Water retention in mushroom during sustainable processing

This thesis deals with the understanding of the water holding capacity of mushroom, in the context of a redesign of their industrial processing. For designing food process the retention of food quality is of the utmost importance. Water holding capacity is an important quality aspect of mushrooms. A convenient process design methodology which accounts also for product quality is Conceptual Process Design (CPD). An approach to follow CPD methodology is first to explore, the material properties of the products to find optimal processing conditions. In this stage the constraints of (existing) processing equipment are not considered. Later in the second stage, suboptimal processing conditions are determined considering the constrains of equipment. In mushroom canning, temperature induced loss of water holding capacity (WHC) of tissue manifests as a lower product yield. This loss of water is accompanied with the loss of nutrients, dissolved in the water. In addition to the loss of product quality like water holding capacity, mushroom canning (with alternating heating and cooling steps) also induces losses of useful resources as (potable) water. In terms of water use, water is added at several steps, and is discarded at other places. This shows that there is opportunity to improve the sustainability of the production system, but with the constraint that product quality is not impaired, or evenly improved. The WHC is an important property that determines several aspects of foods. For example, it determines the juiciness of fruits, vegetables and meat products; the freshness (firmness, or crispiness) of green-leafy vegetables; and the calorie intake per serving for high calorie containing foods such as cheese. Despite being a widely used term in food science, there is no clear definition of water holding capacity and its thermodynamic nature is not fully acknowledged. The understanding of the WHC is even poorer in structured cellular foods such as mushroom, where different water fractions are present in various compartments. In a cellular system water is present as 1) a solution in the vacuoles, 2) water osmotically bound to the cytoplasmic and the cell wall materials and 3) capillary water in pores, which might be filled during processing. Because they have a distinct capillary phase, the mushroom is a good system to study the contributions of various water fraction on the total water retention. The main aim of the current work is to provide insight for the development of canned mushroom processing where: 1) the resources of energy and water are efficiently used, and 2) the quality of mushroom is maintained. These two aims are related to the efficient use of raw materials and maintenance of full weight of mushroom during processing. The WHC is an important quality indicator of mushrooms. The analysis related to water retention of mushroom has been carried out at microscale where molecular and structural interactions in relation to water retention are studied. The micro scale analysis is discussed in chapter 2, chapter 3 and chapter 4. The sustainability analysis is carried out at mesoscale where analysis is carried out at the unit operation level which is described in chapter 5. In chapter 2, the heat-induced change in water holding capacity of particular the gel phase of the mushroom is interpreted with the Flory-Rehner theory, commonly applied to polymer gels. As done earlier for meat, we have first assumed that WHC loss in mushroom can also be attributed to the protein denaturation. This assumption is based on the experimental observation that, like meat, mushroom also follows a typical sigmoid relation with change in temperature. In the theory, we have regarded mushroom as a homogeneous biopolymer hydrogel, in which salt and sugar are dissolved. The water holding capacity is then understood as the swelling capacity of the biopolymer gel. The thermodynamic state of this simplified system is characterized by the so-called swelling pressure, which is decomposed into three independent contributions: 1) the mixing pressure induced by sugars and polymers, 2) the ionic pressure induced by the salt, and 3) the elastic pressure induced by the polymers. An assumption was made that the heat treatment denatures mushroom protein, which is reflected in the change of the Flory Huggins interaction for protein. It follows the same temperature dependency as the WHC loss by mushroom under zero mechanical load. The assumption of the temperature dependency of the interaction parameter is tested with an independent sorption measurement. With the assumption, the sorption curve for mushroom sample which were preheated previously at 30, 60 and 90 °C could accurately be predicted. Curve fitting of WHC under various mechanical loads has shown that model parameters that are associated with the elastic pressure, the crosslink density and fraction of the polymer in the relaxed state, are temperature dependant. The values of increased in contrast to the decrease of upon heating of mushroom tissue up to temperature of 70°C. The result indicates that heat treatment increases the polymer chain length between the cross links as original conformation of mushroom is lost. At the same time, more crosslinks are formed by a polymer because of aggregation of polymers. However, in