Hubbard Foods Limited of Auckland makes a variety of oats-based value-added products. In the preparation of a range of products at Hubbard Foods, technical staff has become aware of a bitterness problem that sporadically appears in toasted oats. Toasting involves dry heating to about 150°C resulting in the golden colour and flavour development necessary for range of products. Bitterness development has been described in the literature, but Hubbard staff is necessarily focussed on production issues, rather than on a sporadic problem seemingly outside the scope of production variables. The author of this thesis set out to identify the cause and suggest a remedy. Prior research with oats has shown that bitterness and associated off-flavours are linked to the accumulation of free fatty acids, their volatile oxidation products, and possibly amino acids and certain phenols. Oats are distinguished from related grains by their high relative fat content, about seven percent, and an associated very active lipase. The free fatty acids stem from the lipase activity that should be, but may not be, inactivated at source in Australia. This is achieved in the milling process by physical disruption and moist heating to a temperature at which the enzyme is denatured. However, residual lipase activity may adversely affect oats quality during time in storage and transit. A number of analytical methods for cereals were adapted to match the constraints of time and resources. These methods were for colour, moisture, peroxidase activity, p-anisidine, and fat and free fatty acids content, composition of fatty acids, total phenols, volatiles, and bitterness as perceived by an analytical sensory panel of four people. Determination of lipase activity is very expensive, so peroxidase activity is commercially used as an indicator. If the latter is inactive, the former will necessarily be also inactive. The designed methods were first applied to 17 oats lots passing through the Hubbard environment, where 14 were paired raw and toasted. The values of moisture, fat content, free fatty acids content and total phenols were within the normal limits expected for commercial lots of oats compared with the previous studies. Not much variation was observed among the 17 oats lots, with the exception of lot DWHE25. Lot DWHE25 was a faulty product, which had high moisture content, high free fatty acids content, and tasted very bitter. The results suggested that moisture content, free fatty acids and bitterness were usually correlated. In spite of the differences encountered and the clues provided by extremes, the data generated from Hubbard oats lots did not provide enough variation in quality to lead to a definitive chemical model of bitter flavour development. But perhaps crucially, it was found that most samples as received from Hubbard Foods were peroxidase-active which conflicted with the results reported on specification sheets prepared by the oats supplier. These specifications accompanied each lot delivered to Hubbard Foods. Therefore, the supplier’s method was examined and was found to be deficient in one critically important respect. Their method omitted the key reactant hydrogen peroxide. Therefore, it is possible that the lipase was active in many of the samples. Therefore, some experiments were conducted where raw oats, from Hubbard Foods and a supermarket, were treated with water additions and stored for a period to examine the effect of moisture content on the quality and flavour deterioration on subsequent storage. Water-treated oats were toasted to simulate a typical Hubbard process, yielding a total of 58 samples with carrying moisture contents. The data set was statistically analysed to identify the cause of bitterness and the means of its control. The free fatty acids content, volatile compounds particularly hexanal, and total phenols increased with moisture content and storage time. The correlations between chemical analysis and sensory test indicated that free fatty acids positively correlated with bitterness (r = 0.71), and hexanal was also positively correlated. Total phenols did not appear to correlate with bitterness. Oats lots with high peroxidase activity tended to have the poorest quality, strongly implicating residual lipase activity as the critical factor. There were no important interactions between water addition and toasting for most of the experiments. Therefore, it seems likely that the toasting procedures at Hubbard Foods are not responsible for bitterness formation. The cause(s) of bitterness is certainly at source, with a faulty peroxidase test strongly implicated.

