7th Conference of Food Engineering, Reno, 2001 Factors that control the phase behavior of a meat-starch extruded system illustrated on a state diagram C.I. Moraru, T.C. Lee, M.V. Karwe and J.L. Kokini Department of Food Science and Center for Advanced Food Technology, Rutgers University
Objectives of the Research v To study the phase behavior of a complex meat-starch extruded system and to illustrate it on a state diagram. v To study the effects of plasticizers (water and glycerol) on the glass transition of the system.
Background Understanding the phase behavior of complex food systems v can help predict and control their texture and storage stability (Kokini et al., 1994). v Carbohydrate-protein mixtures are commonly used for the production of snack foods by extrusion. v The properties of complex systems are not just a sum of their componentsÕ properties. v The interactions of proteins and carbohydrates with water, with the other minor components and with each other govern the structure-property relationships of foods (Matveev et al., 2000).
Glass Transition Temperature (Tg) v Tg is a critical parameter for amorphous food matrices, which controls their processability, properties, stability and safety (Levine and Slade 1993, Roos 1995). v Most foods are mixtures of proteins and carbohydrates, which in many cases are immiscible and retain their own Tg. Tg can be depressed by the addition of plasticizers. v
Plasticization of Biopolymers v Plasticizers increase the workability and flexibility of polymers (Sears & Darby 1982) and decrease their Tg by shielding macromolecular interactions, facilitating segmental motion and decreasing internal friction (Matveev et al., 2000). v Water is the most effective plasticizer for food systems (Roos & Karel 1991, Lillie & Gosline 1993, Brent et al., 1997). v Glycerol is frequently used to plasticize food biopolymers (Lourdin et al., 1997, DiGioia et al., 1998, Forssell et al., 1999, Moates et al., 2001). If a low molecular compound acts as a plasticizer for each v components of a mixed system, both TgÕs are depressed (Matveev et al., 2000).
Formulations v Blends of meat mix and potato granules (1.48/1) with m.c. of 36.5 ± 1%, were prepared (beef jerky analogs). v Meat mix = low fat ground beef (87.99%), oat fiber (5.28%), salt (3.96%), jerky spice mix (1.65%), oil mix (0.48%), liquid smoke (0.48%), sodium tripolyphosphate (0.22%), ascorbyl palmitate (0.04%) and sodium nitrate (0.01%), mixed and cooked for 45min. 2% and 4% glycerol was added to the mixtures before extrusion. v v Starch extrudates were obtained from potato granules.
Sample Preparation Extrusion v ZSK-30 co-rotating, intermeshing twin-screw extruder, in a high shear screw configuration (W&P, Ramsey, NJ) Temperature profile: 25-35-90-135-100-105 ° C (hopper to die). v v Screw speed = 100rpm and specific mechanical energy input of ≈ 1000kJ/kg. Sample equilibration In dessicators over supersaturated solutions of salts, at water v activity (a w ) values between 0 and 0.84.
Analytical Methods v Mechanical Spectroscopy : Rheometrics ARES II (Rheometrics Scientific, Piscataway, NJ) v Differential Scanning Calorimetry - TA 4000 System with DSC 30- S Cell, and a TC11 TA Processor (Mettler Instrument Inc., Highstown, NJ). Samples were inserted in medium pressure stainless steel crucibles and scanned at a heating rate of 5C/min.
