Overview

Yogurt Overview

Although yogurt has been around for many years, it has only become popular within the last thirty to forty years. The surge in yogurt popularity has led to the introduction of many types of yogurt.

Introduction

Yogurt is a fermented milk product that contains the characteristic bacterial cultures Lactobacillus bulgaricus and Streptococcus thermophilus. It was accidentally invented in an early form thousands of years ago from curded goat and sheep milk. By definition, yogurt must contain at least 8.25% solids that are not fat. Full fat yogurt must contain no less than 3.25% milk fat, low-fat yogurt no more than 2% milk fat, and nonfat yogurt must contain less than 0.5% milk. Although yogurt has been around for many years, it has only become popular within the last thirty to forty years. Factors which have contributed to the popularity of yogurt include the introduction of fruit flavors, convenience as a quick and ready-made breakfast food, and marketing as a low-fat and healthy food, especially when compared to other dairy products like ice cream. The surge in yogurt popularity has led to the introduction of many types of yogurt. Traditional yogurt is thick and creamy. Flavored yogurt is typically flavored with fruit, but other flavors have been introduced as well, such as vanilla and chocolate. Yogurt is marketed by different fat levels, with low-fat being the most popular. Creamy yogurt is extra thick and made with whole milk and cream. Bioyogurt uses a different fermentation culture and is supposed to aid digestion. Organic yogurt is made from milk from specially fed cows.

Yogurt usually has a tart flavor because of lactose fermentation and milk is the main ingredient, making it rich in nutrients like protein, vitamins, and calcium. Other dairy ingredients allowed in yogurt include cream to adjust fat content and nonfat dry milk to adjust the solids content, which is typically adjusted above the minimum 8.25% to provide better body and texture. Stabilizers are often used as well to increase firmness, prevent separation of the whey, and to keep the fruit uniformly mixed in the yogurt. These include alginates, gelatins, gums, pectins, and starch. Sweeteners (both natural and artificial), flavors, and fruit preparations can be added as well to make different varieties. All additional ingredients are regulated. Fruit can be added at different points in the manufacturing process. It can be added to the bottom of containers before the finished yogurt is put in them (known as Fruit-On-The-Bottom). It can also be added as a puree to bulk yogurt during fermentation but must also be pasteurized like the milk before fermentation. In some yogurts, the fruit is added in a small separate package to be mixed in the yogurt before consumption.

Yogurt Processing

The first step in yogurt manufacturing is to modify the milk so it is suitable for making yogurt. This typically involves reducing the fat and increasing the total solids. Fat content is reduced using a clarifier and separator that uses centrifugation to separate fat from the milk. The milk is tested for fat and solids content and solid concentration is increased by either evaporating water or adding concentrated milk or milk powder. The typical total solids content of the milk is 16%, with fat from 1 to 5% and the remaining portion being non-fat solids. After the optimal solids content is reached, any stabilizers are added, and the milk is pasteurized. Pasteurization is used to denature whey proteins, forming a more stable gel which prevents separation of water during storage. It also destroys unwanted microorganisms that interfere with fermentation and releases compounds that help stimulate the growth of the starter cultures. The process can be batch or continuous and the exact parameters can vary, but typically involves heating the milk to around 185°F for at least thirty minutes. Homogenization occurs simultaneously with pasteurization and breaks up fat globules to a more uniform dispersion of particles. The milk is forced through small openings at high pressure (2000 to 2500 psi) to break up the fat globules. This makes a smoother, creamier, and more uniform end product that reduces separation to a minimum.

The first step in yogurt manufacturing is to modify the milk so it is suitable for making yogurt. This typically involves reducing the fat and increasing the total solids. Fat content is reduced using a clarifier and separator that uses centrifugation to separate fat from the milk. The milk is tested for fat and solids content and solid concentration is increased by either evaporating water or adding concentrated milk or milk powder. The typical total solids content of the milk is 16%, with fat from 1 to 5% and the remaining portion being non-fat solids. After the optimal solids content is reached, any stabilizers are added, and the milk is pasteurized. Pasteurization is used to denature whey proteins, forming a more stable gel which prevents separation of water during storage. It also destroys unwanted microorganisms that interfere with fermentation and releases compounds that help stimulate the growth of the starter cultures. The process can be batch or continuous and the exact parameters can vary, but typically involves heating the milk to around 185°F for at least thirty minutes. Homogenization occurs simultaneously with pasteurization and breaks up fat globules to a more uniform dispersion of particles. The milk is forced through small openings at high pressure (2000 to 2500 psi) to break up the fat globules. This makes a smoother, creamier, and more uniform end product that reduces separation to a minimum.

