Comparisons of methods for measuring yield stresses in potato puree: effect of temperature and freezing


 


Introduction


            Purees are mashed vegetables and fruits; usually it is pressed and grounded until it reached the consistency of thick liquid. One of the most popular purees is the potato, most commonly known as mashed potatoes. A number of commercial foods like baby foods are solid matters turned into a thick paste. Potato which has a scientific name Solanum tuberosum is a perennial plant from the family Solanaceae. Potatoes are popular for their high content in carbohydrates. The prevalent form of carbohydrates in potatoes is the starch. According to (1996) an important part of the starch in potatoes is the resistant starch, and it is regarded to have the same physiological effects and health benefits. Aside from carbohydrates, potatoes also contain important vitamins and minerals. Due to these important contents found in potatoes, a number of food companies started to produce and manufacture foods in packages such as baby foods. Feeding vegetables is one of the hardest tasks a parent or a caregiver does especially with children that is one of the reasons why food manufacturers developed products that are in the form of puree, another reason individuals who have a hard time macerating foods are benefited by these products. Potatoes are one of the most visible vegetable products that are often turned into aqueous media.


            This paper examines potato purees and the utilisation of rheology as a characterization technique. Moreover, this paper also discusses the advantages and disadvantages of rheology as a characterisation technique, as well as, its basis. Other characterisation techniques will also be discussed in this paper. And finally, a discussion on the microstructure of potato will also be given.


            According to (2007) Rheology is the study of the flow of matter and deformation under the influence of applied stress. Rheology is often applied in engineering, geophysics and physiology. Aside from those, rheology is also applied in various areas of food science. The textural properties of foods such as juiciness, hardness, tenderness, etc. that human beings usually feel are the rheological natures of foods. 


            According to McClements (n.d.) rheology is an important characterisation technique because it aids the food scientists in measuring the properties of food. Moreover, stated that techniques are necessary for the usual analysis and studies in research and development. The research and development of food products are done to determine, observe and identify the intricate correlation of the rhelogical properties and concentrations of foods, which enables the food industry to efficiently use the resources and processing situation in order to manufacture and produce excellent and top notch products.


            Rheological behaviours of foods vary from low viscosity, solid-like and liquid-like behaviour and hard solids. In the case of potato purees, in determining its rheological behaviour, particularly the yield stress different characterisation techniques are being utilised by the food industry and the laboratories.


            Potato is considered a semi-solid material, and often times it is made into puree by the food manufacturers, and because of that the rheological properties such as yield stress, thixotropic flow behaviour and viscoelacity have a strong effect upon food products (2006). Moreover, discussed that semi-solid material such as potatoes display liquid and solid like properties depending upon temperature, stress, and strain. Different rheological applications are used for several reasons and one of those is to determine the yield stress. In the study of yield stress was measured using characterisation technique. But what is yield stress?


            According to  (2006) yield stress is the pressure in which a matter has the capability of supporting without breaking. A typical yield stress is 10¯³ to 10¯² times the shear modulus.      


According to (2004) potato puree was compatible with the Herschel-Bulkley flow behaviour; the measurement of the yield stress using the extrapolation of the flow curves incurring the Herschel-Bulkley model will produce more reliable results.  The  model is suitable for determining the yield stress in potato purees because, as mentioned earlier, potato is a semi-solid material, and  (2005) discussed that this type of material displays a yield stress which relies on the solid volume fraction. During the processing of the semi-solid materials like potato the viscosity transforms considerably because of the segregation effects of the internal microstructure. Moreover,  stated that semi-solid materials function like visco-plastic fluids differentiated by a limited yield stress τand by material properties that rely on time and shear rate, thus Herschel-Bulkley fluid model with its parameters that are dependent on time can be employed to characterize the flow and time depended circumstances of semi-solid materials.


The basis of Rheology is the classical disciplines of elasticity and Newtonian fluid mechanics to materials in which their mechanical behaviours and properties cannot be defined and describe by classical theories. Rheology combines the unrelated fields if plasticity and non-Newtonian fluids.


The bases of the Herschel-Bulkley fluid model are the Bingham models and power-law models. The Herschel-Bulkley fluids show a yield stress, τ0, which is the minimum stress needed for a material to deform (2005).


According to (2006) the model could be described mathematically by the following:


τ = τ0  + k (у)n


Where τ = shear stress


τ0 = yield stress


k= consistency


y= shear rate


and n= power law exponent


 


A number of studies prefer using the Herschel-Bulkley equation over power law and Bingham models because the model yields more accurate results.


            As for the basis on the Bingham Law, it is a two-parameter rheological model that can be described in the following mathematical equation:


τ= YP + PV (y)


Where τ = shear stress


Y = shear rate


UP = yield point


YP = plastic viscosity


             discussed that it is often used in fluids, and fluids that comply with the model are called Bingham plastic fluids and display a linear shear-stress, shear-rate behavior after the primary shear-stress threshold has been attained, PV is the slope of the line while the YP is the threshold stress.


