Similar to fruits, several factors influence the microstructure and quality of frozen vegetables. This mild heat treatment is acceptable for vegetables because they are usually cooked before consumption. Most vegetables are blanched prior to freezing to inactivate the undesirable enzymes in the tissues. In general, freezing destroys the turgidity of vegetable tissues less than it does in fruits because vegetables have less water content and higher fiber content.
Some research studies have shown that water reduction and modification of fruit tissue using osmotic dehydration prior to the freezing process can reduce structural collapse and loss of firmness and increased T g ′ ( Dermesonlouoglou et al., 2007 Forni et al., 1997 Lowithun and Charoenrein, 2009 Siramard and Charoenrein, 2014). This may be extremely damaging to the microstructure and texture of the frozen fruit ( Phothiset and Charoenrein, 2014 Charoenrein and Owcharoen, 2016). After freezing, the frozen fruit may be subject to temperature fluctuation, thawing, and refreezing many times before being consumed. The freezing method and rate of freezing also strongly influence the microstructure, textural properties, and quality of frozen fruit ( Chassagne-Berces et al., 2010 Charoenrein and Owcharoen, 2016). Fruit at its fully ripened stage has a more delicate structure that is more vulnerable to freezing damage than that in its partially ripened stage. Different cultivars have different cellular compositions and water content that affect ice crystal formation. The cultivar and ripening stage have effects on the fruit composition, and cell wall components, which consequently affect ice crystal formation, disruption of the cell wall, turgor loss, and softening after thawing ( Morel et al., 1999 Rimkeeree and Charoenrein, 2014).
The microstructure and quality of frozen fruits are influenced by several factors. (Illustration copyright Sanguansri Charoenrein.) CW, cell wall ICS, intercellular space ML, middle lamella. Light micrograph (A) and transmission eletron micrograph (B) of cell wall microstructure before and after freezing-thawing of mango cultivar Nam Dok Mai.