Encapsulation of lipids and other oxidizable compounds provides a means of protecting these compounds against their environment, including such factors as oxygen and humidity. The physical properties of wall matrices have a major influence on the oxidative stability of encapsulated lipids. This study investigated the effects of composition and processing conditions on physical and chemical stability of encapsulated milk fat powder (dairy creamer). Achieving a stable liquid emulsion is a necessary first step. Emulsifier type and concentration as well as processing conditions affected oil droplet size distribution. Larger oil droplet size led to an unstable emulsion and a consequential increase in surface fat of spray dried powder. At best, about 98% of the milk fat was encapsulated (average oil droplet size ∼0.5 μm, 2.0% lecithin, 36 DE corn syrup solids). The physical properties of spray dried powders that were studied include internal and external morphology, water sorption isotherm, surface fat, true density, and melting and crystallization. Upon storage, physical changes led to a complex change in surface fat with storage humidity, crystallization of carbohydrates (at higher %RH), and caking. These were dependent on maltodextrin dextrose equivalent (DE) and storage humidity. Oxidative stability of milk fat encapsulated in powder with DE-36 corn syrup solids was studied at 14–52%RH and 25°C (Collaborative work with two other investigators; Dr. Nitin Hardas and John L. Foley). Oxidation was negligible for powders stored in the dark at all relative humidities. Under UV light, the encapsulated fat significantly increased in peroxide value (PV) and decreased in 18:2 and 18:3 fatty acids over time. The highest rate of oxidation was found at 52% RH, indicating increased diffusivity of oxygen and other components under this condition. In contrast, surface fat oxidative rate was slower at 52% than at 14% and 44% RH. Overall oxidative stability was dominated by the encapsulated fraction although in general the surface fat fraction was oxidized more rapidly.

Abstract not available

Appearance plays an important role in determining the quality of food products, but little work has been done on the factors that influence the appearance of food emulsions. Appearance depends on the scattering and absorption efficiency of the emulsions. The scattering efficiency is determined mainly by droplet characteristics (size, concentration, aggregation and relative refractive index), while the absorption efficiency is determined by dye characteristics (absorption spectra and concentration). The objective of this study is to investigate the influence of droplet, TiO2 and dye characteristics on the optical properties of oil-in-water emulsions, and apply the knowledge gained from model systems to understand the factors determining the optical properties of a real food system, milk. Droplet and dye characteristics had a pronounced influence on emulsion appearance. The lightness of an emulsion was correlated to the scattering efficiency of the droplets. The color of emulsions (a- and b-values) was mainly determined by the type (red, green, blue) and concentration (0–0.1 wt%) of chromophores present. The lightness of emulsions increased steeply between 0 and 5 wt% oil, and increased slightly at higher concentrations (5–20 wt%). It also increased with decreasing droplet diameter (30–0.2 μm) and increasing refractive index difference between the two phases. The influence of droplet characteristics on the lightness and color of emulsions containing different types of dye was fairly similar. Droplet flocculation did have an impact on emulsion appearance, but the effect was relatively small compared to the effect of droplet size. Small amounts of TiO2 could be used to obtain the same emulsion appearance at lower oil contents. The optical properties of dairy emulsions showed the same trends as those of model ones. The effect was most pronounced below 5% milk fat. There was good correlation between sensory analysis and instrument measurement of emulsion color. Food emulsion appearance therefore can be optimized by carefully controlling droplet and dye characteristics. TiO2 has good potential to improve the appearance of low-fat foods. A mathematical model based on light scattering theory was shown to have good potential for predicting the influence of composition and microstructure on the optical properties of emulsions.