            In addition,  discussed that a number of foods exhibits a rheological behaviour that is called plasticity, and the conceptual plastic material is directed as a Bingham plastic. There are two equations that are necessary to describe the rheological behavior of a Bingham plastic and these are:


t = Gg                                                 (for t < t0)


t  - t0 = hdg/dt                                   (for t ³ t0)


            In the equation G stands for the shear modulus, h is the viscosity, and t0 stands for the yield stress. Moreover, McClements stated that foods that usually exhibit plasticity are composed of a web of aggregated molecules that are distributed in the liquid matrix.


            The other basis for Herschel-Bulkley model is the Power Law model; (2006) defined it as a helpful rheological model that characterizes the correlation between viscosity or shear stress and shear rate over the range of shear rates.


            Rheological characterization of foods has its advantages, according to (2000) rheological techniques provides knowledge on the rheological and mechanical properties of different food systems which is significant in the design of the flow process for the quality control, it is also important in predicting the storage and stability measurements and in understanding and designing the texture. Another advantage of Rheological characterization of foods is that it gives significant information for food engineers and scientists to enhance and optimize the products and the process of manufacturing.


All characterization techniques have also its disadvantages, and according to  the equation of the Herschel-Bulkley model which is often used in Rheological characterisation has two major disadvantages and these are:


 The magnitude of K or the consistency relies on “n,” which is variable and not appropriate for interpretation physically, and the curve is forced mathematically to be shear thinning or shear thickening  when the shear rate is near zero, and because of that the outcome of the equation provides an overestimation of the yield stress in shear thickening.


            Aside from Rheology, there are other forms of characterization technique that can be utilized in determining the yield stress of potato purees.


            Molecular Markers- molecular markers as a characterization technique for potatoes could be utilized. In the study of  (2004) potato genotypes were characterized using molecular markers such as polyacrylamide gel electrophoresis, RAPD techniques and isozymes of esterase, phosphoglucomutase and soluble proteins were analyzed in potato tubers.


X-ray Diffraction- it can also be used as a characterization technique for potato purees. In the study of  (1960) the structures of potato were examined and by using X-ray diffraction, the researchers were able to confirm the diametral dimension og mosit and dry, normal and Lintherized potato starch.


Fluorescence Spectroscopy- In studying the microstructure of potatoes, this method can also be utilized by food researchers. (2005) used fluorescence spectroscopy, together with, reflectance spectroscopy to dtect the compositional nutrient diagnosis N index, which reflect the inadequacy of nutrients and interactions.


 discussed that there are two forms of experimental tests used by a number of food scientists to characterize foods that are viscoelastic or semi-solid like the potato. -


            Transient Experiments- In transient experiments persistent force is used in a material and the outcome strain is measured as a function of time.


·         Creep- In the creep experiment, persistent stress is utilized to the material and the conforming strain is followed as a function of time. The outcome of the experiment is stated in terms of the parameter called the compliance J = strain/stress, due to the fact that the stress remain constant.  Aside from the stress remaining constant, the transformation of strain can also be calculated when the stress is removed. Semi-solid materials are usually characterized through modulus and relaxation time that could be identified by examining the strain curves with time.


·         Stress Relaxation- in this method, constant strain is applied and then calculated the change in stress with time.


Dynamic Experiments- the other experimental test that


discussed is the dynamic experiment. In this experiment a sinusoidal stress is used in the semi-solid material and the outcome sinusoidal strain is then calculated.


In the study of  (1999) the microstructure of potatoes during blanching shows that the swelling and gelatinization of the potato starch has a small impact on the texture on the three various observed stages, and on the first stage it was observed that the breaking forces raises with time. The uptake of water by the cells of the potato was identified gravimetrically. The outcomes reveal that the increase of the breaking force during the first stage is from the water uptake.


As mentioned earlier, the most dominant carbohydrates present in potatoes are the starch. Aside from the starch that can be obtained in potatoes, it also includes important vitamins and minerals. The skin of the potato provides vitamin C, potassium, vitamin B6 and some traces of thiamin, riboflavin, folate, magnesium, niacin, iron, phosphorus and zinc. In addition, the fiber content of potato is higher compare to bread, it also contains phytochemicals.


Conclusion


Potato is one of the most popular and famous vegetable in the world. A number of people consume this product. However, as the time passes by the lives of people have changed drastically and are always in a hurry that they don’t have time to prepare the food.  Due to this, food industry has created products that can be easily consumed by the consumers without the hassle of preparation. Most of the products that were developed by the industry are in the gel or cream form, and a number of products are altered to produce a yield value in order for the suspension of particles and to withhold it from flowing at a low shear stress. (2007). Measuring yield stress in potato purees is an important aspect in the production of process of the product and determining the best characterization technique is significant in order to yield positive results. 


 



Credit:ivythesis.typepad.com


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