yogurt is a fermented milk product widely known and appreciated for its beneficial effects on consumers health . to obtain a first - rate yogurt , excellent quality raw materials and production techniques that follow the good manufacturing practices
its shelf - life could be established based on microbiological data and the use of other parameters such as sensory analysis , aw , ph and titratable acidity ( mataragas et al . , 2011 ) .
chemico - physical and microbiological modifications of yogurt until the expiry date , which can vary from 20 to 40 days from the date of production , can cause undesired changes in the organoleptic properties of the product ( colour , odour , flavour and texture ) . for this reason ,
correct storage conditions until the end of shelf life is essential for the maintenance of product characteristics .
the alteration of dairy products such as yogurt is mainly linked to the development of specific spoilage organisms ( ssos ) such as yeasts and moulds .
the deterioration of yogurt by yeasts is generally identified by the development of abnormal flavours , yeast odour and co2 production with consequent swelling of the lid heat - sealed when used ( vallone et al . , 2001 ) .
in absence of specific guidelines in the italian legislation , some authors report that yogurt cultures ( lactobacillus delbrueckii subsp .
bulgaricus and streptococcus thermophilus bacteria ) should be alive and viable until the time of consumption and in quantities not less than 10 cfu / g ( health ministry , 1972 ) .
the decrease in ph caused by the production of lactic acid is one of the desirable effects of the metabolic activity of lactic acid bacteria .
in fact , the main benefits of yogurt are related to its acidity : the production of lactic acid from lactose can allow the consumption of this product also by lactose intolerant consumers ; moreover , it promotes the absorption of calcium and minerals ( al - kadamany et al . , 2002 ; mataragas et al . , 2011 ) .
the analysis of the aspects described above is essential to determine the conditions of conservation and define the length of shelf - life period of yogurt .
the aim of this work was to analyse some microbiological , chemico - physical and organoleptic parameters in samples of sheep milk yogurt produced in a short supply chain farm , during the shelf - life declared by the dairy farmer .
five samples of a sheep milk yogurt of the same lot , collected from a short supply chain ovine dairy farm of the roman province , were analysed .
the yogurts stored at 4c were examined in 5 sessions of analysis : at 2 , 14 , 30 , 35 and 40 days from the production date .
the following microbiological analysis were performed : enumeration of characteristic microorganisms of yogurt ( iso 7889:2003 ; iso , 2003a ) , identification of lactobacillus delbrueckii bulgaricus and streptococcus thermophilus ( iso 9232:2003 ; iso , 2003b ) , enumeration of enterobacteriaceae ( iso 21528 - 2 : 2004 ; iso , 2004a ) , enumeration of yeasts and moulds at 25c ( iso 6611:2004 ; iso , 2004b ) .
the identification of the isolated microorganisms was performed using miniaturised biochemical tests ( api chl biomerieux , api c aux biomerieux and api ne biomerieux ; biomrieux , marcy letoile , france ) .
dna was extracted from bacterial colonies by using boiling method and subjected to pcr using primers and reaction conditions reported in the literature for the identification of streptococcus thermophilus ( fortina et al . , 2003 ) ,
lactis ( torriani et al . , 1999 ) , and a geneamp pcr system 9700 ( applied biosystems , carlsbad , ca , usa ) machine .
amplified products were visualised on 1.2% agarose gel containing gel red in trisacetate - edta 1x by using a uv - transilluminator . in each analysis session , the organoleptic acceptability of each sample was evaluated by criteria such as discolouration , mould formation , off - odours , abnormal flavours and texture .
the ph of each sample was measured with a mp225 ph meter ( mettler toledo inc . ,
leicester , uk ) with a probe for the detection of the temperature and electrode combined as follows : once the sample reached 25c , the determination of ph was performed repeating the measurements in duplicate for each sample .
the same analyses were also performed on a single sample of bulk milk collected from the same farm .
the analysed product maintained an almost constant amount of lactic acid streptococci ( on the order of 10 cfu / g ) until the end of the declared shelf - life ( figure 1 ) .
the concentration of lactobacilli remained almost constant until the end of shelf - life ( with a concentration on the order of 10 cfu / g ) , followed by a slight decrease after the expiry date ( figure 1 ) .