Escherichia coli O157:H7 is an important foodborne pathogen. The majority of outbreaks of this organism have been attributed to ground beef. In order to improve the sensitivity for detection of E. coli O157:H7 in ground beef by the polymerase chain reaction (PCR), sample preparation methods including differential centrifugation, enrichment, immunomagnetic separation (IMS), and DNA extraction were studied. Quantitative detection protocols based on standard PCR, nested PCR and competitive PCR were developed. Using standard PCR enabled detection of 150 colony forming units (CFU) per gram of ground beef without enrichment and 1.2 CFU/g after 4.5 h of enrichment in modified EC broth plus novobiocin (mEC+n) with SLT I and SLT II primers respectively. The quantitative detection range was 150 to 15,000 CFU/g for protocol without enrichment and 1.2 to 19.2 CFU/g for protocol with enrichment. Standard PCR incorporating IMS detected 70 CFU/g with SLT I and SLT II primers without enrichment and 0.7 CFU/g after 6 h of enrichment in Tryptic Soy Broth (TSB) and IMS. The quantitative detection range was 70 to 24,000 CFU/g for protocol without enrichment and 0.7 to 70 CFU/g for protocol with enrichment. A partial digestion of the cell pellets derived from differential centrifugation was required for IMS. DNA extracted from seeded target cells captured by IMB could be used for PCR amplification without further purification. Nested PCR incorporating differential centrifugation and IMS was capable of detecting 2.4 CFU/g with SLT 1 and SLT 2 primers without enrichment. Within the range of 2.4 to 240 CFU/g, a linear relationship between the log CFU of the inocula and the fluorescent intensity of secondary PCR products was achieved by using 25 cycles for both primary and secondary PCR run. Competitive PCR incorporating IMS enabled quantitative detection of 0.5 to 5.5 CFU per gram of ground beef with SLT I and SLT II primers after 6 hours of enrichment using TSB. The numbers of target organisms after enrichment and IMS determined by competitive PCR were highly correlated to those derived from viable plate counts on violet red bile agar.

Chelators are valuable ingredients used to improve the oxidative stability of many processed foods. Caseins and casein peptides have phosphoseryl residues capable of binding prooxidant metals such as iron and may have antioxidant activity. In 5% corn oil-in-water emulsions CPP (25 μM) inhibited the formation of lipid oxidation at both pH 3.0 and 7.0 as determined by lipid hydroperoxides and hexanal. Calcium (0–100 mM) had no influence on the antioxidant activity of CPP. Casein hydrolysates were tested for their antioxidant properties as hydrolysates represent a more economical source of phosphopeptides. Casein hydrolysates were more effective inhibitors of lipid oxidation than CPP at equal phosphorous content (0–310 μM) suggesting that antioxidant properties were not uniquely related to chelating prooxidant metals but also to free radicals scavenging activity. The antioxidant activity of CPP and casein hydrolysates was studied in phosphatidylcholine liposomes. CPP (<1.0 mg/ml) and casein hydrolysates (0.3–1.7 mg/ml) were effective inhibitors of TBARS development when oxidation was promoted by iron. High amounts of CPP (>1.0 mg/ml) were prooxidant while casein hydrolysates were only observed to be antioxidative. In the presence of peroxyl radicals, casein hydrolysates were more effective scavengers than enriched CPP (3–15 mM). When antioxidant activity was evaluated in cooked ground beef, TBARS formation was inhibited 75 and 39% by 0.5% CPP and casein hydrolysates, respectively, after 4 days of storage. The binding interactions of CPP and casein hydrolysates to iron were studied using isothermal titration calorimetry (ITC) with Fe-nitrilotriacetic acid (NTA) as the titrant. The observed binding constants were in the range of 5.8–1.7 × 103 M−1 for CPP and casein hydrolysates. The number of binding sites was below 1.0 suggesting that more than one phosphoseryl group is needed to bind iron or that other amino acids such as glutamic acid might participate in the binding. The interactions seem to be independent of pH since the apparent binding constants were similar between pH 3.0–7.0. These results show that CPP and casein hydrolysates are promising sources of natural antioxidants in foods since they have the potential ability to act as chelators of prooxidant metals and as free radical scavengers.