Hubbard Foods Limited of Auckland makes a variety of oats-based value-added products. In the preparation of a range of products at Hubbard Foods, technical staff has become aware of a bitterness problem that sporadically appears in toasted oats. Toasting involves dry heating to about 150°C resulting in the golden colour and flavour development necessary for range of products. Bitterness development has been described in the literature, but Hubbard staff is necessarily focussed on production issues, rather than on a sporadic problem seemingly outside the scope of production variables. The author of this thesis set out to identify the cause and suggest a remedy. Prior research with oats has shown that bitterness and associated off-flavours are linked to the accumulation of free fatty acids, their volatile oxidation products, and possibly amino acids and certain phenols. Oats are distinguished from related grains by their high relative fat content, about seven percent, and an associated very active lipase. The free fatty acids stem from the lipase activity that should be, but may not be, inactivated at source in Australia. This is achieved in the milling process by physical disruption and moist heating to a temperature at which the enzyme is denatured. However, residual lipase activity may adversely affect oats quality during time in storage and transit. A number of analytical methods for cereals were adapted to match the constraints of time and resources. These methods were for colour, moisture, peroxidase activity, p-anisidine, and fat and free fatty acids content, composition of fatty acids, total phenols, volatiles, and bitterness as perceived by an analytical sensory panel of four people. Determination of lipase activity is very expensive, so peroxidase activity is commercially used as an indicator. If the latter is inactive, the former will necessarily be also inactive. The designed methods were first applied to 17 oats lots passing through the Hubbard environment, where 14 were paired raw and toasted. The values of moisture, fat content, free fatty acids content and total phenols were within the normal limits expected for commercial lots of oats compared with the previous studies. Not much variation was observed among the 17 oats lots, with the exception of lot DWHE25. Lot DWHE25 was a faulty product, which had high moisture content, high free fatty acids content, and tasted very bitter. The results suggested that moisture content, free fatty acids and bitterness were usually correlated. In spite of the differences encountered and the clues provided by extremes, the data generated from Hubbard oats lots did not provide enough variation in quality to lead to a definitive chemical model of bitter flavour development. But perhaps crucially, it was found that most samples as received from Hubbard Foods were peroxidase-active which conflicted with the results reported on specification sheets prepared by the oats supplier. These specifications accompanied each lot delivered to Hubbard Foods. Therefore, the supplier’s method was examined and was found to be deficient in one critically important respect. Their method omitted the key reactant hydrogen peroxide. Therefore, it is possible that the lipase was active in many of the samples. Therefore, some experiments were conducted where raw oats, from Hubbard Foods and a supermarket, were treated with water additions and stored for a period to examine the effect of moisture content on the quality and flavour deterioration on subsequent storage. Water-treated oats were toasted to simulate a typical Hubbard process, yielding a total of 58 samples with carrying moisture contents. The data set was statistically analysed to identify the cause of bitterness and the means of its control. The free fatty acids content, volatile compounds particularly hexanal, and total phenols increased with moisture content and storage time. The correlations between chemical analysis and sensory test indicated that free fatty acids positively correlated with bitterness (r = 0.71), and hexanal was also positively correlated. Total phenols did not appear to correlate with bitterness. Oats lots with high peroxidase activity tended to have the poorest quality, strongly implicating residual lipase activity as the critical factor. There were no important interactions between water addition and toasting for most of the experiments. Therefore, it seems likely that the toasting procedures at Hubbard Foods are not responsible for bitterness formation. The cause(s) of bitterness is certainly at source, with a faulty peroxidase test strongly implicated.

No description available.

The initial aim of this work was to evaluate processing conditions for extended shelf life (ESL) milk to have a shelf life at refrigeration temperature of 60 d. Milk was processed on a pilot-scale ultra-high-temperature processing plant and evaluated for microbial and sensory quality over 60 d at 7°C storage. Results of this study showed that lower process temperatures were preferable to minimize cooked flavors and that the minimum safe processing temperature was 134°C for 4 s as determined by the destruction of bacterial spores in the processed milk. Consumer preference panel results indicated that consumers preferred milk processed at 134°C for 4 s (those recommended in this study for ESL processing) to commercial UHT milk although there was a slight preference for pasteurized milk. The critical sensory characteristic of the processed milk was a cooked flavor, which decreased with lower processing temperature and shorter storage time; however, a significant increase in flavors that could be associated with lipolytic activity was also noted. This study highlighted deficiencies in existing methods for determining heat-stable bacterial products in thermal-processed foods. No rapid, sensitive assay for detection of heat-stable spores or lipases in milk exists. If such assays were available, it would allow processors to determine Lipase activity and bacterial spore counts before processing and direct raw milk with low spore counts and low lipolytic activity into long-shelf-life products. To this end, assays to rapidly quantitate spores and lipolytic activity in milk were developed. The lipase assay relies on the hydrolysis of p-nitrophenyl caprylate liberating a yellow color that is detected using reflectance colorimetry. The assay is sensitive to 5 mUnits/ml and is linearly correlated to spectrophotometry (r2 = 0.93) and release of titratable free fatty acids (r2 = 0.92 to 0.97). An immunocapture, enzyme-linked immunoassay coupled with a fluorescent detection system was developed for and resulted in a prototype spore assay using Bacillus stearothermophilus spores. This organism was selected because it is extremely heat resistant, is commonly found in milk, and is associated with spoilage of milk and milk products. The assay was able to quantitate spores down to 103 cfu/ml in milk and other products in about 1.5 h. Other detection limits could be set if needed.

