated value Boiling time (min) Fig. 1. Correlation of bulk denity of freeze-dried rice kernel with boiling time, for variou applied preure during freeze-drying Bulk denity (g/cm 3 ) Potato Muhroom Strawberry Calculated value Preure (mbar) Fig. 2. Correlation of bulk denity of freeze-dried agricultural product with applied preure during freeze-drying. A it can be een in Fig. 3 and 4, poroity of freeze-dried material, meaured uing helium tereopycnometer, wa ignificantly (p < 0.001) influenced by freeze-drying condition. At low preure, poroity wa the highet noticed and decreaed a the preure increaed. Muhroom preented more porou tructure among the other agricultural product, wherea potatoe howed the lowet poroity Poroity P=0.04 mbar P=0.13 mbar P=1.25 mbar Calculated value Boiling time (min) Fig. 3. Correlation of poroity of rice kernel, with boiling time for variou applied preure during freeze-drying.

6 652 Vailiki P. Oikonomopoulou et al. / Procedia Food Science 1 (2011) In addition, poroity of freeze-dried rice kernel wa ignificantly (p < 0.001) affected by boiling time, preenting higher value while elongating boiling procedure. Freeze-dried rice boiled for horter time period howed the lowet value of poroity and the highet value of bulk denity. A boiling time increae, there will be a higher water uptake reaching about 2.5 time the ma of dried rice. Therefore, a bulk volume increae will take place. During ubequent freezing and freeze-drying the ice ublimation create pore. The amount of pore (poroity) i related to the water uptake and i higher when the water uptake i increaed. Conequently, if no collaping would happen due to the extenion of boiling time, the poroity would be higher. The increae of poroity of rice kernel with increaing boiling time, varied from 40 to 50%, uing a a reference the hortet boiling time, depending on the preure applied Potato Muhroom Strawberry Calculated value Poroity Preure (mbar) Fig. 4. Correlation of poroity of freeze-dried agricultural product with applied preure during freeze-drying. The reult of parameter etimation of the mathematical model for poroity and bulk denity of freezedried food product, meaured uing helium tereopycnometer, are ummarized in Table 2. Table 2. Parameter etimation for poroity and bulk denity of freeze-dried food product Material k o m o n o R 2 Rice 0.208± ± ± k 1 m 1 R 2 Potato 0.876± ± Strawberry 0.929± ± Muhroom 0.947± ± Scanning Electron Microcopy The alteration of poroity of freeze-dried product a a function of boiling period and drying condition are viualized with SEM image and preented in Fig. 5 and 6. Scanning Electron Microcope wa operated at 100x magnification for trawberrie and 500x magnification for the other food product. A it can be een, the increae of boiling time lead to an increae in poroity and pore ize of rice kernel. In addition, a far a freeze-drying preure i concerned, when the abolute preure in the freeze-drying chamber i higher, then the poroity i lower.

7 Vailiki P. Oikonomopoulou et al. / Procedia Food Science 1 (2011) a. b. c. d. Fig. 5. Microtructure of freeze-dried rice a a function of boiling time and applied preure during freeze-drying mbar, t=4 min, b) P=0.04 mbar, t=20 min, c) P=1.25 mbar, t=4 min, d) P=1.25 mbar, t=20 min. a) P=0.04 a. b. c. d. e. f. Fig. 6. Microtructure of freeze-dried agricultural product a a function of the applied preure during freeze-drying a) Potato, P=0.06 mbar, b) Potato, P=1.00 mbar, c) Strawberry, P=0.06 mbar, d) Strawberry, P=1.00 mbar, e) Muhroom, P=0.06 mbar, f) Muhroom, P=1.00 mbar. Fig. 7 preent the pore ize ditribution of freeze-dried product, etimated uing image analyi. The percentage of large pore i higher at the lower applied preure, where the poroity i higher P=0.04 mbar P=0.13 mbar P=1.25 mbar P=0.06 mbar P=0.30 mbar P=0.60 mbar P=1.00 mbar % % Pore Radiu (μm) 20 a. b Pore Radiu (μm) Fig. 7. Pore ize ditribution of freeze-dried rice and potatoe a a function of the applied preure during freeze-drying a) Rice, t=20 min, b) Potato. 4.Concluion Freeze-drying condition ignificantly affected bulk denity and poroity of dried food product. Boiling time alo affected the tructural propertie of freeze-dried rice kernel. Bulk denity decreaed and poroity increaed when decreaing the applied preure during freeze-drying. In addition, rice ample boiled for lower time period preented the lowet value of poroity and the highet value of bulk denity, while thoe boiled for higher time period howed the highet value of poroity and the lowet

8 654 Vailiki P. Oikonomopoulou et al. / Procedia Food Science 1 (2011) value of bulk denity, repectively. The above reult were viualized with Scanning Electron Microcopy and image analyi. Acknowledgement The author would like to thank the Reearch Committee of the National Technical Univerity of Athen (NTUA) for their financial upport. In addition, we would like to thank Agrino Ev.Ge. Pitiola S.A for providing rice raw material. Reference [1] Krokida MK, Marino-Kouri D. Rehydration kinetic of dehydrated product. J Food Eng 2003;57:1-7. [2] Koc B, Eren I, Kaymak Ertekin F. Modelling bulk denity, poroity and hrinkage of quince during drying: The effect of drying method. J Food Eng 2008;85: [3] Marque LG, Silveira AM, Freire JT. Freeze-drying characteritic of tropical fruit. Dry. Technol. 2006;24: [4] Krokida MK, Karathano VT, Marouli ZB. Effect of freeze-drying condition on hrinkage and poroity of dehydrated agricultural product. J Food Eng 1998;35: [5] Regier M, Hardy EH, Knoerzer K, Leeb CV, Schuchmann HP. Determination of tructural and tranport propertie of cereal product by optical canning, magnetic reonance imaging and monte carlo imulation. J Food Eng 2007;81: [6] Rahman MS. Toward prediction of poroity in food during drying: A brief review. Dry. Technol. 2001;19:1-13. [7] Karathano VT, Anglea SA, Karel M. Structural collape of plant material during freeze-drying. J Therm Anal Calorim 1996;47: Preented at ICEF11 (May 22-26, 2011 Athen, Greece) a paper EPF375.