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Keywords:

  • freezing point;
  • freezable water;
  • ration component;
  • thermal property;
  • water activity

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

ABSTRACT:  Beef roast with vegetables is an example of a meal, ready-to-eat (MRE) ration entrée. It is a mixture of meat, potato, mushroom, and carrot with a gravy sauce. The thermal properties of each component were characterized in terms of freezing point, latent heat, freezable and unfreezable water contents, and enthalpy during freezing using differential scanning calorimetry. Freezing and thawing curves and the effect of freezing and thawing cycles on thermal properties were also evaluated. The freezing points of beef, potato, mushroom, and sauce were all in the range of −5.1 to −5.6 °C, but moisture content, water activity, latent heat, freezable and unfreezable water contents, and enthalpy varied among these components. Freezing temperature greatly affected the unfrozen water fraction. The unfreezable water content (unfrozen water fraction at −50 °C) of ration components was in the range of 8.2% to 9.7%. The freezing and thawing curves of vegetables with sauce differed from those of beef but took similar time to freeze or thaw. Freezing and thawing cycles did not greatly affect the thermal properties of each component. Freezing point and latent heat were reduced by decreasing moisture content and water activity of each component. Water activity was proportionally linear to freezing point at aw > 0.88, and moisture content was proportionally linear to freezable water content in all ration components. Water was not available for freezing when moisture content was reduced to 28.8% or less. This study indicates that moisture content and water activity are critical factors affecting thermal behavior of ration components during freezing.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Meal, ready-to-eat (MRE) is a ration designed to help sustain an individual engaged in heavy activity, such as military training or actual military operations, when normal food service facilities are not available. However, in recent years, there has been a strong civilian demand for MRE in disaster relief operations. Large stocks of MRE can be ferried and distributed easily and they provide basic nutritional support to victims before kitchens can be set up to produce fresh food. The MRE is a totally self-contained, flexibly packaged full meal, which contains an entrée, side dish, and dessert that can be eaten hot or cold and requires no special preparation or storage. An entrée, such as beef roast with vegetables, includes meat and vegetables in a sauce, composing a complex food system of carbohydrates, proteins, lipids, water, and other minor components. Like all food materials, the MRE components are also subject to various time-dependent changes that affect food stability and quality, and the change rate is dependent on its physical state and water content. Thus, in a nonequilibrium state, MRE components tend to form amorphous, noncrystalline structures (Roos 1995, 2003). Characterization of thermal properties of an MRE system is essential to understand the relationship between food physicochemical properties and food stability and the interactions among components during temperature changes, thus, to further control food quality during processing and food storage. Thermal properties of pure or simple materials have been commonly reported in literature (Rahman 1995), but properties of complex, multiple components of processed foods were rarely reported due to the heterogeneity in chemical composition and structure and the various physical and chemical reactions among components. Information on thermal properties of individual ration components is not available in literature.

Freezing point, water activity (aw), freezable water content, latent heat of fusion, and enthalpy are important thermal properties of a food that potentially affect manipulation of freezing temperature, thermal stability during storage, frozen product quality during processing, and end uses by customers. A single differential scanning calorimetry (DSC) thermograph can be used for characterization of these thermal properties (Wang and Kolbe 1991). DSC has been used extensively in characterization of phase transition and thermal properties of food materials, such as fresh meat, fruit, vegetables, and cereal products at subzero temperatures (Roos 1986, 1987; Wang and Kolbe 1991; Rahman 1995; Tocci and Mascheroni 1998; Hamdami and others 2004; Fasina 2005; Wang and others 2007). This study focused on thermal properties of a ration component model system as a function of moisture and temperature and the relationships between thermal characteristics. Findings further understand the mechanism involved in controlling freezing in subzero climates, which reduce heating time required for MRE to reach serving temperature and army personnel's time away from mission critical tasks.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Ration component model—beef roast with vegetables

A MRE entrée, beef roast with vegetables, packed in flexible pouches (Ameriqual Foods, Evansville, Ind., U.S.A.) was used as a ration component model. Each pouch contains 1 piece of braised beef roast with diced potatoes, diced carrots, and sliced mushroom in sauce. Pouches were stored in a refrigerator for later analysis.

