State diagram and water adsorption isotherm of raspberry (Rubus idaeus)

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Abstract

Thermal transitions of freeze-dried raspberry powder (Rubus idaeus) were analyzed by using differential scanning calorimetry. Freeze-dried raspberry powders containing unfreezable and freezable water were examined to develop the state diagram of raspberry. The state diagram of freeze-dried raspberry powders included the glass line; glass transition temperature versus solids content, freezing curve; initial freezing point versus solids content; end point of freezing Tm, corresponding solids content Xs, characteristic glass transition Tg and corresponding solids contents Xs of maximally-freeze-concentrated raspberry. The conditions of the maximal-freeze-concentrate obtained from freezing curve corresponded to Tm=-38°C and Xs=0.78kg solids/kg raspberry and Tg=-47°C and Xs=0.82kgsolids/kg raspberry. The Tg was determined by extending the freezing curve to glass line. The quantities of unfreezable water identified from enthalpy data and the freezing curve were comparable. Adsorption isotherms of freeze-dried raspberries were determined at room temperature by the isopiestic method and the data was modeled with BET and GAB equations. The BET and GAB monolayer moisture contents were observed to be 0.045 and 0.074 kg water/kg dry raspberry solids, respectively. The state diagram and water sorption properties of raspberries are useful in optimizing the retention of anthocyanins, phenolics concentrations and antioxidant activities in freeze-dried and frozen raspberries during storage.

Introduction

A state diagram of food presents different physical states of food as a function of solids content and temperature. The role of the state diagram of food materials in determining processing and storage stability is highlighted in a number of studies (Rahman, 2006, Sablani et al., 2004, Champion et al., 2000, Goff and Sahagian, 1996, Sa and Sereno, 1994, Roos and Karel, 1991, Slade and Levine, 1991). The state diagram consists of a freezing curve of initial freezing point versus solids content, a solubility curve of solids fraction in a saturated aqueous solution at a given temperature, the eutectic point, a glass line of glass transition temperature versus solids content, and conditions of maximal-freeze-concentration (Rahman, 2006). The concept of glass transition was investigated extensively in polymer, material, pharmaceutical and food sciences to relate physical, chemical and structural changes in the physical state of material. Glass transition is a nature of second order time-temperature dependent transition of physical state of a material. During glass transition temperature change, material transforms from a relatively stable glassy state to a metastable rubbery state or vice versa. As a result of the industrial relevance and scientific interest of glass transition research, researchers continue to discuss the application of glass transition as a tool for predicting the microbiological, physical and chemical changes that occur during processing and storage (Sablani et al., 2007a, Sablani et al., 2007b, Sablani et al., 2007c, Kasapis et al., 2007, Rahman, 2006, Khalloufi and Ratti, 2003, Champion et al., 2000, Karel et al., 1994, Kerr et al., 1993, Roos and Karel, 1991, Slade and Levine, 1991).

Raspberries (Rubus idaeus) are commercial fruits used industrially for formulating jam, jelly, sauce, puree, topping, syrup or juice concentrates. Raspberry fruit is well recognized for health promoting constituents. Raspberries are rich in potential antioxidant phenolic compounds including anthocyanins. Studies evaluated the potential role of raspberries in preventing chronic stress, cancer and heart diseases (Zhang et al., 2005, Wang and Lin, 2000). Anthocyanins and phenolic compounds are susceptible to deterioration during processing and storage conditions (Sadilova et al., 2006). Stability of bioactive compounds during processing and storage is important to the food industry.

Glass transition temperature data are reported for several fruits (tomato, dates, pineapple and grapes) but a complete state diagram using glass lines and freezing curves are reported only for selected fruits (apples, strawberries, grapes and dates) (Bai et al., 2001, Kasapis et al., 2000, Rahman, 2004, Sa and Sereno, 1994, Sa et al., 1999). Khalloufi et al. (2000) examined glass transition temperatures of raspberries, blueberries, strawberries and blackberries as a function of water contents. The glass transition temperatures of the berries decrease as water contents increase. Since soluble solids of berries are mostly sugars, the glass transition temperature of the freeze-dried powder is associated with the glass transition temperatures of glucose and fructose. However the studies related to freezing curve and conditions of maximally-freeze-concentration for berries including raspberries are not reported in the literature. This information on maximal freeze concentration of berries is important to develop a complete state diagram useful in studying stability of anthocyanins and other bioactive compounds in frozen and dried raspberries.

The objective of the current study is to develop a state diagram for freeze-dried raspberries by determining glass line (Tg versus total solids content), freezing curve (initial freezing temperature versus total solids content) and maximal-freeze-concentration conditions (Tm, Tg and Xs). In addition, a water adsorption isotherm is determined to evaluate and compare a stability criterion with the concept of glass transition.

Section snippets

Materials and methods

Red raspberry fruits (Rubus idaeus) grown in Vancouver, WA were collected and frozen immediately at −37 °C for 48 h. The frozen raspberries were layered in the metal trays of freeze dryer (Virtis freeze mobile 24 with Unitop 600 L, VirTis SP Industries Co., New York) to decrease the water content. The shelf temperature was set at −20 °C with a vacuum of 20 Pa. The temperature of the condenser was adjusted to −60 °C. After 48 h of freeze drying, the raspberries were removed and ground immediately to a

Adsorption isotherm of freeze-dried raspberries

The water adsorption isotherm of freeze-dried raspberry powders at 23 °C followed a typical type II behavior as presented in Fig. 1. The adsorption isotherm of freeze-dried raspberry powders exhibits a sigmoid shape with three distinct regions aw = 0.0 to 0.25, 0.25 to 0.6 and 0.6 to 0.8 typical to type II isotherm. The sigmoid shape of sorption isotherm is common for many food and biological materials (Rahman, 1995, Rahman and Labuza, 1999).

The sorption isotherm data was modeled using BET and GAB

Conclusions

The state diagram of raspberries was developed by determining glass line, freezing curve and the conditions of maximally-freeze-concentration. The initial glass transition temperatures of freeze-dried raspberries decreased linearly from 17.5 °C to −65.5 °C as the total solids content decreased from 0.966 to 0.758 kg dry solids/kg sample. The initial freezing point of freeze-dried raspberries decreased from −2.45 °C to −17.4 °C as total solids content increased from 0.30 to 0.70 kg dry solids/kg

Acknowledgements

This activity was funded, in part, with an Emerging Research Issues Internal Competitive Grant from the Washington State University, College of Agricultural, Human, and Natural Resource Sciences and Agricultural Research Center.

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