INTRODUCTION
General overview
Preparation of yoghurt takes place through milk fermentation process with various cultures of bacteria comprising of streptococcus subspecies mixed with lactobacillus delbrueckii subspecies and thermophiles (Barbano, 2009). Basically, yoghurt comes in two types: distilled and set (Augustin et al., 2003). The author further explains there is a difference in the main procedure through which these yogurts are manufactured. In the set yoghurt (inclusive of fruit on the bottom), the formation takes place in retail pots as the fermentation of the lactic acid bacteria proceeds into lactic acid generating a structure of continuous gel in the container of the consumer. In the processing of the stirred yoghurt, the acid gel whose formation takes place during the stage of maturation in the large tanks of fermentation undergoes disruption through the process of stirring—agitation process (Jaros & Rohn, 2003). After stirring, pumping of the product takes place through a screen in which the product is given a viscous and smooth texture. The yoghurts’ physical attributes are inclusive of perceived viscosity and important features when it comes to the overall quality and sensory acceptance of the product by the consumers that are mostly the youths (Cheng et al., 2000). A clear understanding of the mechanisms through which the yoghurt texture is enhanced together with the texture development conditions will be essential when it comes to the incorporation of arabinogalactan for improvement of Yoghurt quality. In order to provide a more clear insight on the effect of introducing arabinogalactan on the yoghurt sensory properties, it is important to begin by understanding the process of yogurt manufacturing.
Yogurt varieties
In order to ensure that the natural sourness is offset, sweetened yogurt has dominated most of the markets in the United States. Yogurt is also flavored or stored in a container with fruit flavor (Serra et al., 2009). In case of stirring of the yoghurt before being purchased then the yogurt is referred to as Swiss- Style in the United States. In Northern America, most yogurt manufacturers have a tendency of adding Pectin; which is mainly found in the fruits (Serra et al., 2009). Gelatin is also added to enhance artificial creaminess and thickness at a reduced cost. Other commonly consumed yogurt varieties are referred to as as cream line. These varieties are manufactured by whole milk that is yet to be homogenized (Chandan et al., 2006). This process prevents the cream from sinking at the bottom. Instead of using pieces of raw fruit in fruits yogurts, fruit jam is mainly used to ensure that it can be stored for weeks (Islaten & Karagul-Yuceer, 2006).
Arabinogalactan and dietary fiber foods
Currently, the food marked has been characterized by having various dietary fibers that have claimed to have dietary and health benefits. However, the dietary fibers are mainly limited when it comes to the extent of affecting the digestive tolerance (Richardson & Morris, 2001). Worse still, these dietary varieties tend to be unstable at low pH. Arabinogalactan is one of the products that are unique when it comes to enhanced tolerance and low pH stability (McKinley, 2005). One of the arabinogalactan varieties is Larch Arabinogalactan (AG); a non-starch polysaccharide that has received approval by the Drug and Food Administration. Taking advantage of AG, Larex Inc.; which is a company based in United States has taken major steps in the effort to ensure that this material is extracted and commercialized in large scale. In addition from various studies, it is proved that AG has the prebiotic role.
A study established that the harmfulness to benefits ratio of colon microflora may amplify because of ingestion with AG (Chandan et al., 2006). With the benefit of enhancing improvement in the colon health, ingestion of AG in subjects portrayed an increase in the trend of softer stools, increased fecal masses together with better habits of bowels. Therefore it is apparent that arabinogalactan has uniqueness as a result of low pH stability. This implies that this polysaccharide can possibly be integrated in a variety of beverages (Chandan et al., 2006).
The yogurt physical and rheological properties
Food rheology involves study of the food materials flow and deformation (Richardson & Morris, 2001). In addition, yoghurt can be classified as the pseudoplastic materials that can be viscoelastic liquid when dealing with set yogurt (Truong & Marshall, 2011). Viscoelastic is an indication that there are some elastic properties in the material with a perfect solid coupled with some of the property flow of viscous fluid. Furthermore in yogurt, a shear thinning is time dependent if exhibited although it is clarified that yogurt is rarely a true thixotropic material (Barnes & Nguyen, 2001).