Sample preparation

Ration components in 3 pouches were manually separated into beef, potato, mushroom, and sauce. The sauce was considered to be the mixture of smooth and thin fraction within a pouch except beef, intact potato dices, and sliced mushrooms. Thus, the sauce fraction included seasonings, spices, and small amount of broken vegetables, such as carrots, potatoes, and mushrooms. Separated ration components were directly used for DSC, moisture, and water activity analyses. To study moisture content in relation to thermal properties, the separated ration component was blended in a mixer, and frozen in a blast freezer (−29 °C) for 12 h, and freeze dried in a freeze dryer (FreeZone 6 Freeze Dry System, Labconco, Kansas City, Mich., U.S.A.) at −40 °C for 3 d to remove moisture. The freeze-dried samples were used for preparation of model systems at various moisture contents as described by Roos (1987). The freeze-dried samples were ground using pestle and mortar. The moisture of each powder sample was adjusted to 25% to 80% with distilled water in 50 mL tubes and equilibrated at 4 °C for 24 h for further characterization.

Differential scanning calorimetry

A DSC (Pyris-1, Perkin-Elmer, Norwalk, Conn., U.S.A.) was used to measure onset freezing point temperature, freezable water content, unfreezable water content, latent heat, and enthalpy of ration components. The instrument was calibrated for 2 temperature points and heat flow using indium and deionized water. Onset freezing temperature of deionized water was set at 0 °C. Samples were taken either from the center of beef chunk, vegetable dices, and sauce, or from equilibrated samples after freeze-drying. Weighed samples (10 to 18 mg) were sealed in aluminum pans, quenched to −60 °C, held for 10 min, and then scanned from −60 to 50 °C at a heating rate of 2 °C/min, as described by Roos (1986). An empty aluminum pan was used as reference. Each component from a single pouch was analyzed at least in triplicates, and 3 pouches were analyzed. Onset temperatures of freezing points, latent heat of fusion, and total enthalpy (from −50 to 40 °C) of ration components were calculated using Pyris analysis software. Freezable water and unfreezable water contents were calculated based on the latent heat data following the method described by Ross (1978).

Moisture content and water activity

Moisture content and water activity of ration components were determined by a oven (Econotherm oven, Precision Scientific Inc, Chicago, Ill., U.S.A.) at 105 °C for 24 h and a water activity meter (Aqualab Series 3, Decagon Devices, Pullman, Wash., U.S.A.). Surface condensate of samples was gently removed using a piece of Kimwipes® (Kimberly-Clark Corp., Roswell, Ga., U.S.A.) before measurement to reduce the variation of analysis.

Freezing and thawing cycle analysis

The whole pouches of ration entrée were used for the freezing and thawing cycle study. Thermometer probes were inserted into the center of either the beef portion or vegetable portion of a whole pouch. All samples were put into a blast freezer (−29 °C) for 12 h and then in ambient temperature for 12 h. Temperature changes were recorded every min at the first 1.5 h and every 30 min for the next 10.5 h. Measurements were done in triplicate for 3 pouches.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Thermal properties

Beef and potato had lower moisture contents (75.3% to 77.4%), latent heat of fusion (205.2 to 207.3 J/g), and enthalpy (289.3 to 299.4 J/g), compared with mushroom and sauce (Table 1). Sauce showed the highest water activity at aw= 0.999, whereas, beef, potato, and mushroom had similar aw in the range of 0.993 to 0.995 (Table 1). All components showed similar freezing points in the range of −5.1 to −5.6 °C (Table 1). Additionally, beef and potato had a lower proportion of freezable water (0.670 to 0.677 g/g) compared with mushroom and sauce (0.761 to 0.769 g/g). Unfreezable water contents (unfrozen water fraction at −50 °C) of all components were in the range of 0.082 to 0.097 g/g (Table 1).

