The effect of mechanical shear in ambient yoghurt

Abstract

Yoghurt is a fermented product form the milk, well known in the world. The fermentation is carried out by Lactic acid bacteria (LAB). The most common are Streptococcus thermophilus and lactobacillus bulgaricus, these bacteria make the milk acid, generating a disturbance in the casein micelles making a solid mass called coagulum. This specific LAB also produces expopolysaccharides (EPS) that form the characteristic ropy in the yoghurt. There are many types of yoghurt based in its consistency; the most popular are the set and the stirred type. Set yogurt is the solid coagulum where the yoghurt is incubated, cooled and retailed to the consumer in the same container. The stirred yoghurt is fermented in a big container and after fermentation the coagulum is destroy gentle, cooled and pumping to the final package to the consumer.

Ambient yoghurt production is the same as stirred type whit the difference of the ambient yoghurt is heat-treated after fermentation in order to inactivate the LAB and so therefore prolong the shelf life of the product; due to this, ambient yoghurt doesn’t needs to be cooled. Heat-treatment and pumping have an impact in the final viscosity and stability as water retention in the product since the yoghurt gel is shear sensitive. The use of different stabilizers is needed to maintain the quality parameters of the yoghurt as the viscosity perceived as smoothness and avoid the syneresis (separation of the whey). The correct mix in the stabilizers added to the yoghurt and to know how yoghurt behaves during shearing in the process presents new challenges in the dairy industry with the finality to give the consumer a product with long shelf life that keeps the smoothness and stability characteristic in the yoghurt.

Taking into account these considerations, the purpose of this study was to evaluate the rheological behaviour of the yoghurt after applying mechanical shear to the stirred yoghurt. The procedure of the experiment consist in 3 parts: 1) the setting of the rig to simulate the shear rates in the industry (called as low, medium and high shear rate), from low shear rates, medium and high 2) Yoghurt production based in two different standard ambient yoghurt formulations with stabilizers: a) Formula 1 (F1) and its control (F1C) and Formula 2 (F2) and its control (F2C). F1 and F2 had modified starch and pectin, with the difference that F1 had gellan gum and F2 gelatin 3) applying the shear rates in different times (30 and 60 seconds) to the yoghurt in the rig and in the rheometer. Before and after the shearing of the product in the rig and in the rheometer, it was measured the percentage of water retention (%WR) and the viscosity in a rheometer to see how were the changes of F1 and F2 after shearing in rig and rheometer.

The results showed that both formulas and controls were sensitive to shear but behave different due to the stabilizers used. F1 viscosity after shearing shows better stability, it can be attributed to the gellan gum, on the other hand it doesn´t have good %WR even though with the addition of stabilizers and had a continuous decreasing after medium and high shearing. F2 has higher viscosity and better higher retention than its own control but does not have a good viscosity stability having a constant decreasing when shear was applied, however the %WR was higher and more stable at high mechanical shear; gelatin could be the answer for this behaviour. The different times that the same shear rates were applied to the samples (30 and 60 sec) doesn´t show a significative difference in the viscosity and the %WR. Finally the viscosity results of the yoghurt which run in the rig are not the same as the ones that run in the rheometer, even though the formulas used in the development of the rig were correct, the prove is the %WR in both samples which run in different systems (rig and rheometer) has not significative difference.

Popular Abstract

Yoghurt is a well known product, for its healthiness, taste and pleasant texture in the mouth. As yoghurt is gaining more popularity the necessity to reach more consumers is evident. Maintaining the cool chain, that keeps the yoghurt fresh, sometimes is not easy and even more if the transportation of the product takes long time, this could result that shelf life of the yoghurt is shorter when it reaches the consumer. The ambient yoghurt is yoghurt that can be in ambient temperature and has a longer shelf life than the traditional cooled yoghurt.

The yoghurt characteristics are formed due to fermentation of milk; fermentation is the process in which a complex substance breaks down into single substances. In case of the yoghurt Lactic Acid Bacteria (LAB) are bacteria that transform the milk to yoghurt, prefers to grow at temperatures around 40 to 44°C. LAB likes to ferment the sugar of the milk, called lactose, when LAB ferments lactose generates acid; this acid formation cause disturbance in the substances that form the milk structure. Casein, which is the protein of the milk are one of the many molecules that integrate the milk, as fat and whey. When the milk is not acid all these substances are dispersed, that is why the milk is liquid, and some of these substances repels each other as the caseins but when the milks acidifies, caseins attracts grouping and forming nets, this aggregation is called coagulum.

When yoghurt is formed it is needed to low the temperature (4-6°C) in order to slow or put in stand by the activity of the LAB and avoid more acidification of the product. The cool chain is needed to maintain the shelf life of the yoghurt around 16-21 days. In the market, there is principally two types of yoghurt, the set and the stirred from these types there are many variants. The set yoghurt is the one that the fermentation was done into the recipient to the final consumer without breaking the coagulum and the stirred yoghurt is when the fermentation was made in big tanks and after fermentation the coagulum is broken gently to fill the product in the individual packages to the consumer.

Ambient yoghurt doesn´t need refrigeration and has an extended shelf life (around 6 months) due the inactivation of the LAB. It means that the yoghurt has an extra step in the process and after fermentation the yoghurt has a heat treatment, killing the LAB. This process generates a lot of shear in the product causing disturbance in the yoghurt structure. The use of natural stabilizers as starch (the one used to make sauces thicker), pectin (ingredient used to make marmalades and jellies firm), gelatin and gellan gum (a gelling product from a bacteria) are used in ambient yoghurt to conserve the firmness and delays the syneresis after the processing.
Yoghurt structure is very delicate and it is needed to treat it gently in order to avoid the disturbance in the casein net. If the yoghurt is highly damage the whey goes out and the coagulum surface seems watery (called syneresis), giving a bad first impression into the product, also the texture is damage. The damage can be done in the process plant when the yoghurt is pumped or heat treated due to the yoghurt in the line run in different shears.
This work describes how the yoghurt texture and its capacity to retain water in its structure (percentage of water retention) after applying different shears. In this experiment two yoghurt formulations both with starch and pectin but one with gellan gum and the other gelatin.

Both yoghurts show disturbances in their texture after shear. However the one with gellan gum maintains more stability against shearing than the one with gelatin. In case of the percentage of water retention the formula that has gelatine has a better stability and its structure maintain more water than the one with gellan gum.

These results can give a better vision about how the ambient yoghurt with stabilizers behave after shearing. This work opens more possibilities to work more in mix stabilizer for ambient yoghurt and give a forecast how it could be the final product after shearing in an industrial way.

• author: Magdaleno Morales Henrysson, Emilia
• supervisor: Maria Glantz; Katarina Lindgren
• organization: Food Technology and Nutrition (M.Sc.)
• course: KLTM01 20161
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering; Agriculture and Food Sciences
• keywords: long life yoghurt, stabilizers, viscosity, Yogurt, shear rate, food technology, livsmedelsteknologi
• language: English
• id: 8881577
• date added to LUP: 2016-06-27 08:59:03
• date last changed: 2016-06-27 08:59:03