الفهرس 
المقدمهOnly 14 pages are availabe for public view
 |  | from | 288 |  |  |
| | | from | 288 | | |
Abstract
Yoghurt is a very popular cultured dairy product in which milk is fermented using Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus as the main starter cultures. Such popularity is due to, at least in part, to various health claims and therapeutic benefits added to its nutritional impact. On the other hand, the consumer acceptance and marketability of yoghurt are dependent upon four factors, namely, body, texture, flavour and keeping quality or shelf-life.
from the rheological point of view, rheology - in general - deals with the relationship between three variables: strain, stress and time. In this respect, it was reported (Lee and Lucey 2010) that yoghurt can be classified as pseudoplastic material (contain a yield stress that has to be exceeded for flow to be initiated) that can be either a viscoelastic fluid if we are dealing with stirred or drinking yoghurt or a visoelastic solid if we are dealing with set yoghurt. However, yoghurt quality and rheology create an exponential increase in complexity.
In unfortified yoghurts - due to the low total solids content - there are quality concerns, such as, weak body, poor texture, whey separation and variations in consistency. Therefore, it is common practice to add some stabilizers or hydrocolloids (such as pectins) to overcome such problems.
Pectin is anionic carboxylated polysaccharide found in most plant tissues and fruits. This polysaccharide has been widely studied because of its ability to form gels in certain conditions and concentrations. The molecule can contain from a few hundred units up to approximately one thousand units corresponding to molecular weight of up to 150000, depending on the raw material used and the final pectin types (Gregory, 1986). It consists mainly of α-(1–4)-D-galacturonic acid units. The carboxyl groups of pectin are often esterified with methanol and the percentage of groups esterified allows for the classification of pectins according to their degree of methoxylation (degree of methyl esterification, DE). In nature, pectins are usually highly methoxylated (HM), with more than 50% of the carboxyl groups esterified. Low methoxylated (LM) pectins (<50% esterified) are generally obtained by controlling acid de-esterification of HM pectins. De-esterification using ammonia produces a different type of LM pectins in which some carboxylic acid groups have been amidated, referred to as amidated LM pectins (LMA) (DeVries et al., 1986).