Table 1—.  Thermal properties of ration components.
ComponentsMoisture (%)Water activityFreezing point (°C)Latent heat (J/g)Freezable water (g/g)Unfreezable water (g/g)Enthalpy (J/g)
Beef75.3 ± 0.40.995 ± 0.002−5.6 ± 0.4205.2 ± 5.60.670 ± 0.0180.088 ± 0.015289.3 ± 23.1
Potato77.4 ± 0.30.994 ± 0.001−5.8 ± 0.5207.3 ± 5.80.677 ± 0.0190.097 ± 0.019299.4 ± 24.9
Mushroom84.3 ± 0.30.993 ± 0.001−5.2 ± 0.2233.3 ± 6.50.761 ± 0.0210.082 ± 0.021346.3 ± 18.1
Sauce85.0 ± 0.10.999 ± 0.001−5.1 ± 0.3232.2 ± 7.10.769 ± 0.0110.082 ± 0.011340.7 ± 10.2

During freezing, the unfrozen water fraction in each component was diminished rapidly at freezing point and then decreased gradually to −50 °C (Figure 1). Beef, vegetable, and sauce had a similar amount of unfrozen water fraction at the same temperatures below freezing points. Free water in the components was crystallized rapidly from the soluble phase in the form of ice at and below its freezing point. An unfrozen phase containing concentrated soluble solids also was formed, which correspond to low freezing points (Franks 1985). With further decrease in freezing temperature, the viscosity of the unfrozen phase increased. This greatly retarded water molecule mobility and delayed ice formation and crystal growth, resulting in a slow reduction of unfrozen water content (Goff 1995; Roos 1995) in the frozen ration components. Thus, the amount of unfrozen water fraction varied depending on freezing temperature and gradually decreased to the amount of bound water with decreasing freezing temperature. Many researchers have used −40 °C as a reference temperature for measurement of unfreezable water content, which covers the typical temperature range of industrial processes (Fasina 2005).

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Figure 1—. Unfrozen water fraction of ration components during freezing.

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The freezing point of a food material is the initial freezing point at which ice formation begins. It is commonly measured by either DSC or the freezing curve method (Rahman 1995). In the DSC method, the freezing point is reported as the temperature at endothermic peak (Wang and Kolbe 1991; Fasina 2005) or at the point of maximum slope of the endothermic peak (Rahman 2004). The freezable and unfreezable water contents of food materials have been calculated based on enthalpy (total enthalpy at a reference temperature range) and latent heat of fusion (estimated from the area of the melting endotherm) (Roos 1986). Thus, the values of thermal properties in food materials reported in literature varied depending on material constitute, analysis method, and calculation variation (Tocci and Mascheroni 1998). Generally, the freezing points of ration components reported in this study are lower than those reported in literature for fresh beef and vegetables (Rahman 1995; Wang and others 2007) but similar to the data on cured meat and meat simulants in literature (Pham 1996; James and others 2005). This suggests that the heating process caused tissue and cell structure damage, protein denaturation, and starch gelatinization and retrogradation, which might affect freezing point of products. James and others (2005) reported that the freezing point of meat cured with 3% to 5% salt ranged from −4.1 to −6.3 °C.

Freezing and thawing behavior

Beef and vegetable portions showed different stages of freezing (Figure 2A). At the first stage, temperatures of beef and vegetables quickly decreased to ice crystallization temperature showing a supercooling phenomenon at −2.1 ± 0.0 °C and −4.3 ± 0.2 °C, respectively, and then rapidly rose to the equilibrium freezing points around −2.0 °C. At this temperature, ice crystals began to form and latent heat of crystallization was released. Supercooling varied with cooling rates, which is often observed at a low cooling rate (Rahman and Driscoll 1994). High concentrated solutes also rarely induce supercooling (Franks 1985; Goff 1992). At the 2nd stage, the temperature of beef remained nearly constant for about 1 h due to ice crystallization. After completion of ice crystallization, the temperature of beef continued to drop down as sensible heat was released.

image

Figure 2—. Freezing (A) and thawing (B) curves of ration components.

