Glass transition and state diagram for freeze-dried Agaricus bisporus

Abstract

Sorption isotherms and state diagram of freeze-dried Agaricus bisporus were developed to further investigate the connection between the two distinct criteria of food stability. Sorption isotherms of freeze-dried A. bisporus were determined by the gravimetric method and the data were modeled by GAB model. The GAB monolayer moisture content was calculated to be 6.2 g/100 g (d.b.). The glass transition temperature of freeze-dried A. bisporus was determined by differential scanning calorimetry. The state diagram was composed of the freezing curve and glass transition line, which were fitted according to Clausius–Clapeyron model and Gordon–Taylor model, respectively. The state diagram yielded maximally-freeze-concentrated solutes at 0.704 solids with the characteristic temperature of glass formation being −77.9 °C. The state diagram and sorption isotherms of freeze-dried A. bisporus are useful in evaluating the storage stability as a function of temperature and moisture content as well as optimizing drying and freezing conditions.

Introduction

Water activity (a_w) and moisture content, correlated through sorption isotherms, have been considered relevant parameters to describe food stability. Information on moisture sorption isotherms are extremely important in modelling drying process, designing and optimizing drying equipment, predicting shelf-life stability, and calculating moisture changes during storage and selecting appropriate packaging material (Yazdani et al., 2006). Water activity depends on the composition, temperature and physical state of the compounds. The physical state of food compounds is also related to the stability (Fabra et al., 2009).

Recently the limitations of a_w are pointed and alternatives are proposed. These limitations are: (i) a_w is defined at equilibrium, whereas foods may not be in a state of equilibrium, (ii) the critical limits of a_w may also be shifted to higher or lower levels by other factors, such as pH, salt, anti-microbial agents, heat treatment, and temperature, (iii) nature of the solute used also plays an important role, and (iv) it does not indicate the state of the water present and how it is bound to the substrate (Rahman, 2006). Thus, glass transition concept was put forward considering the limitations of a_w (Rahman, 2006, Rahman et al., 2009). Glass transition is a nature of second order time- temperature dependent transition of physical state of a material. The glass transition temperature (T_g) is defined as the temperature at which an amorphous system changes from the glassy to the rubbery state. As the temperature increases above T_g, various changes in system such as increase in free volume, decrease in viscosity and increase in thermal expansion, are noticed. The most important changes affecting food behavior are related to the exponential increase in molecular mobility and decrease in viscosity. These factors govern various time-dependent and often viscosity-related structural transformations, such as stickiness, collapse, and crystallization during food processing and storage (Shi et al., 2009). The importance of T_g of amorphous food materials for processing and storage stability has been recognized and emphasized by many researchers and a wide range of potential food applications of the glass transition phenomenon have been identified (Bhandari and Howes, 1999, Matveev et al., 2000, Kasapis, 2006). An important utility of T_g is made in the state diagram which, in its simplest form, represents the pattern of change in the state of a material as a function of increasing levels of solids (Roos and Karel, 1991, Sablani et al., 2004). State diagram are graphical representations of the physical states of food constituents with respect to the temperature, moisture or solids content of foods at constant pressure for equilibrium and non-equilibrium systems (Sablani et al., 2010). The state diagram of a food over the whole range of moisture content gives full information about the temperature of the different phase transitions at given moisture content and assists in predicting food stability during storage as well as selecting a suitable condition of temperature and moisture content for processing (Fabra et al., 2009, Shi et al., 2009, Sablani et al., 2010). Rahman (2010) combined both the water activity and glass transition concepts in the state diagram. The BET-monolayer value was also plotted in the state diagram and macor-micro region concept was put forward. Further applications of macro–micro region concept in the state diagram including drying process, baking process, and stability of enzymes are also interpreted in detail (Rahman, 2010).

As the importance of state diagram is better recognized, more studies have been carried out for real foods during recent years. State diagrams have been reported for grape (Sá and Sereno, 1994), carrot (Georget et al., 1999), dates (Kasapis et al., 2000, Rahman, 2004, Guizani et al., 2010), apple (Bai et al., 2001), persimmon (Sobral et al., 2001), Pineapple (Telis and Sobral, 2001), mango (Ayala et al., 2002), tomato (Telis and Sobral, 2002), strawberries (Moraga et al., 2004), garlic powder (Rahman et al., 2005), kiwifruit and gooseberry (Moraga et al., 2006, Wang et al., 2008), grapefruit (Fabra et al., 2009), raspberry (Syamaladevi et al., 2009), tuna (Rahman et al., 2003), abalone (Sablani et al., 2004), horse mackerel (Shi et al., 2009). However, based on the authors’ knowledge, no data are available either for edible fungi in general or for A. bisporus, in specific.

A. bisporus is the most widespread with artificial cultivation in the world with yield account for 70% of total edible fungi. A. bisporus is considered as particular high-ranking nutritional vegetable in Europe and America with excellent source of several essential amino acids, vitamins (B₂, niacin, and folates) and minerals (potassium, phosphorus, zinc and copper) (Manzi et al., 2001). Consumption of A. bisporus has increased rapidly due to their tender texture, flavor and nutritional value. However, A. bisporus is one of the most perishable products and its commercial value decreased or fully lost within 1–3 days at ambient temperature. Drying especially freeze-drying and quick freezing are the main preservation methods for A. bisporus. During these processes, the formation of non-equilibrium amorphous states is usual (Roos, 1995a). Several authors have discussed the importance of the glassy-rubbery state in food products as it relates to collapse, stickiness, caking and re-crystallization phenomena (Bhandari and Howes, 1999, Kasapis, 2006, Ohkuma et al., 2008). The formation of solute crystals will imply an increase in free water and thus in a_w, with the consequent increase in the rate of deteriorative reactions. In this sense, it is very important to know the moisture content–water activity–glass transition temperature relationships to determine storage temperature and relative humidity (RH) that insure the stability of freeze-dried or frozen products avoiding the change from the stable glassy state to the rubbery one (Fabra et al., 2009).

Therefore, the aims of this work were (1) to establish the state diagram and the moisture sorption isotherm of freeze-dried A. bisporus, (2) to evaluate and compare a stability criterion with both the glass transition theory and the water activity concept.