The pH of beef muscle affected water-holding capacity (WHC) and tenderness. When the pH was brought to 4.0 and below by immersion in citric acid or lactic acid or brought to ∼7.0 after base immersion, WHC and tenderness of beef muscle increased. When the pH was brought back to the isoelectric point of the major myofibrillar proteins after either acid or alkaline treatment (e.g. from ∼3.7 or 7.0 to ∼5.5), most of the weight gained from prior acidic or basic treatment was lost. The Warner-Bratzler shear force values (WBSF) returned at least to the level of the WBSF of the controls after the readjustment of pH to 5.5. This reversibility strongly suggests that tenderization is not caused primarily by proteolysis. It is proposed that myofibrillar structures could swell and disrupt at acid/neutral pH due to electrostatic repulsion. When the pH was brought back near the isoelectric point, the myofibrillar structure might collapse and/or aggregate. Less water could be held with the aggregated proteins than in the initial myofibrillar structure of raw beef. Lactic acid and citric acid produced essentially the same shear force if WBSF values after marination were compared at a similar pH value. CaCl 2 showed a synergistic effect and negative effect, respectively, on the tenderization induced by citric acid and lactic acid. The solubility of beef myofibrillar proteins increased steeply from pH 4.27 to 4.03, where a significant decrease in WBSF was often observed. The solubility of beef muscle protein was below 20% between pH 6.9 and 7.6. The relation of protein solubilization to tenderization observed at acid pH was not seen in the tenderization induced by base treatment. Elastic gels from washed chicken breast muscle with a moisture content of 88% were formed at an acid pH (∼3.7 and lower) and low ionic strength. A decrease of pH from 4.1 to 3.7 or below increased gel elasticity and decreased water loss under pressure. In the presence of NaCl, gels made at pH ∼3.7 had decreased elasticity and increased water loss under pressure compared to those prepared without salt.

Proteins and protein-derived materials, such as enzymatic hydrolysates, have been shown to improve the oxidative stability of food lipids. However, the mechanisms of protein antioxidant action are complex, particularly in heterogeneous, dynamic environments such as food systems. The goal of this research is to provide a fundamental understanding of the various mechanisms by which proteins inhibit oxidative reactions in food lipid dispersions. The presence of low concentrations of the whey protein β-lactoglobulin (β-Lg) present in the continuous phase of oil-in-water emulsions was observed to inhibit lipid oxidation. Furthermore, the antioxidant activity of continuous phase β-Lg was significantly improved by thermal denaturation (95°C; 15 min) at pH 7.0, despite having lower iron binding values and concentration of reactive sulfhydryls than untreated, native β-Lg. The observed enhancement in antioxidant activity of heated β-Lg seems to result from improved solvent accessibility of radical scavenging amino acids, and/or changes in β-Lg's surface activity, leading to increase interactions with lipophilic oxidants. Cysteine and tryptophan residues in native β-Lg dispersed in the continuous phase of emulsions were observed to oxidize prior to emulsified lipid, suggesting that these amino acids possess antioxidant activity. However, methionine residues in β-Lg did not appear to participate in free radical scavenging in this system, despite its reported oxidative lability. This may be due to the nature of the endogenous oxidant (i.e. lipid-derived radical species) or to the fact that β-Lg's methionine residues have low solvent accessibility when the protein is in its native conformation. Finally, it was observed that β-Lg's antioxidant activity is enhanced with enzymatic (e.g. chymotrypsin) hydrolysis. Increased peroxyl radical scavenging activity and iron chelation seem to be responsible for the increased antioxidant activity of hydrolyzed β-Lg. Liquid chromatography - mass spectrometry (LC-MS) was used to assess the oxidative modification of three specific amino acid residues in the selected chymotrypic hydrolysates of β-Lg during the period preceding the formation of lipid oxidation products. Significant oxidation of tyrosine and methionine were observed during this period, suggesting that these residues are capable of free radical scavenging, thereby conferring improved oxidative stability to oil-in-water food emulsions.