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Comparing with the freezing curve of beef, vegetables had a very short temperature stable stage and subsequently decreased due to continuous crystallization of freezable water and, possibly, multiple solutes (Franks 1985; Goff 1992). Vegetables without heat treatment showed a long temperature stable stage during freezing (Wang and others 2007), suggesting that the short temperature plateaus of vegetables possibly resulted from extensive cell structure damage during heating process. As compared to semirigid nature of plant cells, meat muscle is more flexible and resistant to freezing and thawing (Franks 1985). Ohnishi and others (2004) reported that the dynamic elasticity and viscosity of fruit and vegetables (carrot, potato, apple, and broccoli) were drastically decreased after freezing and thawing due to cell structure injury, but those of meat (chicken and fish) were scarcely changed, and those of processed gel food (soy curd and yam pasta) increased after freezing and thawing, indicating high freezing tolerance of muscles during freezing and denaturation of protein and mannan in processed foods. The difference between freezing and thawing curves of beef and vegetable in this study supports previous findings. For both beef and vegetables, it took 6 h to drop the temperature from ambient temperature to about −30 °C in a blast freezer. The thawing curves of ration components showed a reverse freezing process (Figure 2B). Average freezing or thawing rate was about 0.14 °C/min for a 6 h period. Numerous studies showed that a slow freezing rate causes formation of large ice crystal size, leading to serious changes in tissue and cell microstructures, which is detrimental to food quality (Goff 1992). Therefore, rapid freezers are commonly used in frozen food industry.

Up to 20 freezing and thawing cycles in pouches did not greatly affect water activity and freezing points of ration components but slightly decreased latent heat of fusion and freezable water content, thus increasing unfreezable water content (Table 2). Freezing and thawing cycles did cause protein curdling and drip in thawed process, fat coalescence, starch retrogradation, tissue and cell structure damage, and molecule degradation, which led to release of small molecules into the unfrozen water phase, potentially decreasing latent heat and freezable water content.

Table 2—.  Effect of freezing and thawing cycles on thermophysical behavior of ration components.
ComponentFreezing/thawing cyclesWater activityFreezing point (°C)Latent heat (J/g)Freezable water (g/g)Unfreezable water (g/g)
Beef00.995 ± 0.002−5.6 ± 0.4205.2 ± 5.60.670 ± 0.0180.088 ± 0.015
100.993 ± 0.002−4.9 ± 0.2214.6 ± 6.00.701 ± 0.0200.056 ± 0.020
200.994 ± 0.002−5.5 ± 0.3197.1 ± 4.90.643 ± 0.0160.114 ± 0.016
Potato00.994 ± 0.001−5.8 ± 0.5207.3 ± 5.80.677 ± 0.0190.097 ± 0.019
100.995 ± 0.001−4.9 ± 0.1187.0 ± 4.40.610 ± 0.0140.157 ± 0.010
200.994 ± 0.001−5.0 ± 0.3189.2 ± 3.40.618 ± 0.0110.155 ± 0.011
Mushroom00.993 ± 0.001−5.2 ± 0.2233.3 ± 6.50.761 ± 0.0210.082 ± 0.021
100.994 ± 0.001−5.1 ± 0.1243.7 ± 6.10.796 ± 0.0200.049 ± 0.020
200.993 ± 0.002−5.2 ± 0.3222.3 ± 4.90.726 ± 0.0160.119 ± 0.016

Relations between moisture content and water activity

The relationship between aw and the moisture content of ration components was obtained by analyzing aw of each component at different moisture levels after freeze-drying and moisture adjustment. A similar method has been used for characterization of thermal properties of strawberries (Roos 1987). The aw of all components were nonlinear with their moisture content (Figure 3). Reducing the moisture content of ration components decreased aw. At the same moisture level, the aw of sauce was much lower than that of other ration components, which maybe due to high salt content and other minor components. Moisture content and water activity have been used largely to predict freezing points of frozen foods in many empirical, theoretical, and semi-empirical models (Chen 1987; Rahman and Driscoll 1994; Rahman 1995; van der Sman and Boer 2005). Those models are established by linear, quadratic, or polynomial correlation equations and are quite complex due to the large variation of food compositions.