The purpose of the study
In the recent past, there has been an increase in the popularity of the soluble dietary fiber solely based on the fact that the consumption of these fibers has been proved to enhance health improvement. From the demonstrations that have been depicted by in vitro and in vivo studies, it is apparent that the risk of being exposed to cardiovascular attacks can be reduced based. These attacks result to a decline in the cholesterol blood, tardy digestion of the carbohydrate, regulation in the levels of the insulin and postprandial blood together with enhancement of a healthy microflora gut balance (Laws & Marshall, 2001).
In previously conducted researches, it is apparent that an increasing number of teenagers and children in the United States are increasingly becoming addicted to fruit drinks and soft drinks among other beverages (Serra et al., 2009). It is hence apparent that this trend results in most of these students drinking less milk. Therefore, as a result of reducing the amount of milk consumption, the weakening of the bones is likely to be witnessed on the long term basis (Katz, 2001). Moreover as high consumption of carbolic soft drinks has continued being witnessed, the same author further explains that more cases of obesity have been reported. From a study conducted in 2002 it is apparent that the consumption of yoghurt contains similarly energetic nutrients that are essential for the growth of health children (Jaros et al., 2002). Through affecting the sensory properties of yogurt, there is a high likelihood of the children developing an association with the product and hence increased consumption (Schorch et al., 2000). Hence development of yoghurt with different flavor and taste from the obvious can be one of the ways through which the consumption of this milk can be boosted.
Objective
The main aim of this research was to evaluate the possibility of development of yogurt that contains the improved arabinogalactan nutrients. Therefore, the study objective was to define the sensory and rheological properties of milk with arabinogalactan elements and hence to enhance evaluation of the parameters in the processing through which these properties can be maximized.
METHODOLOGY
This chapter provides a comprehensive discussion on the effect of arabinogalactan fiber on the rheological and sensory properties of yogurt. In this chapter, the methodology of yogurt preparation is provided. It also provides a description of the methods through which the yogurt textural characteristics can be measured through the use of vane rheometry.
This experiment incorporated four main ingredients: (a) Milk protein concentrate, (b) low heat skim milk in powder state from Fresno’s Dairy America Inc. (California USA), (c), Lactic DVS culture from Dairy Connections Inc. (found in Wisconsin, USA), and (d) distilled water.
Preparation of milk base
This experiment involved preparation of three various milk bases- arabinogalactan base, NFDM base together with the control base. The preparation of these bases was enhanced by combining commercial nonfat dry milk powder, commercial low heat together with the distilled water. This process did not incorporate addition of protein milk concentrate. From two arabinogalactan, there was preparation of separate yogurt bases. In the first base that contained 12 percent of the total solid, preparation was enhanced through gradual addition of the skim milk powder to distilled water that was in a beaker of a 2 liters capacity. At 800 rpm, agitation was done continuously by use of a magnetic stirrer. After covering the beaker, the mixing of the base lasted for sixty minutes to completely ensure that the skim milk powder was completely dehydrated. The next step involved preparation of a control yogurt base with solids amounting to 8.8 percent. This preparation took place in a similar way through the use of 120 gm. NFDM in 880 gm. distilled water. In preparation of the last yogurt base which contained arabinogalactan, arabinogalactan amounting to 12 grams together with 108 grams of NFDM were added to the distilled water by following the above description. This resulted in a base that contained 12 percent of the solids. In the final mix, the base that contained 12 percent of the total solids together with the other 12 percent of total solids were observed to have similar contents of protein.
