A new method for producing superior set yogurt, focusing on heat treatment and homogenization.

Discipline: yogurt; Keywords: set yogurt, extended shelf life processing, homogenization, fat globule size.

Yogurt is a very popular fermented dairy product that is widely consumed around the world. Various forms of yogurt can be found, including set yogurt, stirred yogurt, and drink yogurt. Set yogurt is usually the most preferred, but texture is an important feature in acceptance. Good texture leads to superior ratings for a product. Consumers tend to prefer a soft and smooth texture, and a too hard or rough texture is not desired. It is thus necessary to continuously develop methods to obtain smoother set yogurt. However, in most countries, food products are also required to include no harmful microorganisms and no foreign substances but should also have no off-taste or off-flavour, yet a normal appearance. The better set yogurt for a consumer is the one that has both a pleasing texture (especially smoothness) and sufficient curd strength. In this study, Dr T. Ichimura and colleagues investigated heat treatment as in extended shelf life processing and homogenization conditions as a means to obtain a smoother texture for set yogurt compared with the conventional method.  Extended shelf life (ESL) processing (i.e., heat treatment at 130°C for 2 seconds) is usually not used for producing set yogurt because of the fragility of the curd structure.The results of the authors were published in the Journal of Dairy Science, Volume 105, page 2978 to 2987, under the title: A new method for producing superior set yogurt, focusing on heat treatment and homogenization.

In the study, the effects of homogenization conducted at higher pressure than the conventional (10 megapascal [MPa] for the first stage and 5 MPa for the second stage) on curd structure of set yogurt was investigated, the focus being on the fat globule size. Each yogurt mix was adjusted at the range of fat globule sizes from 0.45 μm to 1.1 μm by a homogenizer and then heated at 95°C for 5 minutes (conventional heat treatment), 120°C for 2 seconds (ESL processing) or 140°C for 2 seconds. The yogurt mixes were fermented by a common yogurt starter, and the curd texture of the obtained yogurts was evaluated.

Curd hardness and curd firmness of the yogurt were each negatively correlated with the fat globule size regardless of the heat-treatment temperature. Compared with the curd obtained with conventional heat treatment, the ESL-processed curd was extremely fragile, but significantly smooth. With ESL processing, a curd hardness of more than 40 g, which is of sufficient strength for commercial transport systems, was obtained by making the fat particle size less than 0.6 μm, using a two-stage homogenization pressure: 35 MPa for the first stage and 5 MPa for the second stage. Microscopic analysis indicated that the smaller fat globules reinforced the network structure. The yogurt made by ESL processing and that created with 35 + 5 MPa homogenization showed better sensory evaluation scores.

A method was developed to manufacture set yogurt that achieves both good texture and high curd strength by focusing on the heat-treatment temperature and homogenization protocol in the yogurt manufacturing process. Other studies have shown that the yogurt curd strength was reduced by ESL processing, but the present findings demonstrated that a higher-than-conventional homogenization pressure condition increased the curd strength of the ESL-processed yogurt to a sufficient level. The yogurt made by ESL processing together with 35 + 5 MPa homogenization had significantly higher scores for smoothness, richness, mildness, and overall acceptability than those of the yogurt made under control conditions. ESL processing along with 35 + 5 MPa homogenization enables the shared use of a pasteurizer for milk and yogurt and improves the efficient operation of equipment and the hygiene of the process. It can thus be concluded that ESL processing plus 35 + 5 MPa homogenization, is a manufacturing method that provides yogurt with excellent palatability, curd quality, and productivity.