From the point of view of texture, vegetables represent a very diverse group of products.They can be consumed in many different textural forms. For instance, cooked asparagus is soft, fibrous and pulpy, cucumber is crisp, firm and juicy, raw carrots are hard and crisp, and cooked peas are soft and mealy. The tissue structure in each case determines the texture as received by the consumer.
Fresh vegetables
As opposed to fruits, which are generally picked when mature but unripe, many vegetables (apart from legume seeds, tubers, bulbs and roots) are picked and consumed when still immature, tender and succulent. This is largely due to the fact that while fruits generally soften on maturation and storage, young vegetables become tougher when they age. This toughening is due to the lignification of the primary cell wall and formation of the secondary cell wall.
Many of the form, structure and physiological considerations discussed in connection with fruit texture also apply to vegetable texture. The main difference is that, while on maturation the fruit tissue stops growing and suffers enzymatic degradation, the vegetable tissue proceeds to differentiate and cell growth and enlargement continue. The amount of fibrous tissue increases. Toughening may reach the point where the vegetable is no longer suitable for human consumption.
Fibre formation is a particularly important problem with asparagus. Asparagus fibrousness can be measured using a cutting test with a wire or blade. The Warner Bratzler Blade is ideal for round samples due to its V-shaped notch, and it can be used along with a slotted insert to guide the blade into a Heavy Duty Platform for total shearing of an asparagus spear. The further the fibrous tissue has developed, the higher this shearing force will be.
Most vegetables are cooked before consumption, a process that softens their texture (unless there is a lot of lignification) and gelatinises the starch deposited as food for the plant. Starch and protein deposits are particularly abundant in beans, peas, and corn picked at maturity. A sugar to starch equilibrium reaction occurs during the storage of some vegetables, and this can affect their texture.
Sweetcorn and sweet peas when picked immature have much sugar and a succulent texture. Within hours the texture of sweetcorn can become tougher and drier due to sugar conversion to starch. Sweetcorn toughness can be measured using Volodkevich Bite Jaws, a fixture that performs an imitative test by simulating the action of an incisor tooth biting through food. A kernel is placed centrally on the lower jaw and the upper jaw moves down to shear the sample. Toughness is measured by using an automatic analysis macro to calculate the area under the force-time graph on the downward stroke.
In general, lowering the temperature at which fresh vegetables are stored will decrease their rate of textural changes by slowing down the metabolic processes. However, exceptions are potatoes, cucumbers and tomatoes. White potatoes, which contain a lot of starch at harvest, will undergo a starch to sugar conversion when stored below 10°C. This conversion leads to textural deterioration in both raw and cooked potatoes, a sweet taste, and a proneness to browning on dehydration, which can cause problems in the manufacture of potato chips. The texture of both raw and cooked potato can be measured using a Knife Blade test. Samples are cut to constant, known dimensions for high repeatability and sheared. Maximum force is seen to decrease substantially as the length of cooking time is increased.
When cucumbers are stored below 7°C, their texture breaks down due toexcessive
softening and exudation of liquid as tissue undergoes a structural
collapse. Tomatoes are also susceptible to low-temperature injury.
Dehydration is another cause of textural changes during the storage of fresh vegetables. It can lead to the wilting of leafy vegetables and loss of plumpness in fleshy vegetables. It can be prevented by waxing some vegetables, such as cucumbers and aubergines. Probe penetration tests are well suited to measuring the texture of vegetables with tough skins and fleshy internal tissue. For example, a penetration test into a pepper sample shows a linear force increase as the probe presses into the surface and causes some caving in. The force drops suddenly as the probe penetrates into the skin, which recovers and the probe quickly pushes through the brittle underlying flesh. The force continues to drop gradually as the probe penetrates through the remaining flesh before emerging through the other side. All of these graph properties can be assessed separately during analysis for a full picture of the pepper’s texture.
Processed vegetables
From a textural standpoint, vegetables can be divided into several categories:
1: Those that are eaten raw and valued primarily for their crispness. such as radishes and lettuce. These products lose their firm and crisp texture on heating because of loss of turgor and cannot be fully preserved by canning (involving a heat sterilisation step) or freezing (involving blanching, an enzyme inactivation heating pretreatment). Some, such as cucumbers, are preserved by brining/fermentation and turned into a crisp new pickled product.