Yeasts are traditionally used as part of the surface microflora in surface-ripened cheeses, where they contribute positively to the flavour of the cheese. The primary objective of this study was to investigate the potential of three dairy yeasts to provide attributes as adjuncts in anaerobically ripened cheeses. Geotrichum candidum (B9001), Yarrowia lipolytica (B9014) and Candida kefyr (B9006), obtained from the Fonterra Co-operative Group Ltd, Palmerston North, New Zealand, were studied. They showed diverse metabolic activities in laboratory media, which were influenced by the growth conditions. The metabolic activities of special interest were the lipase and proteinase activities and the production of volatile compounds, as these are important for cheese ripening and flavour development. Lipase activity (p-nitrophenyl butyrate assay) and proteinase activity (fluorescein isothiocyanate β-casein assay) were determined in three fractions prepared from yeast cultures and designated as extracellular fraction, washed-cell fraction and intracellular fraction. Lipase activity of G. candidum was detected only in the extracellular fraction and increased five fold when induced by safflower oil in a shake culture (0.16 µM/min/mL supernatant at 24 h). Lipase expression was delayed in static cultures. Y. lipolytica showed lipase activity in extracellular, washed-cell and intracellular fractions under all conditions. Static cultures in both glucose and safflower oil media showed higher lipase activity than shake cultures. The lipase activity of Y. lipolytica was higher in the late stationary phase than in the log phase under all conditions tested. The highest lipase activity was detected in a 192 h static culture grown in safflower oil medium (0.13 µM/min/mg dry cell weight, 0.3 µM/min/mg dry cell weight and 4.29 µM/min/mL supernatant in the intracellular, washed-cell and extracellular fractions respectively). C. kefyr did not show any lipase activity (< 0.03 µM/min/mL culture) under any of the growth conditions tested. Proteinase activity was detected in the intracellular fraction of 72 h shake cultures of G. candidum grown in both glucose medium and safflower oil medium (154 and 122 RFU/min/mg dry cell weight respectively) but was not detected in static cultures. Proteinase activity was absent in the Y. lipolytica cultures under all conditions tested (< 10 RFU/min/mL culture). C kefyr showed low proteinase activity (12-74 RFU/min/mL supernatant) in the extracellular fraction only in shake cultures grown in glucose medium. Volatile compounds of the headspace were sampled and analysed using solid phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS). The concentrations of volatile compounds were highest in shake cultures grown in glucose medium for all three yeasts. All yeasts produced several alcohols. Several esters were also detected in the G. candidum and C. kefyr cultures whereas aldehydes were detected only in the G. candidum cultures. G. candidum and Y. lipolytica were selected for cheese production trials because of their active cheese ripening enzymes. These yeasts, grown under different conditions, were added to Cheddar cheese (10 L vat). The yeast adjuncts influenced the cheese ripening by lipolysis [in terms of the production of free fatty acids (FFAs) analysed by gas chromatography-flame ionisation detector (GC-FID)] and the production of volatile compounds (SPME-GC-MS), whereas proteolysis (analysed by size-exclusion high performance liquid chromatography) by yeast enzymes was not obvious. The influence of Y. lipolytica as an anaerobic adjunct to cheese ripening was dependent on the growth conditions used during its propagation in laboratory media. The concentration of total FFAs was very high (37.1 mg/g cheese at 6 months) when a 192 h Y. lipolytica culture grown in safflower oil medium was added to a cheese make, whereas the cultures grown in glucose medium did not have any detectable effect. Addition of G. candidum culture to the cheese curd was more effective than its addition to the cheese milk. Both G. candidum and Y. lipolytica lipase(s) selectively hydrolysed the long-chain unsaturated fatty acids from the milk triglyceride in the cheese environment. Also, Y. lipolytica lipase exhibited some selectivity towards hydrolysis of butyric acid from the milk fat in the cheese. 2-Heptanone, 3-methyl-2-butanone and 2-nonanone were detected (1-10 x 106 relative peak area) only in the cheeses with yeast adjuncts but not in the control cheese. Enhancement of the production of both conjugated linoleic acid (CLA) and ethyl esters in a washed-curd, dry-salted cheese (375 L vat), made with G. candidum, Y. lipolytica, Propionibacterium freudenreichii ssp. shermanii, Lactobacillus fermentum and Lb. rhamnosus, was only partially successful. Higher concentrations of ethyl esters (> five fold; analysed by SPME-GC-MS) were produced in the cheeses made with yeast adjuncts. However, the concentration of total CLA (free plus esterified; analysed by GC-FID) did not increase although a higher concentration of free linoleic acid (> 10 fold), the substrate for CLA synthesis, was produced in the cheeses made with yeast adjuncts. A study of the formation of aromatic volatile compounds by C. kefyr in a medium containing L-phenylalanine (L-phe) showed that the yeast's ability to produce phenyl ethanol, phenyl ethyl acetate and benzaldehydc (analysed by SPME-GC-MS) was enhanced with an increase in the initial L-phe concentration (in the experimental range; analysed by enzymatic assay using phenylalanine ammonia lyase), but the yield was very low (20-27%). The initial concentration of glucose (in the experimental range; analysed by enzymatic assay using Peridochrom glucose reagent) did not affect the production of these aromatic volatile compounds. This study successfully showed that the yeasts G. candidum and Y. lipolytica, when used as anaerobic adjuncts, can influence the ripening and flavour development in Cheddar and washed-curd, dry-salted cheeses. The study also showed the capability of C. kefyr to produce aromatic volatile compounds from amino acid fermentation but the yields need to be increased by further manipulation of the medium components and the culture conditions before this capability can be used commercially.