concerning lactic acid bacteria , on the totality of the bacterial strains identified as lactobacillus delbrueckii subsp .
bulgaricus and streptococcus thermophilus , there was a 100% concordance of results by using biochemical and pcr methods .
the concentration of enterobacteria was found to be < 10 cfu / g in all samples .
the presence of yeasts was detected starting from 14 days after production , with a concentration of 40 cfu / g ; during the subsequent analyses they increased , until a concentration of 10 cfu / g at the end of the shelf - life .
the result of the yeast typing on the isolates revealed the presence of candida famata species only .
the presence of moulds was observed starting from 30 days of production ( expiry date of the product ) at a concentration of 4.310 cfu / g .
overall , the microbiological analyses performed during the shelf - life period revealed a modest increase of undesirable microorganisms such as yeasts and moulds , with a constant concentration or a slight decrease in lactic acid bacteria . from the examined yogurts and from the bulk milk sample as well , the presence of ralstonia picketii , i.e. a contaminating microorganism probably of environmental origin , was detected .
the organoleptic characteristics of the products remained unchanged until the end of the declared shelf - life .
therefore , the presence of yeasts and moulds produced no substantial organoleptic variations up to 30 days from the production date .
however , a bitter / cheese flavour and an acrid smell was perceived in the yogurts after the expiry date of the product .
the ph values measured from the examined yogurts showed minimal variations until the end of the shelf - life ( from 4.56 to 4.61 ) , while a decrease ( until 4.35 ) occurred after the end of shelf - life ( figure 2 ) .
as reported in the literature , our results showed that the process of deterioration in yogurt is due primarily to the development of ssos , such as yeasts and moulds ; these organisms are able to ferment lactose and sucrose also at refrigeration temperatures and low ph values .
the presence of these organisms can be determined by : addiction of characterising ingredients , such as cereals , fruit not properly pasteurised ; application of improper production and/or storage practices ; use of inadequate raw materials . in this case , it is important to evaluate the starting microbiological quality of the milk ( mataragas et al . , 2011 ) . only in very few cases
the source of contamination of yogurt are the starter cultures ; thus , a correct monitoring of the production and storage areas of the products is pivotal .
good sensory characteristics and an almost constant concentration of lactic acid bacteria were maintained during all the commercial life of the products .
predictive models might be useful to study the process of deterioration of yogurts by contaminants .
such information could be extremely relevant for dairy farmers to determine a suitable shelf - life of their products ( al - kadamany et al .
, 2002 ; mataragas et al . , 2011 ; sofu and evinci , 2007 ) . in order to evaluate meaningfully chemical - physical and microbiological modifications during the shelf - life period of this product
 aim of this work was to analyse some microbiological , chemico - physical and organoleptic parameters of sheep milk yogurt during and after its declared shelf - life . 
 five samples of a sheep s milk yogurt of the same lot , collected from a short supply chain ovine dairy farm of the roman province , were analysed . 
 declared shelf - life of the product was 30 days . 
 the products were examined at 2 , 14 , 30 , 35 and 40 days from the production date , performing the following microbiological analyses : enumeration of i ) colony - forming units characteristic of the yogurt , ii ) enterobacteriaceae , iii ) yeasts and/or moulds at 25c . 
 microbiological identification was performed by miniature biochemical tests and for the lactic acid bacteria also by pcr . at every test interval , 
 evaluation of organoleptic parameters and ph was also performed . 
 the analysed product maintained an almost constant amount of lactic acid bacteria until the end of the declared shelf - life . 
 concerning lactic acid bacteria , a 100% concordance of the results observed by using biochemical identification methods and pcr assays was obtained . 
 after 14 days from the production , the presence of yeasts ( candida famata ) was revealed , while the presence of moulds was detected after 30 days . 
 ralstonia picketii , an environmental microorganism , was also isolated . 
 the results obtained in this study indicate that yogurt spoilage is mainly due to the growth of specific microorganisms of spoilage , such as yeasts and moulds . 
Introduction
Materials and Methods
Results
Discussion
Conclusions