EFFECT OF THERMIZATION AND STORAGE ON THE NUTRITIONAL PROPERTIES AND SHELF-STABILITY OF YOGHURT FROM BLENDS OF COW MILK AND COCONUT MILK

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ABSTRACT

Yoghurts were produced by blending reconstituted powdered cow milk (100 g of powdered cow milk (CM) : 0.5 litres of water) with 25 % and 50 % coconut milk (CCM) and a control (100 % cow milk) before fermentation. After fermentation for 16 h at room temperature, the yoghurts were subjected to two heat treatments (thermization) namely, 75 0C thermization and 80 0C thermization and a control (unthermized). The proximate, physicochemical, vitamin and microbiological quality of yoghurt samples were investigated during storage at room temperature for 5 weeks. The results obtained, revealed that in each of the three formulations of yoghurt, proximate composition, titratable acidity (TTA), total solids, free fatty acids (FFA), viscosity, microbial load and vitamins decreased as the temperature of thermization increased. The pH and moisture increased from 4.30 and 82.36 %, respectively with increase in thermization temperature to 4.59 and 83.39 %, respectively. Yoghurts samples from 50 % cow milk (CM) + 50 % coconut milk (CCM) had the highest moisture content (83.39 %) and lowest total solid content (17.84 %) compared to 82.98 % and 18.16 %, respectively from 75 % cow milk (CM) + 25 % coconut milk (CCM) and 82.92 % and 18.28 %, respectively from 100 % cow milk (CM). Titratable acidity increased significantly (p < 0.05) with increase in percentage coconut milk ( 0.77 for 100 % cow milk (CM), 0.82 for 75 % cow milk (CM) + 25 % coconut milk (CCM) and 0.87 for 50 % cow milk (CM) + 50 % coconut milk (CCM)). There was significant (p < 0.05) decrease in pH as the percent coconut milk increased (4.48 for 100 % cow milk (CM), 4.40 for 75 % cow milk (CM) + 25 % coconut milk (CCM) and 4.30 for 50 % cow milk (CM) + 50 % coconut milk (CCM)). The highest viscosity of 282.59 ± 2.46 cPs was obtained for unthermized 100 % cow milk which also had the highest protein content of 3.84 ± 0.11 %. Free fatty acid increased significantly (p < 0.05) with increase in percent coconut milk and during storage. B-complex vitamins (B1, B2 and B3) increased significantly (p<0.05) as the storage time increased while the antioxidant vitamins (vitamin C and E) decreased significantly (p < 0.05). Vitamin A was not detected in any of the samples. Total bacterial count increased significantly (p < 0.05) from 2.27 x 102 cfu/ml ± 0.14 to 6.77 x 102 cfu/ml ± 0.10 on storage within 3 weeks. Thereafter, their numbers reduced. Fungi were detected in all the yoghurt samples but they reduced after one week of storage. Sensory results indicated that yoghurts with coconut milk (CCM) and those thermized at 80 0C had significant (p < 0.05) lower consumer preferences.

                                                    CHAPTER ONE

                                                   INTRODUCTION

  1.  Background of the study

Yoghurt is a fermented product obtained through an anaerobic fermentation of lactose in milk by relevant micro-organisms, most of which are classified as probiotic (Tull, 1996). It is known and consumed in almost all parts of the world. Most Nigerians regularly take yoghurt either as a dessert, snack or as a probiotic food drink to aid digestion and to re-establish a balance within the intestinal microflora. The tangy sour taste and aroma of yoghurt are due to the activities of most commonly mixed cultures of Streptococcus thermophilus and Lactobacillus  bulgaricus which remain active in the product after its production (Bille and Keya, 2002; Douglas, 2005).

Yoghurt making involves the introduction of friendly bacteria in pasteurized milk under controlled temperature and environmental conditions (Granessha, 2005). The bacteria ingest the milk sugar and release as much as 0.9 to 1.2 % lactic acid as waste product, which reduces pH from 6.7 to below 4.6. The increased acidity causes milk protein to coagulate. According to Early (1998) and Douglas (2005), the increased acidity also prevents proliferation of other potentially harmful bacteria. The optimum fermentation temperature for short incubation of 3.5 – 5.0 hours is 42 0C and for long period of 14 – 16 hours is 30 0C.

Yoghurt can be used to prevent and control diarrhea due to its therapeutic effect. It is capable of modulating the inflammatory response produced by carcinogen. It reduces the inflammatory response through an increase in apoptosis. Proteins in yoghurt are more digestible (Jacobson, 1978) than that in raw milk. Yoghurt was found to improve iron status by improving the absorption of iron from other foods (Branca and Ross, 2001).

On the basis of shelf-life, yoghurt can exist as fresh or thermized. Thermized yoghurt is a product which has been heat treated to reduce bacterial load so as to extend the shelf-life of the product outside refrigeration. Incessant power outage in Nigeria and adverse marketing and distribution conditions have stimulated research interest in thermized yoghurt production and its quality improvement. Heat treatment of yoghurt, however, destroys most of the natural viscosity of the original product due to shock and thinning effect (Early, 1998).

Coconut (cocos nucifera) milk is used by confectionaries worldwide to enhance quality, flavour and taste of various products (Perseley, 1992). Coconut milk has been found to be rich in calcium. The milk was reported to be high in minerals and vitamin content (Nieuwentus and Nieuwentus, 2002) while contributing about 10% of the total energy

(Thai Food Composition, 2004). Coconut milk is a complete protein food when taken in its natural form (Ukwuoma and Mauanya, 2003) and it helps in fighting heart disease. The most popular yoghurts known on the Nigerian market are those obtained from cow milk (Sackey- Addaguay, 2008). Yoghurt obtained by using coconut milk has been found to be a delicious and nutritional product (Imele and Atemnkeng, 2001). The nutritional quality of yoghurt can, therefore, be enhanced by addition of coconut milk.

  1. Statement of Research Problem

From the foregoing, it does seem evident that the substrate usually employed in yoghurt production can be evaporated whole milk, skimmed milk solids, powdered milk or fresh milk from cow. Although these substrates produce good quality yoghurt, milk extracts (coconut milk) from plant produce can be used as milk to formulate yoghurt products of good quality and stability, thereby, bringing their potentials to limelight.

In view of the above, this research was designed to evaluate the quality and stability of thermized yoghurt products, produced from blends of cow milk and coconut milk. This goal was achieved by evaluating the physical, chemical, microbiological and sensory properties of thermized yoghurt composites stored at room temperature.

1.3   Specific Research Objectives

The specific objectives of this study are:

  1. To access the possibility of the use of coconut milk in yoghurt production.
  2. To evaluate the physical, chemical, microbiological and sensory properties of thermized yoghurt.
  3. To determine the storage stability of the products.
  4. To determine the vitamins in the product.

1.4   Significance of the Study

Yoghurt from cow milk has been in existence for a very long time. It is realized that strict vegetarians and people allergic to dairy products are also limited in their quest for probiotic yoghurts when only animal-based yoghurt are available. It is, therefore, of great importance to find out and diversify the feasibility of the use of coconut milk extract, which is plant- base for yoghurt production. Yoghurt obtained by using coconut milk has been found to be a delicious and nutritious product (Imele and Atemnkeng, 2001).

It is, therefore, hoped that this research work would motivate and promote the use of coconut milk in the production of yoghurt so as to enhance the nutritional quality of yoghurt.

EFFECT OF THERMIZATION AND STORAGE ON THE NUTRITIONAL PROPERTIES AND SHELF-STABILITY OF YOGHURT FROM BLENDS OF COW MILK AND COCONUT MILK