Plesiomonas shigelloides has been recognized as a potential human and animal pathogen in the past six decades. In this study, RAPD typing was performed to determine the genetic variability among the isolates of P. shigelloides from different sources. In addition, quantitative methods to detect P. shigelloides based on PCR were developed. Also, factors affecting PCR detection were examined, methods to reduce the inhibition of PCR by PCR inhibitors were tested, and a method for quantitative detection of viable P. shigelloides in a mixture of viable and dead cells was developed. RAPD analysis showed that there was notable genetic variability among most of the strains of P. shigelloides isolates from fresh water, fish and human clinical sources. The isolates from fish had a closer linkage to the human clinical isolates than did the freshwater isolates, suggesting that fish may be the more serious source for potential risk of infection. Using standard quantitative PCR, the lowest level of detection for P. shigelloides, without enrichment, was 60 CFU/g in clams with the treatment of BSA and 200 CFU/g in oysters with the treatment of formaldehyde and BSA respectively. Using conventional quantitative PCR in combination with ethidium bromide monoazide (EMA) enabled quantitative detection of viable P. shigelloides cells in varying ratio's of viable to dead cells at a minimum of 24 CFU per PCR reaction. Competitive PCR was capable of detecting DNA derived from 2.4 × 102 to 2.4 × 105 CFU/g of clam tissue without enrichment, and 4.0 × 101 to 1.2 × 104 CFU/g of clam tissue with 7 hrs. of nonselective enrichment at 37°C respectively. Real-time PCR quantitatively detected 3 to 1.0 × 104 CFU per gram of oyster after 7 hrs. nonselective enrichment. Without enrichment, the minimum detection level was 1.0 × 103 CFU per gram of oyster tissue with the addition of formaldehyde following by the treatment with coated charcoal.

This work focused on the factors that affect the cook yield and moisture retention of different muscle foods at reduced salt concentrations (≤ 150 mM). Before studying the water-holding capacity (WHC) of different muscles, which was evaluated by cook yield and moisture retention upon heating and pressing, solubilization of myofibrillar proteins of chicken and cod muscles in water were first investigated. Certain possible solubility inhibiting (PSI) polypeptides and exposure to low pH affected the water solubility of myofibrillar proteins. The PSI proteins acted like a barrier that prevented the rest of the myofibrillar proteins from disorganization, swelling and final solubilization in water. M-protein (160 kDa), α-actinin (95 kDa), amorphin (87 kDa) and desmin (56 kDa) were tentatively identified as the PSI polypeptides in the chicken breast muscle. Re-addition of these PSI polypeptides to the PSI-extracted chicken mince inhibited the full solubilization of the chicken mince. Exposure of the cod myofibrillar proteins to a low pH could cause protein denaturation and subsequently the loss of extractability in water. M-protein (160 kDa), a 103 kDa polypeptide and α-actinin (95 kDa) were tentatively identified as the PSI polypeptides in cod muscle. By extracting the acidified cod mince with a solution of physiological ionic strength at neutral pH promoted the process to reverse the low pH effect on these three PSI polypeptides, thus the muscle proteins solubility was re-established. The cook yield and moisture retention of the washed chicken mince at low ionic strength (25-150 mM sodium chloride) was governed by the balance between the driving force and the constraints against swelling of the myofibrils. Washing the chicken mince with moderate salt at neutral pH increased electrostatic repulsive force and the osmotic potential of myofibrillar proteins, and also solubilized the constraint components associated with myofibrillar structure. Therefore, improved water-holding capacity was observed. By lowering the pH of chicken mince previously extracted with moderate salt at neutral pH to 6.3, the improved WHC was lost, suggesting that pH might be a dominant factor in controlling WHC. Examining the aqueous phase (press juice) after centrifugation showed that pH adjustment from 5.8 to 7.0 can selectively solubilize some proteins in chicken muscle. The adjustment of pH to neutrality also improved the cook yield and moisture retention of unwashed chicken muscles. It is hypothesized that these polypeptides represent proteins which are soluble in the aqueous phase of the muscle at neutral pH but much less so at pH 6 and their solubilization in situ may be involved in the water-holding properties of the muscle. After pH adjustment, the net negative charges of muscle proteins increased; and also removed some of the polypeptides that hold structural elements like thick filaments and Z-disks together. Removal of these polypeptides from where they are normally located in the muscle cell structure at neutral pH allows the components of the structural elements, for example, myosin of the thick filaments, to be released or to expand and to pull in water by the increase of electrostatic repulsive forces and the entropy of the system. The proteins thus unfold more extensively during heating resulting in a better cook yield and moisture retention upon heating and pressing. Mechanical force such as mincing and homogenizing, can damage the muscle cell structure, thus removing some of the constraints associated with the cell membrane or the linkage between the myofibrils. Therefore, cook yield and moisture retention after heating and pressing were improved in the ground and homogenized muscle.

Abstract not available