After pasteurization and homogenization are complete, the milk is cooled to around 110°F before adding the starter culture to begin fermentation. If the milk is not adequately cooled, the cultures will be inactivated when added to the milk. Incubation can occur either in bulk or in the individual containers the yogurt is sold in. Stirred yogurt is fermented in bulk and then poured into containers. Set yogurt ferments in the containers. The concentration of fermentation culture added is around 2%. Milk is held at a consistent temperature for three to four hours during the incubation process. As fermentation of the lactose takes place, the bacteria metabolize compounds in the milk that form a soft gel and the characteristic flavor of yogurt. One important byproduct is lactic acid, which is measured to determine when the yogurt is ready. The standard method for determining acidity is titration with sodium hydroxide, which is time-consuming and requires sample preparation. Regulations in the United States require yogurt to have at least 0.9% acidity and a pH around 4.4-4.5. Once the acidity and pH reach the desired level, the yogurt is cooled to around 45°F to stop fermentation. The finished containers of yogurt are then shipped to stores in refrigerated trucks.

Conclusion

As with any dairy product, yogurt is subject to many safety tests. Some include microbial quality, a degree of pasteurization, and the presence of contaminants, such as antibiotics, pesticides, and radionuclides. Microbial quality is determined by a dye reaction test and a count that is too high makes the milk unsuitable for manufacturing. The degree of pasteurization is measured by an enzyme known as phosphatase and performing this test is required before fermentation may proceed. The final yogurt products undergo many safety and quality tests as well, such as pH, rheology, taste, color, and odor. NIR spectroscopy has emerged as a tool for rapid, non-invasive, and cost-effective analysis of parameters of interest in yogurt that could potentially replace traditional reference methods. The two main parameters for flavor in yogurt are sugar and pH. Traditional methods for measuring these are time-consuming, can require the use of wet chemistry, and alternative methods may not be suitable for on-line measurements. Two separate studies used NIR spectroscopy to measure both sugar and pH using chemometric models correlating the spectra to sugar and acidity. Both studies showed good results and excellent predictive performance from the models. Adulteration is a major problem in the food and dairy industries and yogurt is no exception. One study examined the feasibility of determining the presence of three well-known non-milk protein adulterants in yogurt using various pre-processing techniques and data classification methods on NIR spectra. Results proved that adulteration of yogurt by edible gelatin, industrial gelatin, and soy protein can all be detected using NIR spectra and classification methods. All of these parameters and measurements have been studied using NIR spectroscopy with results showing the potential to replace traditional reference methods.

Scientific Reference

Milk Facts: Yogurt Production
http://www.milkfacts.info/Milk%20Processing/Yogurt%20Production.htm

How Products Are Made: Yogurt
http://www.madehow.com/Volume-4/Yogurt.html

Yogurt: The Product and Its Manufacture
Corrieu G., and Beal C., (2016) Yogurt: The Product and Its Manufacture. In: Caballero, B., Finglas, P., and Toldra, F. (eds.) The Encyclopedia of Food and Health vol. 5, pp. 617-624. Oxford: Academic Press. https://www.researchgate.net/publication/301702346_Yogurt_The_Product_and_its_Manufacture

Measurement of Yogurt Internal Quality Through Using Vis/NIR Spectroscopy – Shao, He, Feng, Food Research International 40 (2007) 835-841
https://www.sciencedirect.com/science/article/pii/S0963996907000294

The Feasibility of Using Near-Infrared Spectroscopy and Chemometrics for Untargeted Detection of Protein Adulteration in Yogurt: Removing Unwanted Variations in Pure Yogurt – Xu, Yan, Cai, et al., Journal of Analytical Methods in Chemistry, Volume 2013, Article ID 201873
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3697415/

Commercial References

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