Shelf-life studies are often used to test the emulsion stability of salad dressing and the staling properties of baked goods, as well as the integrity of packaging films, to cite a few examples. For some products, such as meat, controlled storage has a beneficial (tenderising) effect. For others, such as fruit that is picked mature but unripe, storage develops proper (softer) texture followed by its deterioration.
For still others, storage is always detrimental, but the rate of deterioration varies from very rapid (e.g. bread) to very slow (e.g. butter). With ripened cheeses, storage is part of processing that develops proper flavour and texture.
Types of Change that occur during Storage
Textural changes on storage are generally caused by enzymatic action, changes in moisture, or reactions in food polymers leading to crosslinking and toughening. Enzymatic action can be beneficial, such as occurs in ripening of fruits, meat, and cheese, or detrimental, such as occurs in senescence of fruits (the process of deterioration with age). The effect of changes in moisture depends on the original moisture level in the food and, thus, on its textural character.
Moisture can be lost, absorbed or rearranged. Loss of moisture from high moisture foods (e.g. apples) leads to loss of juiciness and crispness. Loss from medium moisture systems (e.g. cakes) leads to the development of dryness; in the mouth, moisture will be absorbed by the food rather than released from it. Moisture rearrangement is believed to be involved in the staling of bread – quite probably the most researched storage-induced textural deterioration due to its high economic impact. Moisture gain by dry systems valued for their crispness (e.g. snacks, breakfast cereals) results in loss of crispness, development of toughness and, ultimately, sogginess.
Loss of Crispness
Snack products and breakfast cereals are highly valued for their crispness detected as mechanical (stiffness and ease of fracture) and auditory (sound production) sensations. It is interesting that dry/crisp foods lose their crispness on gaining moisture, while wet/crisp foods lose their crispness on losing moisture. When stored under high humidity conditions without proper package protection, crackers and similar products will absorb moisture and become deformable and less prone to fracture. Sensorially, this will be felt as softness, sogginess, or toughness and will lead to product rejection.
There are also multi-textured products, such as pies, in which moisture gradients exist between the different components. The crust is a dry/crisp component that loses its crispness on gaining moisture from the filling. Because of its high fat content, the structure and crispness of pie crust are different from those of crackers. The moisture transfer between the components is governed not so much by their actual moisture contents, but by the difference in their water activity. It may be stopped either by formulating the components to identical (or closely similar) water activity levels or by separating them with a moisture-impermeable barrier.
Increase of Firmness
The effect of storage on butter and margarine involves growth of fat crystals and strengthening of the network bonds. Fat crystals growth in margarine leads to increasing firmness (resulting in increased difficulty to spread) and development of graininess (which is only accentuated by low temperature storage).
Increase of Softness
Commercial canning is a popular way of preserving fruit and vegetables. Because it involves heat processing, the resulting texture resembles that of cooked fruit/vegetables in terms of cellular disintegration and softening. Textural damage on canning manifests itself in the development of softness, a process that sometimes continues on storage.
Although freezing preserves more of the original sensory attributes of fruit than drying or canning, its effect on textural degradation has been well recognised (albeit to different degrees depending upon the fruit type). The extent of textural degradation is diminished by rapid freezing rates, low freezing temperatures, and addition of sugar syrup/brine. It is increased by fluctuating storage temperatures such as are normally encountered in transport and home freezers.
Increase of Toughness
The effect of storage on the texture of meat may be divided into three stages: pre-rigor - where toughening due to cold storage shortening occurs, rigor mortis – where substantial toughening occurs due to extensive biochemical changes, and ageing – where the meat relaxes and develops the desired tenderness. A full understanding of the biochemistry of meat is required to optimise the texture of the meat at each stage of its processing.
Frozen and Fresh Storage Issues
Frozen storage is always detrimental to texture due to ice crystal formation and growth, a process that is usually controlled with formulation in products especially designed for frozen storage, such as ice cream. The severity of textural damage depends upon the structure of the product. It can vary from total product failure (e.g. mayonnaise) to a reduction in quality, such as the development of toughness and drip in meat and fish. Fresh produce, such as fruit and vegetables may also suffer structural damage on impact and static loading, resulting in cell rupture followed by enzymatic degradation.
Shelf-life testing of food systems now commonly employs texture analysis as a testing protocol. Here, the food material is subjected to different environmental conditions and the product is evaluated for changes in physical properties. Whilst unpleasant odours and appearance of a food provide signals that the food is unsafe to eat and represent a potential health hazard, texture deterioration is related to general quality decrease.
The importance of texture in consumer acceptance of foods has gained significant recognition and an eye on consistent textural quality leads to consumer loyalty and subsequent repeat purchases of that product.
If you don’t already know how a texture analyser can be employed to measure, compare and provide a means of product quality acceptance/rejection Stable Micro Systems would be only too happy to help. No-one knows texture measurement and analysis like we do!
Contact Stable Micro Systems today to discover the full range of Texture Analysis solutions.
For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.
The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is
available to measure and analyse the textural properties of a huge range of
food products. Our technical experts
can also custom design instrument fixtures according to individual
specifications.No-one understands texture analysis like we do!
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