STARTER CULTURES AND FERMENTED MILK PRODUCTS

Module 4. Metabolism of starters and biochemistry of fermentation

Lesson 6
VITAMIN METABOLISM IN STARTER CULTURES

6.1 Introduction

Milk and yoghurts contains both fat- and water-soluble vitamins and various antimicrobial compounds synthesised by the starters. The content of these vitamins changes during the growth of starter cultures in fermented milk products preparation. The change in vitamin content could be either an increase or decrease in the finished product as explained below.

6.2 Increase in Vitamin Content

• Vitamins which increase during the actual manufacture of yoghurt are niacin and folic acid, because they are actively synthesised by the starter cultures. The increases in folic acid and niacin in yoghurt (made from whole milk fortified with 2 % solids-not-fat and incubated for 3 hours at 42°C) amounts to 3.946 and 22µg/ 100 g respectively and the losses in storage may exceed these gains in due course. Although there is a general agreement that vitamin B₁₂ decreases during yoghurt production, but some species of Lactobacillus and strains of yoghurt starter culture synthesise vitamin B₁₂.
• S. thermophilus and Lactobacillus delbrueckii subsp bulgaricus synthesise niacin and folic acid and, to a lesser degree, than vitamin B6 during the production of yoghurt.

6.3 Decrease in Vitamin Content

An excess of dissolved oxygen and/or a moderate heat treatment of milk can reduce significantly the vitamin content, and the most susceptible ones are vitamin C, B₆, B₁₂ and folic acid.

• Excessive heat treatments of the milk, e.g. boiling for 5 minutes, cause even greater losses of the above vitamins; for example, vitamin B₁₂ is reduced to 1.78 µg/litre.
• The yoghurt starter bacteria utilise some of the vitamins present in milk during the fermentation period to meet their growth requirements. This factor contributes, to some extent, to a reduction of the nutritional properties of the product. However, the quantities consumed are dependent on the rate of inoculation, the strain of yoghurt starter and the conditions of fermentation.
• Some vitamins decrease during the storage of yoghurt at 4°C. During the storage of yoghurt at 5°C for 16 days, loss of folic acid and vitamin B₁₂ is 28.6 and 59.9 % respectively. A decrease in the biotin, niacin and pantothenic acid contents, and these losses were due to the combined effect of microbial catabolism during the incubation period, and chemical decomposition of these vitamins during cold storage. This latter aspect was confirmed in yoghurt made by the direct acidification method rather than by microbial fermentation.

6.4 Biosynthesis of Folic Acid (Folacin)

The "Folic acid group" (or "folates") is a generic name given to around ten different compounds which share a basic structural unit connected to "conjugates" of different numbers of glutamic acid residues. Many organisms require folacin as a growth factor. It functions as a coenzyme in many different biochemical reactions, i.e. as an activator and carrier of carbon units during oxidation and it participates in the metabolism of purines, pyrimidines and some amino acids.

6.5 Bio-Synthesis of Niacin

The niacin activity was exhibited by nicotinic acid and nicotinamide. The former compound constitutes part of the structure of the two important coenzymes, i.e. NAD and nicotinamide adenine dinucleotide phosphate (NADP). These two coenzymes are composed of adenylic acid and nicotinamide ribotide linked through their phosphate groups. As NAD and/or NADP are essential for many oxidative/reduc­tive biochemical reactions, the niacin synthesised by S. thermophilus and Lactobacillus delbrueckii subsp bulgaricus may originate from the nicotinamide fraction arising during the formation of NAD and/or NADP. The biosynthesis of these nucleotides involves, basically, the following steps: firstly, the synthesis of a sugar moiety and secondly, the synthesis of the pyrimidine or purine base. Alternatively, after this formation of NAD and/or NADP, the nicotinamide fraction could be released as a result of the degradation of these nucleotides.

Nicotinic acid is derived by a few bacteria from the metabolism or breakdown of tryptophan, a pathway which is dependent on the availability of certain vitamins, e.g. thiamine, riboflavin and vitamin B₆, to activate the required enzymes. As S. thermophilus and Lactobacillus delbrueckii subsp bulgaricus utilises these vitamins and tryptophan does not accumulate during yoghurt production, it is possible that these organisms use the vitamins for the synthesis of niacin.

6.6 Biosynthesis of Vitamin B₆

The activity of vitamin B₆ is exhibited equally by the following compounds: pyridoxine, pyridoxal and pyridoxamine. The basic structure of these compounds is similar which consists of a pyridine ring, but they differ in the respect of the radical components as follows:

No information is available on the biosynthesis of the pyridine ring in microorganisms, plants or animals; however, the different forms of vitamin B₆ are inter convertible by microorganisms. In view of the limited knowledge of the synthesis of vitamin B₆ in general, it is difficult to suggest any possible metabolic pathway by which S. thermophilus and Lactobacillus delbrueckii subsp bulgaricus might synthesis this vitamin.