our fitting procedure, we have excluded WHC data at low external pressure values, as water is present in both gel phase as in in the capillaries. This is done as the pores in this range are not fully collapsed and the Flory Rehner theory is valid only for the gel phase In chapter 2 mushroom is simplified in the sense that only compositional contribution is considered in WHC but not the structural contributions. In subsequent chapters we have acknowledged that mushroom has a cellular structure with a distinct pore phase. The pores are intentionally filled during processing via vacuum impregnation. The contribution of water present in the capillaries due to vacuum impregnation of mushroom has been discussed in chapter 3. Both the temperature of heat treatment and the initial porosity of mushroom contribute independently to water holding capacity of heat treated mushroom. The hydration of heat treated mushroom increases linearly with the initial porosity of mushroom for all the temperatures from 30 to 90 °C. The porosity of mushroom can also largely explain the increase in hydration of heat treated mushroom with storage as both porosity and the hydration increase simultaneously with the storage days. The fluid that filled in the capillaries acts against collapse of the hyphae which have inherent elastic force that works in the other direction. The initial porosity of mushroom is an important aspect that determines the hydration of the heat treated mushroom and therefore, cannot be ignored. In addition, the Flory-Rehner theory alone cannot capture the contribution of the capillary water. Hence an addition is needed in the theory to capture this effect. The cellular phase in mushroom tissue is even more complicated because water is present in this phase in two other forms, as gel water and the intracellular water. Chapter 4 takes into account the role of structure in the WHC. The role of cell membrane integrity and the cell-wall structural components is investigated for retention of the water fraction. The cell membrane integrity is calculated from the conductivity measurement of the fluid that leaches out from the vacuole that has salts in it. The loss of the cell membrane integrity largely explains the water loss from heat treated mushroom sample. The loss of cell membrane integrity is also related with the water loss from frozen mushroom, but additional losses occur during freezing due to novel crosslinks formed during the growth of ice crystals compressing the unfrozen cell wall material. The enzymatic hydrolysis of mushroom cellular components shows that chitin and mushroom protein both contribute to the water holding capacity either via osmotic binding or by their role to provide the mechanical strength to the mushroom hyphae. In addition, proteins have additional contributions to water retention by mushroom because of their electrostatic interaction as polyelectrolyte. This is evident as the hydration of the mushroom increases with pH of mushroom. In chapter 5, the efficiency of the use of the resources (raw materials, energy water) is investigated. The mass and exergy flow in the current production system is visualized with the Sankey diagrams. The sustainability of unit operations involved in the current production system of canned are analysed with the second law efficiency using exergy. Using ideas from Process Intensification three alternative routes are proposed for the production of canned mushrooms namely: 1). Slicing before vacuum hydration, and 2) Using hot water for vacuum hydration and 3) Using blanch water for vacuum hydration. Using hot water for vacuum hydration is not seen as a feasible option, since it consumed more resources. Slicing mushroom before their vacuum hydration and using blanch water for hydration of mushroom lowers the resources requirement for production. In addition, using blanch water for hydration also increases the final product yield. Finally, the main findings of this thesis are summarized in the general discussion in chapter 6. The findings from previous chapters are combined to an overall description of water loss from heat treated mushroom. The overall description of water holding capacity in mushrooms is given in terms of the thermodynamic conditions for equilibrium between the different compartments holding water. The two dimensions of the thesis, the higher water retention of processed mushroom and more sustainable operation are discussed in the light of conceptual process design, using a micro/mesoscale approach. At the microscale material properties of mushroom are discussed. The biggest effect comes from cell membrane integrity loss. The porosity of fresh mushroom and the ionic interactions of polymers are the other effects that influence the WHC. Mesoscale analysis shows that shifting the sequence of unit operations and reusing the blanch water that is discarded in the current production process can improve the sustainability. Finally based on outcome of current work, future perspective of current work is discussed briefly. Overall, this thesis demonstrated that there is substantial scope in improving the efficiency in the use of resources in producing preserved mushroom. Also scope in retention of water in the mushroom tissue is demonstrated. Thus this thesis shows that both aspects, product and process efficiency, can be improved at the same time.