2: Those that are eaten cooked and valued for their softness/mealiness, such as aubergines and potatoes.
3: Those that can be consumed in either form, such as carrots and cauliflower. These vegetables have relatively thick cell walls that provide rigidity and crispness in the absence of turgor.
Canning
It is taken for granted that the texture of canned vegetables resembles that of overcooked vegetables. Just like with fruit, the softening effect is due to pectin degradation and calcium displacement. It can be reduced by addition of calcium to the canning liquid. Once degraded, the texture of canned vegetables is fairly stable on storage. Vegetables stored for a significant period of time prior to processing are more prone to softening on canning.
Freezing
Frozen vegetables have a better appearance than canned and their texture is expected to resemble that of fresh, firm-cooked vegetables. Additionally, the texture of vegetables is less susceptible to damage on freezing than that of fruit texture.
As opposed to fruits, which are generally picked when mature but unripe, many vegetables (apart from legume seeds, tubers, bulbs and roots) are picked and consumed when still immature, tender and succulent. This is largely due to the fact that while fruits generally soften on maturation and storage, young vegetables become tougher when they age. This toughening is due to the lignification of the primary cell wall and formation of the secondary cell wall.
Fibre formation is a particularly important problem with asparagus. Asparagus fibrousness can be measured using a cutting test with a wire or blade. The Warner Bratzler Blade is ideal for round samples due to its V-shaped notch, and it can be used along with a slotted insert to guide the blade into a Heavy Duty Platform for total shearing of an asparagus spear. The further the fibrous tissue has developed, the higher this shearing force will be.
Sweetcorn and sweet peas when picked immature have much sugar and a succulent texture. Within hours the texture of sweetcorn can become tougher and drier due to sugar conversion to starch. Sweetcorn toughness can be measured using Volodkevich Bite Jaws, a fixture that performs an imitative test by simulating the action of an incisor tooth biting through food. A kernel is placed centrally on the lower jaw and the upper jaw moves down to shear the sample. Toughness is measured by using an automatic analysis macro to calculate the area under the force-time graph on the downward stroke.
Dehydration is another cause of textural changes during the storage of fresh vegetables. It can lead to the wilting of leafy vegetables and loss of plumpness in fleshy vegetables. It can be prevented by waxing some vegetables, such as cucumbers and aubergines. Probe penetration tests are well suited to measuring the texture of vegetables with tough skins and fleshy internal tissue. For example, a penetration test into a pepper sample shows a linear force increase as the probe presses into the surface and causes some caving in. The force drops suddenly as the probe penetrates into the skin, which recovers and the probe quickly pushes through the brittle underlying flesh. The force continues to drop gradually as the probe penetrates through the remaining flesh before emerging through the other side. All of these graph properties can be assessed separately during analysis for a full picture of the pepper’s texture.
Processed vegetables
From a textural standpoint, vegetables can be divided into several categories:
1: Those that are eaten raw and valued primarily for their crispness. such as radishes and lettuce. These products lose their firm and crisp texture on heating because of loss of turgor and cannot be fully preserved by canning (involving a heat sterilisation step) or freezing (involving blanching, an enzyme inactivation heating pretreatment). Some, such as cucumbers, are preserved by brining/fermentation and turned into a crisp new pickled product.
2: Those that are eaten cooked and valued for their softness/mealiness, such as aubergines and potatoes.
3: Those that can be consumed in either form, such as carrots and cauliflower. These vegetables have relatively thick cell walls that provide rigidity and crispness in the absence of turgor.
Canning
It is taken for granted that the texture of canned vegetables resembles that of overcooked vegetables. Just like with fruit, the softening effect is due to pectin degradation and calcium displacement. It can be reduced by addition of calcium to the canning liquid. Once degraded, the texture of canned vegetables is fairly stable on storage. Vegetables stored for a significant period of time prior to processing are more prone to softening on canning.
