Microbiological Studies On Frozen Foods And Canned Foods:

Osama Abdel-salam Mahmoud Sharif Superviser

The studies aimed for the microbiological evaluation of somefoods available in the local market to determine their acceptabilityfor human consumption. The study included the following:-I- Survey samples :1. Canned foods (low acid canned foods) :2. Frozen foods: A- Vegetables (green beans - okra)B- Frozen meat, frozen minced meat.11-Storage samples :To study the effect of storage temperature and period ofstorage:1- low acldCnnned food.2- Frozen vegetables and frozen meat.III- Longevity samples :To study the longevity of Staph. aureus, E. coli andSalmonella sp. in frozen minced meat and frozen vegetables.Survey samples :I. Canned foods : The cans were opened and examined forcolour, appearance, ... etc. and the study of the contents of the cansrevealed the following.-- pH decreased in some samples, but pH was normal in themajority of samples.and the microbiological examination of the contents of thecans showed the following:- Samples of the canned fish were free from the mesophilicanaerobes and thermophilic anaerobes also the examined cans werefree from yeasts and moulds and free from H2S producing bacteriaexcept one sample contained H2S producing bacteria.- The aerobic bacteria was present in low numbers and themesophilic aerobes were higher than thermophilic aerobes.2. Frozen vegetables (green beans and okra) :a- Frozen green beans:- Total count:Samples taken in 1994 were within the valid limits (100 000cel1/g of sample) in the majority of samples. However, somesamples exceeded the valid limits of total count.- In 1995 and 1996 the examination of the samples showedthe same trend i.e the majority of the sampleswere within the validlimit, but some samples were out of limit.The results of T.C. showed that counts varied from companyto another company and also differed from year to year for thesame company.- Concerning yeasts and moulds counts in frozen green beansit was found that some samples of 1994 and 1995 contained higheryeasts and moulds counts than the valid limits (less than 50 cell/g),but the remaining samples were within the valid limits. Butsamples examined during 1996 were within the valid limits ofyeasts and moulds.- Samples of frozen green beans were free from Salmonellasp. and Clostridium perfringens during 1994, 1995 and 1996.- Staph. aureus was present in frozen green beans samplesduring 1994. However, Staph. counts were reduced greatly insamples taken during 1995 and 1996.-E. coli was present in some samples of 1994 and was lessfrequent in samples of 1995 and disappeared completely insamples of 1996.Concerning frozen okra which was examined during 1994,1995 and 1996, T.C. for the exam ined samples during 1994 waswithin the valid limits (100000 cell/g) except some samples whichexceeded the valid limits. T.C. of samples of okra during 1995 and1996 were within the valid limits.- Yeasts and moulds in frozen okra increased in the samplesexamined during 1994 and 1995, but their numbers decreasedgreatly during 1996.- Samples of frozen okra examined during 1994, 1995 and1996 were free from the pathogens Salmonella and Clostridiumperfringens.- E. coli which is an indicator of pollution was detected intwo samples during 1994 and one okra frozen sample during 1995and disappeared completely in all investigated okra frozen samplesduring 1996.b - fiot.zcn IIlinced meat S~1I11plesr- T.e. of the investigated minced meat samples was withinthe valid limits (less than t 06/g) for all examined samples during1994, 1995 and 1996.- Yeasts and moulds counts were higher than the valid limits(50 cell/g) for samples of 1994, 1995 and 1996.- All frozen minced meat samples examined during 1994,1995 and 1996 were free from Salmonella and ct. perfringens.II· Storage samples :- Canned sardine:Canned sardine stored at 37°C yielded the following:• No gas production in the stored cans during the storageperiod (6 months) indicating the absence of gas producingbacteria and no reactions between the food material and themetal of the cans.• No pronounced change In the pH of the examined sardinecans indicating the absence of acid-producing organisms.• T.e. of stored sardine cans were low at the beginning due tothe drastic effect of the heat treatment. However. there was atrend of increasing T.C. by increasing the storage period.• Yeasts and moulds disappeared completely from theexamined sardine cans.• All investigated samples were free from I-hS producingbacteria except one sample showed H2S production after 4months storage.- Canned sardine at 55°C showed the following :-• Aerobes counts were higher than the counts of anaerobes.• Thermophiles counts were higher than mesophiles.• There was a slight change in pH in the majority ofinvestigated samples. Only one sample- in which pHdecreased greatly.• There was coincidence between microbial counts and pH. Astotal count increased pH decreased.• Yeasts and moulds didn’t appear in samples stored at 55°C.• Gas formation appeared at the late periods of storage (4-6months) in cans stored at 55°C.• H2S appeared only in one can after 4 months storage.The storage of frozen vegetables showed the following results :-- Total microbial counts of frozen vegetables, which were storedfrozen for 18 months, were within the valid limits (100 000 cell/g).95Generally, there was a trend, which showed that as storage periodincreased r.c. decreased.-Psychrophiles counts in frozen vegetables were moderate andthere was a general trend, as storage period increased Psychrophilicorganisms increased.- Yeasts and moulds In frozen vegetables were within the validlimits (less than 50 cell/g) and decreased as storage periodincreased to disappear completely after 12 - 15 months storage.- Coliforrns counts decreased greatly as storage period increased.- E. coli didn’t appear in some frozen vegetables samples andappeared in some samples where it decreased by increas ing storageperiod to disappear completely after 15 - 18 months.- Concerning pathogens, Salmonella and Clostridium perfringenswere absent in frozen vegetables, but Staph. aureus was found andtheir counts decreased by increasing the freezing period todisappear completely after 15 - 18 months.Concerning the storage of frozen meat, results showed thefollowing:- T.e. decreased as freezing period increased.- Psychrophilic bacteria increased as freezing period increased.- Yeasts and moulds counts decreased as freezing period increased.- Coliform bacteria, in frozen meat, decreased as freezing periodincreased.III- Longevity of inoculated pathogens in frozen vegetablesand minced meat:Longevity of inoculated pathogens in frozen vegetables andfrozen minced meat, stored for 3 months under freezing conditionsshowed the following results:The freezing process decreased Salmonella counts in frozenminced meat to 50%.Storage for 3 months freezing conditions decreased thecounts of Salmonella to disappear completely after 3 months infrozen vegetables and to decrease greatly in frozen minced meatafter 3 months storage.It seems that the components of minced meat protectedSalmonella to some extent from the drastic effect of freezing.Concerning the longevity. of inoculated Staph. aureus tofrozen vegetables and frozen minced meat during freezing andduring storage at freezing conditions, results show the following:- The freezing process caused .reduction in Staph. aureuscounts in vegetables and minced meat.- The storage of frozen vegetables and frozen minced meatunder freezing caused continuous reduction in Staph. aureuscounts along the storage period (3 months).- Concerning the longevity of inoculated E. coli in frozenvegetables and frozen minced meat, it behaved as Staph. aureus,since freezing caused reduction in E. coli counts and storage atfreezing conditions caused continuous reduction of E. coli counts.