Evaluating the quality and shelflife of yoghurt from three different sources under uniform storage condition for a period of 42 days

ABSTRACT

Not all yoghurts produced and sold can be considered as probiotic yoghurts, simply because probiotic yoghurts should conform to standards in terms of the sensory, proximate, and microbial count. The aim of this study is to conduct comparative evaluation on the sensory, proximate composition, and microbial count of some commercially produced yoghurts from two sources (dolait and camlait) and homemade yoghurt stored under uniform temperature condition (6⁰C). Two commercially sold brands of yoghurts (dolait and camlait) were purchased, and one homemade yoghurt   was produced locally using commercial starter cultures. Enumeration of microbial count and evaluation of proximate analysis and sensory properties of the samples was carried out in triplicate at in intervals for a period of 42 days (0day, 21 days, and 42 days).   The was an observable decrease in the LAB count of the samples from 21 – 42 days across the whole samples. Homemade yoghurt samples got bad even before day 21. The pH decreased with corresponding increase in titratable acidity and the nutritional content of the samples also decrease with prolonged storage. Based on the sensory properties of the samples, the whole samples were accepted within a day with homemade yoghurt being the least accepted. It is advisable to consume unrefrigerated yoghurt from the time of production within a day. Dolait and camlait yoghurt Samples are the best products in terms of proximate, sensory properties and lactic acid bacteria (probiotic) count. This study was carried out using the completely randomized experimental design and data was analyzed using one- way ANOVA and turkeys test. Results of the data were discussed based on specific objectives and concluded accordingly.

CHAPTER ONE

INTRODUCTION

1.1 Background of study

Yoghurt is a semisolid fermented product made from a standardized milk mix by the activity of a symbiotic blend of Streptococcus salavarius subsp. thermophilus and Lactobacillus delbruechii subsp. bulgaricus cultures. For the sake of brevity, we shall term the yogurt culture organisms as ST and LB. Milk of various mammals is used for making yogurt in various parts of the world. However, most of the industrialized production of yogurt uses cow’s milk. It is common to boost the solids-not-fat fraction of the milk to about 12% with added nonfat dry milk or condensed skim milk.  The increased protein content in the mix results in a custard like consistency following the fermentation period (Hui, 1992).  In general, yogurt contains more protein, calcium, and other nutrients than milk, reflecting extra solids-not-fat content. In recent years, a dynamic increase in the production of fermented milk drinks has been observed. The factors that caused the interest in consumption of yoghurt include its organoleptic characteristics, nutritional, dietary and medical value as well as its wide assortment in the market (Zareba and Ziano. 2014, Brodziak and Krol 2016, Jakubowska and Matusevicius 2018). Natural yoghurt is a fermented product that is obtained by acidifying milk with specific yoghurt bacteria (Lactobacillus delbreucckii Spp. bulgaricus and Streptococcus Salivarius Sop. thermophilus). According to Zareba and Ziano, 2013, yoghurt products can also be supplemented with lactobacillus acidophilus and Bifidobacterium strains.

An appropriate selection of raw materials and method of production as well as storage conditions significantly affect physicochemical, sensory and microbial characteristics of yoghurt. Yoghurt belongs to the group of dairy products with a short shelf life which is normally about 3 weeks or less (Mojka 2013). Improper storage conditions can lead to a decrease in nutritional value and deterioration of sensory characteristics of yoghurt, as well as a reduction in bacteria content. The storage temperatures of yoghurt products also significantly determine their quality and durability. The recommended storage temperature of yoghurt ranges from 1⁰C to 8⁰C but its durability can be enlarged by storing at 4⁰ C or less during the whole shelf life (Pikul, 2014).