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Figure 3—. Relationship between water activity and moisture content of ration components.

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Effects of moisture content and water activity on DSC thermal graphs

As moisture content decreased from 67.1% to 29.3%, aw of beef was reduced from 0.984 to 0.877. As shown in Figure 4, the peak area of the DSC thermograph of beef became smaller and the peak moved to lower temperature, indicating that latent heat of fusion, enthalpy, freezing point, freezable water, and unfreezable water contents of beef were affected by moisture content and water activity. Potato, mushroom, and sauce showed similar trends (data not shown). Results from this study are in agreement with results reported by Roos (1986) that there were high correlations among freezing point, latent heat, enthalpy, and moisture of carrot, reindeer meat, and white bread. Roos (1987) also found that ice freezing/melting occurred only at aw > 0.75 for humidified strawberries, whereas glass transition was observed at aw < 0.75. In this study, all samples were analyzed at aw > 0.80, and only ice melting was observed.

image

Figure 4—. DSC thermal graphs of beef at various moisture and water activity levels.

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Effect of moisture content and water activity on freezing point

The freezing points of all ration components decreased as moisture content decreased (Figure 5A). A similar trend also was observed by Rahman and Driscoll (1994) in fresh seafood. At the same moisture level, sauce had the lowest freezing point among all ration components due to salt and seasoning (Figure 5A). Water activity of ration components were proportionally linearly correlated with their freezing point at aw > 0.88 (Figure 5B). Such linear relation also was observed by Chen (1987) between aw and freezing point depression for frozen food at temperatures ranging from 0 to −40 °C.

image

Figure 5—. Relationship of freezing point of ration components to moisture content (A) and to water activity (B).

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Effect of moisture content and water activity on freezable water content

Freezable water content increased as moisture content, aw, and freezing point increased (Figure 6A, 6B, and 7), while a proportionally linear relation was observed between freezable water content and moisture content (Figure 6A). At a moisture content of 28.8% or less, no free water was available for freezing. In other words, when 78% of water was removed from original ration components, only about 10% of water remained unfreezable at −50 °C. The water to solid ratio of ration components was 0.43 g: 1 g, which is close to published data for surimi (0.48 g: 1 g) (Wang and Kolbe 1991) and fish (0.40 g: 1 g) (Pham 1987). However, a broad water: solid ratio was reported from 0.14 to 0.65 g: 1 g in various foods with variations in sources, composition, reference temperature, analysis method, and calculation model (Rahman and Drisoll 1994).

image

Figure 6—. Relationship of freezable water to moisture content (A) and to water activity (B) of ration components.

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image

Figure 7—. Relationship between freezing point and freezable water content of ration components.

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Sauce and beef had the highest and lowest freezable water content at the same level of aw, respectively (Figure 6B). However, as aw decreased to 0.88, only a small amount of freezable water could be detected. Sauce also had the highest freezable water content at the same freezing point among ration components (Figure 7). When freezable water content was reduced to 0, freezing point was not detectable anymore. Thus, unfreezable water does not contribute to freezing point or influence the molecular mobility of the overall matrix system.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Thermal properties of each MRE ration component varied depending on composition and moisture content. Moisture content was found to be linearly related to freezable water content of ration components during freezing (up to −60 °C). Water activity (aw > 0.88) also was correlated with freezing point of ration components. When moisture content was reduced to 28.8% from the original 75% to 85%, no water was available for freezing. This study indicates that moisture content and water activity of each ration component are critical factors affecting thermal behavior of ration component during freezing. Thermal behavior of ration components can be used to improve the quality and prevent freezing of MRE ration components.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

This study is financially supported by DOD/BAA and US Army Natick Soldier R, D & E Center, Natick, Mass., U.S.A. Kansas Agricultural Experiment Station contribution number: 08-182-J.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References