Yoghurt culture preparation
Using DC1612 Direct Vat Culture Set (DVS) in the experiment, the milk was acidified during the process of manufacturing the yogurt. In this culture, Lactobacillus bulgaricus and Streptococcus commercial strains were used. These are the strains that have commonly been used in the industry. The storage of the culture was before use maintained at a temperature of – 16 degrees centigrade in a flaky form. In order to make sure that the yogurt bases are continuously inoculated, there was a preparation of a standard culture. Preparation of a milk base was done by addition of NFDM (70 grams) into 430 milliliters of distilled water. The ingredients were then mixed using a magnetic stirrer for a period of 45 minutes. The heating of this base took place at a temperature of 90 degrees centigrade for a period of three minutes through immersing a water bath containing hot circulating water. At a temperature of 35 degree centigrade in the jar, the prepared milk base was mixed with the DC1612. The sealing of the jar was followed with incubation for a period of 9 hours at 35 degree centigrade. As the pH approached 4.5, the fermentation process was terminated through refrigeration to 5 degree centigrade. The subsequent culture was incorporated for the fermentation step as an inoculum.
The process of pasteurizing the yogurt mix
Through pasteurizing, the enzymes are deactivated coupled with destroying the microorganisms found in the milk (Sodini et al, 2004; Lee & Lucey, 2004). Accomplishment of this process which also involves denaturing of the whey proteins was enhanced in a Stephan Universal machine. The heating of this machine was enhanced through circulation of water bath. Holding of all the bases followed for a period of 25 minutes at 80 degrees centigrade. Using the Stephen machine which is a kettle jacketed with agitation, uniform heating is enhanced on the milk bases (Lucey, 2001, Bangari, 2011). Once pasteurization has taken place, the milk that had already been heated was transferred into a glass beaker. A plastic foil was used to cover this glass beaker, followed immediately by refrigeration at a temperature of 5 Degree Celsius. The bases that had been pasteurized were put in storage for an overnight to ensure that the proteins underwent a complete process of hydration coupled with minimization of any foam that might have been generated in the process of cooling/ heating.
Sample preparation, fermenting and storing
After pasteurization, the warmed milk was stored at a temperature of 45 degrees Celsius. After the milk temperature attained a temperature of 45 degrees, addition of mother culture was done to it at 2 percent incubation rate followed with agitation of the mixture. Replicates measuring 200 ml were transferred to the plastic cups (250 ml). The cups were covered with lids, immediately followed by the transfer of the cups to an air incubator. The temperature was sustained at 42 degrees. For each of the three bases, seven replicates were generated. The onset of the incubation was followed by recording of time in intervals of one hour. On attaining a pH of 4.6, the cooling of the cups immediately followed to 5 degrees Celsius. Evaluation of the gel pH and strength was carried out after 24 hours.
Data analysis
PH determination: The pH measurement was done in the entire process of experiment for each of the variables of the milk base. In all the yogurt samples, pH was measured through insertion of pH probe into the sample followed by lightly shaking for a certain period of time before making the readings. In each of the samples, the pH was determined and documented at the onset of the incubation and at intervals of one hour until when the pH struck 4.5. This is the time when the termination of the fermentation took place. Measurements were also carried out for the pH values of all the samples that had been stored at temperature of 10 degrees. This measurement took place after 24 hours and five days after the onset of the experiment.
Brookfield Viscometer: The yield values of the prepared samples of yogurt were determined through the use of Brookfield DV-III Ultra Programmable Rheometer. The DV-III’s operating principle involves driving a spindle which is inserted into the sample using a calibrated spring (Bangari, 2011). A spring deflection enhances measurement of the food sample viscous drag alongside the spindle. Through a transducer, the spring deflection is measured. The calculation of the viscosity is done from the shape and size of the spindle, the spindle speed, the spindle container in which the rotation of the spindle is taking place together with the calibrates spring full scale torque (Bangari, 2011). Calculation of all the measurement of shear stress, shear rate together with the torque takes place in compliance with the SI or CGS system.
Yield measurement: The yield strain and stress of the static shear test samples was determined using Brookfield DV-III Ultra Rheometer. Two values of the yields readings were collected from every sample. During the analysis, there was also a recording of the temperature of the sample. As a time function, there was also a recording of the torque that was required to uphold a fixed rotation. The calculation of the yield strain, yield stress and torque data was recorded. Having completed the process, the figures were saved followed by conversion into the format of MS Excel Format. The viscosity measurements later took place at 25 degrees sample temperature. Every sample test was followed by a careful rinsing of the spindle with water and moderate wiping out before being used in the next step.
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