Yoghurts are usually spoiled by yeasts and the introduction of
fruits into the yoghurts reinforces the spoilage by yeasts because
additional sources of contamination and fermentable substrates are
provided. Yeasts growth has been linked to the ability of these
micro-organisms to grow at refrigeration temperatures to ferment
lactose and sucrose, and to hydrolyse milk casein (Suriyarachchi
and Fleet, 1981). Indeed, in the present study lactose concentration
decreased during storage of the product as well as fructose and
glucose. The latter two sugars could also function as fermentable
substrates for yeasts growth and for this reason yoghurt with fruits
are likely to support the growth of a wider variety of yeast species
(Suriyarachchi and Fleet, 1981). However, the presence of potassium
sorbate in the product considerably delayed the growth of
yeasts. The result of this was the population of yeasts being still in
lag phase or in early exponential phase whereas the product being
already spoiled (evident spoilage such as mould formation on the
surface of the product, off-odours, discolouration or syneresis).
According to other studies, yeasts achieved 106 CFU/g after 21 d in
the presence of potassium sorbate up to 150 mg/kg. The effect of
potassium sorbate on the micro-organisms growth was more
pronounced at low pH values (Mihyar et al.,1997). When acid pKa is
high (potassium sorbate pKa ¼ 4.75) and product pH is low
(yoghurt with fruits pH ¼ 4.00e4.25) then the effect is more
intensive because higher proportion of acid is in its un-dissociated
form which has higher antimicrobial effect than its dissociated
form.