Tg Analysis by Mechanical Spectroscopy Endset 10 11 0.24 Onset of relaxation: 0.22 Onset onset of GÕ drop 0.2 G' ( dyn/cm 2 ) G" ( dyn/cm 2 ) ) Midpoint of relaxation : 10 10 0.18 tan_delta GÓ peak 0.16 Endset of relaxation : Midpoint 0.14 Tan δ peak 9 10 0.12 0.1 0.08 10 8 0.06 -50.0 -18.0 14.0 46.0 78.0 110.0 Temp [¡C] Example of dynamic temperature sweep test (Extruded potato granules, a w =0.84)
Tg Analysis by DSC DSC thermogram showing an endothermic event and a Tg (extruded potato granules, a w =0.26)
Transition identification for the complex biopolymer matrix v Identification of transitions was made by comparing thermo-mechanical spectra and DSC plots of extruded starch and extruded S-P matrix [ ] tan_delta ( [dyn/cm 2 ) G' ( bQ ] bQ ) 0.05 10 8 0.0 -40.0 -20.0 0.0 20.0 40. 0 60.0 80.0 100.0 120.0 Temp [Á C] Extruded S-P Extruded starch Comparative temperature sweeps Comparative DSC thermograms
Mechanical relaxations of the matrix Two major relaxations (Tg 1 , Tg 2 ) and two secondary relaxations were v observed. The major relaxations were influenced by moisture content/a w . v aw<0.58 aw>0.67 0.35 0.35 0.35 0.35 0.3 0.3 0.3 0.3 a w =0.57 0.25 0.25 a w =0.32 Tg 2 tan_delta tan_delta Ice a w =0.21 0.25 0.25 T g2 T g1 0.2 0.2 melting a w =0.75 a w =0.17 tan_delta tan_delta a w =0.0 7 Sub Tg 2 0.15 0.15 a w =0.84 0.2 0.2 relaxation a w =0.88 0.1 0.1 0.15 0.15 a w =0.90 Tg 1 0.05 0.05 0.1 0.1 0.0 0.0 -50.0 -30.0 -10.0 10.0 30.0 50.0 70.0 90.0 110.0 -50.0 -30.0 -10.0 10.0 30.0 50.0 70.0 90.0 110.0 130.0 150.0 Temp [¡C] Temp [¡C]
Moisture had a depressing effect on both major relaxations Tg 1 Ð meat proteins Tg 2 Ð starch 50 160 40 140 Temperature of Tg2, C 30 Temperature, C 120 20 10 100 0 80 -10 -20 60 -30 40 -40 20 -50 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 a w a w y = -86.08x + 46.86,R2 = 0.98 y = -91.65x + 150.57, R2 = 0.96 Tan delta(endset) Tandelta peak(endset) y = -86.09x + 15.13, R2 = 0.88 y = -70.68x + 106.92, R2 = 0.92 G' onset G' onset
The magnitude of the relaxations was influenced by moisture 0.4 0.25 Starch Tg Protein Tg 0.35 0.2 Tan delta, Tg1 0.3 0.15 Tan delta, Tg2 0.25 0.1 0.2 y = 0.1688x + 0.0648 0.05 0.15 2 = 0.9934 R 0 0.1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Aw Aw Tan δ peak increased with increased moisture content for Tg of v proteins - typical for polar polymers (Kalichevsky et al.1993). v For starch, the decrease of relaxation magnitude could be explained by increased crystallinity.
The secondary relaxations were not influenced by moisture 90 Sub Tg relaxation Tan delta peak (endset) 80 G' onset Temperature of transition 3, C 70 60 50 40 Ice melting 30 20 10 0 0 0.2 0.4 0.6 0.8 1 Aw v Moisture content did not change the location of the sub Tg relaxation and ice melting
State diagram of the extruded system 160 Flow state+chemical degradation 140 120 100 Starch Tg Temperature, C 80 60 Rubbery texture 40 20 Protein Tg 0 G l a s s + I c e -20 G l a s s y s t a t e -40 -60 0 0.2 0.4 0.6 0.8 1 a w v Immiscibility of the two biopolymers is highlighted.
Effect of glycerol on the thermo- mechanical properties of the matrix 10 11 0.35 GÕ 0%gly 0.3 4%gly 0%gly 2%gly 2%gly 0.25 [ ] tan_delta ( 4%gly ) ] ] 2 [dyn/cm 2 0.2 [dyn/cm bQ 10 10 bQ G' ( 0.15 ) 0.1 Tan delta 0.05 10 9 0.0 -50.0 -10.0 30.0 70.0 110.0 150.0 Temp [ÁC] v Glycerol decreased the modulus and increased the magnitude of the relaxations
Glycerol depressed slightly both TgÕs at low moisture content / a w 50 160 Tg 2 Ð starch Tg 1 Ð meat proteins 150 40 Temperature of Tg2, C Temperature of Tg1, C 140 30 130 20 120 10 110 100 0 90 -10 80 -20 70 -30 60 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Aw a w 2 = 0.9603 Reference y = -91.648x + 150.57, R y = -86.08x + 46.86, R2 = 0.98 Reference 2 = 0.9036 2% glycerol y = -83.381x + 141.67, R y = -67.74x + 32.25, R2 = 0.94 2% glycerol y = -72.910x + 136.64, R 2 = 0.8722 y = -86.14x + 42.77, R2 = 0.90 4% glycerol 4% glycerol
Practical conclusions v In the studied system, starch and meat proteins were immiscible and retained their own Tg. v Water was found to be the major plasticizer for both components of the system. v Glycerol had also a plasticizing effect on the matrix.
Fundamental conclusion v Complex food systems show similar transitions and relaxations as synthetic polymers, but their interpretation is complicated due to multiple, overlapping transitions.
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