The studies on the impact of storage temperature of yoghurt are aimed at finding a method of handling yoghurt that will maximally limit the changes in quality parameters (e.g. color, taste, aroma, pH, microbial load etc.) during retail storage. Therefore, in my study, I aim at assessing the physicochemical, microbiological and organoleptic features of commercial natural yoghurt immediately after purchase from three different sources and during storage at uniform temperature

1.2 Ingredient used in yoghurt production

Yogurt is the food produced by culturing one or more of the optional dairy ingredients with a characterizing bacterial culture that contains the lactic acid-producing bacteria, Lactobacillus bulgaricus and Streptococcus thermophilus.  One or more of the other optional ingredients described below may also be added. All ingredients used are safe and suitable. Yogurt, before the addition of bulky flavors, contains not less than 3.25% milk fat and not less than 8.25% milk-solids- not-fat, and has a titratable acidity of not less than 0.9%, expressed as lactic acid (Hui, 1992).

 Dairy Products: Fresh bovine (cow) milk is usually the base material for making yoghurt in the Western world, although ovine, caprine or buffalo milks can also be employed. The fat content of bovine milk tends, depending on breed of cow and diet, to be in the range 3.0–3.5 g 100 mL–1, and this value has to be reduced by separation or supplemented with cream according to consumer taste and/or market demand; the fat content of most retail yoghurts lies in the range 1.0–4.5 g 100 mL^–1. However, the critical feature of the milk is the level of solids-non-fat (SNF), which, in bovine milk, varies from 8.5–9.0 g 100 mL^–1 according to the season of the year, with around 4.5 g being lactose, 3.3 g being protein (2.6 g casein and 0.7 g whey proteins) and the balance being minerals (Tamime, 2006). Yogurt  is  generally  made  from  a  mix  standardized  from  whole,  partially  defatted  milk, condensed  skim  milk,  cream,  and  nonfat  dry  milk. 

 Stabilizers: The primary purpose of using a stabilizer in yogurt is to produce smoothness in body and texture, impart gel structure, and reduce wheying off or syneresis. The stabilizer increases shelf life and provides a reasonable degree of uniformity of the product. Stabilizers function through their ability to form gel structures in water, thereby leaving less free water for syneresis.  A good yogurt stabilizer should not impart any flavor, should be effective at low pH values, and should be easily dispersed in the normal working temperatures in a dairy plant. The stabilizers generally used in yogurt are gelatin; vegetable gums such as carboxymethyl cellulose, locust bean, and Guar; and seaweed gums such as alginates and carrageenans (Hui, 1992). Gelatin is derived by irreversible hydrolysis of the proteins collagen and ossein. It   Other types  of stabilizer that have been used in yoghurt making include carboxymethyl cellulose, locust bean gum, alginates, carrageenan, starch and modified starch (Tamime, 2006).

 Sweeteners:  Nutritive carbohydrates used in yogurt manufacture are similar to the sweeteners used in ice cream and other frozen desserts described by Arbuckle. Sucrose is the major sweetener used in yogurt production. Sometimes corn sweeteners may also be used, especially in frozen yogurt mixes. The level of sucrose in yogurt mix appears to affect the production of lactic acid and flavor by yogurt culture. A decrease in characteristic flavor compound (acetaldehyde) production has been reported at 8% or higher concentration of sucrose.1 Sucrose may be added in a dry, granulated, free- flowing, crystalline form or as a liquid sugar containing 67% sucrose. Liquid sugar is preferred for its handling convenience in large operations. Commercial yogurts have an average of 4.06% lactose, 1.85% galactose, 0.05% glucose, and pH of 4.40 (Hui, 1992). Sweetening agents, such as sucrose, high-fructose corn syrup or honey, are usually added to stirred yoghurts to mask the acidity for acid-conscious consumers and, perhaps, produce a firmer texture.