Made available in DSpace on 2015-04-17T15:03:08Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 760407 bytes, checksum: 36ff52186c6a7e31beb2bd5ddc4a7fa0 (MD5) Previous issue date: 2011-03-25 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / This study aimed to isolate strains of Staphylococcus aureus in ricotta cheeses and to characterize them respecting phenotypic markers of lipolytic activity, and bacteriocin typing and resistance typing. For isolation, were acquired 11 samples of different brands of ricotta cheese sold in supermarkets in João Pessoa - Paraíba. Among the 41 strains, 82.9% proved positive lipase (Lip+) and 17.1%, negative lipase (Lip-). In relation to bacteriocin typing, 9.76% showed bacteriocin producing against a strain of Staphylococcus aureus isolated from surfaces processing food and 55% were susceptible to a strain of Staphylococcus aureus isolated from ricotta cheese. For resistance typing, 87.8% of strains were resistant to streptomycin, 26.59% to penicillin and 19.51% to erythromycin. None of the strains were resistant to tetracycline. Variability was observed between the minimal inhibitory concentrations (MIC), both in resistant strains as sensitive ones, so the variability showed more prevalent with penicillin (ranging from 0.015625 to 16 mg / mL) and the variability between strains of the same cheese. Only erythromycin-resistant strains were subjected to "D-test" to determine the type of resistance, of which all presented inductive resistant. From this results obtained, can conclude that although the phenotypes show an adaptive answer on related to the peculiarities of local environmental pressures, it is clear the importance of these genetic markers to discern new strains of S. aureus for those endemic, as well as to investigate possible epidemiological impacts and on food safety. / Este estudo teve como objetivo isolar cepas de Staphylococcus aureus de queijos ricota e caracterizá-las com relação a marcadores fenotípicos de atividade lipolítica, bacteriocinotipagem e resistotipagem. Para o isolamento, foram adquiridas 11 amostras de queijo ricota de diferentes marcas comercializadas em supermercados da cidade de João Pessoa-PB. Entre as 41 cepas isoladas, 82,9% revelaram-se lipase positiva (Lip+) e 17,1%, lipase negativa (Lip-). Com relação à bacteriocinotipagem, 9,76% mostraram-se produtoras de bacteriocinas frente a uma cepa de Staphylococcus aureus isolada de superfícies de processamento de alimentos e 55% mostraram-se sensíveis a uma cepa de Staphylococcus aureus isolada de queijo ricota. Para resistotipagem, 87,8% das cepas apresentaram-se resistentes a estreptomicina, 26,59% a penicilina e 19,51% a eritromicina. Nenhuma das cepas foi resistente a tetraciclina. Observou-se a variabilidade entre as Concentrações Inibitórias Mínimas, tanto em cepas resistentes como sensíveis, sendo tal variabilidade mais predominante com penicilina (variação de 0,015625 a 16 &#956;g/mL), bem como a variabilidade entre cepas do mesmo queijo. Apenas cepas resistentes a eritromicina foram submetidas ao D-teste a fim de determinar o tipo de resistência, das quais todas as cepas mostraram-se com resistência indutiva. A partir dos resultados obtidos, pode-se concluir que embora os fenótipos revelem uma resposta adaptativa relacionada às peculiaridades das pressões ambientais locais, é evidente a importância destes marcadores genéticos a fim de se distinguir novas linhagens de S. aureus daquelas endêmicas, bem como investigar possíveis impactos epidemiológicos e na segurança alimentar.