Freezing
Cell breakage causes a lot of this damage and is related to the rate of freezing. As with other high moisture products, slow freezing causes the formation of large ice crystals leading to destruction of the cell and leaching out of cell contents. Small ice crystals do not rupture the cell membrane, so the texture of fast-frozen products is less degraded. The texture of fresh and defrosted peas can be measured in comparison using the Multiple Pea Test Rig. This rig measures the force to simultaneously penetrate up to 18 samples.
Puncture strength and penetration force are measured and the penetration distance set so that the probes penetrate completely through the sample. Two peaks are seen on the force-time graph – one each as the probes break through the top and bottom surface skins, either side of a plateau as the probes push through the mealy flesh.
Pickles
Pickled and / or fermented vegetables such as cucumbers and olives soften during storage, and this is considered undesirable by the consumer. A test ideally suited to measuring pitted olive texture is a Kramer Shear Cell test. The cell is used for analysing multi-particle products such as cereals and pickles in sauce together with fruit and vegetables. This test applies a combination of compression, shearing and extrusion and allows an averaging effect by measuring multiple samples in bulk.
Mechanical damage
Seeds of low moisture content handled in bulk, such as soya beans and corn, are often subjected to conditions in storage and transport that result in kernel breakage, which is of substantial concern to agricultural engineers.
Pickles
Mechanical damage
Factors affecting the susceptibility to breakage as well as equipment to quantify it continue to be researched. The susceptibility to breakage of soya beans can be measured using multiple methods, such as penetration, bulk shear or compression. In each case, a bean sample is particularly prone to breakage if its fracture force and distance, or area under the force-distance graph, are low.
Hard-to-cook defect in legumes
The inability of beans to soften during a reasonable cooking time can develop in beans when they are stored at high temperature and high relative humidity. This defect results in poor quality products and increased fuel costs, lowering the commercial value of the beans. Much research has been and continues to be done on the mechanism of bean hardening. However, there is no general agreement on the exact mechanism. Cooking quality is an important quality characteristic for pulses (e.g. peas, lentils, chickpeas, and beans) because they are commonly consumed after cooking in various types of foods. Cooking is required to ensure acceptable sensory quality.
Texture is one of the most important quality factors influencing consumer acceptance of cooked pulses, and so the evaluation of the texture and firmness of cooked pulses is critical to the determination of cooking quality. An AACCI method was developed in 2013 for the determination of the firmness of cooked pulses. A collaborative trial based on ten laboratories analysing 26 blind duplicates of thirteen different samples were tasked to evaluate the repeatability and reproducibility of the method.
After cooking according to the standard preparation procedure, the firmness of pulse samples isdetermined using a TA.XTplus Texture Analyser loading approximately 7.5g of cooked sample into a Mini Kramer Shear Cell. The sample is compressed and extruded at 1.5mm/s.
Hard-to-cook defect in legumes
The inability of beans to soften during a reasonable cooking time can develop in beans when they are stored at high temperature and high relative humidity. This defect results in poor quality products and increased fuel costs, lowering the commercial value of the beans. Much research has been and continues to be done on the mechanism of bean hardening. However, there is no general agreement on the exact mechanism. Cooking quality is an important quality characteristic for pulses (e.g. peas, lentils, chickpeas, and beans) because they are commonly consumed after cooking in various types of foods. Cooking is required to ensure acceptable sensory quality.
Texture is one of the most important quality factors influencing consumer acceptance of cooked pulses, and so the evaluation of the texture and firmness of cooked pulses is critical to the determination of cooking quality. An AACCI method was developed in 2013 for the determination of the firmness of cooked pulses. A collaborative trial based on ten laboratories analysing 26 blind duplicates of thirteen different samples were tasked to evaluate the repeatability and reproducibility of the method.
The firmness of the cooked sample is defined as the maximum force required to shear the cooked sample and expressed as the maximum shear force per gram of cooked sample (N/g of cooked sample). Firmness values should be an average of six determinations.
Reference: Food Storage Stability, Chapter 8 “Effect of storage on texture” – Alina S. Szczesniak
Reference: Food Storage Stability, Chapter 8 “Effect of storage on texture” – Alina S. Szczesniak
There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of 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!
To discuss your specific test requirements click here...
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!
To discuss your specific test requirements click here...
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