Starter Culture:  The starter is a critical ingredient in yogurt manufacture. The rate of acid production by yogurt culture should be synchronized with plant production schedules.  Using frozen culture concentrates, incubation periods of 5 hr at 45°C, 11 hr at 32°C, or 14 to 16 hr at 29 to 30^oC are required for yogurt acid development. Using bulk starters at 4% inoculum level, the period is 2.5 to 3.0 hr at 45°C, 8 to 10 hr at 32°C, or 14 to 16 hr at 20 to 30°C. The production of flavor by yogurt cultures is a function of time as well as the sugar content of yogurt mix. Acetaldehyde production in yogurt takes place predominantly in the first 1 to 2 h of incubation. The milk coagulum during yogurt production results from the drop in pH due to the activity of the yogurt culture. The streptococci are responsible for lowering the pH of a yogurt mix to 5.0 to 5.5 and the lactobacilli are primarily responsible for further lowering of the pH to 3.8 to 4.4. The texture of yogurt tends to be coarse or grainy if it is allowed to develop firmness prior to stirring or if it is disturbed at pH values higher than 4.6 (Hui, 1992).  The two thermophilic lactic acid bacteria, S. thermophilus and L. delbrueckii subsp. bulgaricus, which trigger yogurt fermentation, are considered as generally recognized as safe. They convert lactose into galactose that is not metabolized and glucose that is fermented predominantly to lactic acid, thus corresponding to homofermentative metabolism (Corrieu & Béal, 2015).

1.3 Major producing countries

France is the leading consumer of yoghurt in Europe, followed by Ireland, with the average person consuming 21.3kg and 13.2kg per annum, respectively. The yoghurt market has been growing steadily in recent years, benefitting from yoghurt’s image as both a healthy alternative for snacking or meals, and also as a treat or dessert.

1.4 Composition of yoghurt

Yogurt (plain yogurt from whole milk) is 81% water, 9% protein, 5% fat, and 4% carbohydrates, including 4% sugars (table). A 100-gram amount provides 406 kilojoules (97 kcal) of dietary energy. As a proportion of the Daily Value (DV), a serving of yogurt is a rich source of vitamin B12 (31% DV) and riboflavin (23% DV), with moderate content of protein, phosphorus, and selenium (14 to 19% DV).

1.5 Importance of yoghurt

1. Nutritional benefits of yoghurt

–  A research carried out by Megan Ware, RDN, L.D. on January 11, 2018, yogurts can be high in protein, calcium, vitamins, and live culture, or probiotics, which can enhance the gut micro biota. These can offer protection for bones and teeth and help prevent digestive problems.

Low-fat yogurt can be a useful source of protein on a weight-loss diet. Probiotics may boost the immune system.

2. Health benefits of yoghurt

Yogurt is packed with source of protein, carbohydrates, fats, minerals and vitamins. It has various uses and creamy texture. Healthy bacteria are used for the fermentation of milk. It maintains the balance between bacteria (good and bad).  Yogurt is also helpful for the lactose intolerant patients. It also maintains the overall health of the people.

1. Assist digestion

Yogurt contains healthy bacteria which help to promote gut’s micro flora that is the reason for the healthy digestive tract and digestion. It is essential for the gastrointestinal ailments such as IBS, diarrhea, constipation, colon cancer and intolerance of lactose. Yogurt has soothing properties that soothes the digestive problems.

1. Type 2 diabetes

The study shows that the high intake of yogurt helps to lower the chances of Type 2 diabetes. It promotes digestion and nutrient absorption in the digestive tract which is vital for the regulation of blood sugar.

1. Treats colorectal cancer

The study shows that the intake of yogurt prevents the chances of colorectal cancer. It is effective for the maintenance of digestive tract due to the presence of probiotics as well as healthy bacteria present in yogurt.

1. Treats osteoporosis

It prevents the occurrence of osteoporosis due to the presence of Vitamin D and calcium. It is helpful for the skeletal health due to the sufficient presence of Vitamin D.

1. Enhance immunity

Yogurt contains probiotics that enhance the gut and immune system by raising the production of cytokine cells in intestine. It prevents the chances of immune diseases in the children. It also prevents the chances of digestive problems causing bacteria

1.6 Problem statement

1.7 Main Objective

An evaluation of the quality and shelf life of yoghurt from 3 different sources (dolait, camlait and homemade), as sensory qualities, proximate analysis and microbial load, under uniform storage condition.

1.8 Specific Objective

1) To evaluate the sensory qualities of the 3 different yoghurts (dolait, camlait and homemade) over time

2) To evaluate the proximate analysis of the 3 different yoghurt (dolait, camlait and homemade) over time

3) To evaluate microbial load of the 3 different yoghurts (dolait, camlait and homemade) over time