FOOD 4150 Lecture 8

Milk as a Growth Media

• major nutritional components:

  • proteins

  • fats

  • carbon and non-protein nitrogen sources

  • minerals and micronutrients

• main carb: lactose

• pH: near neutrality

• aw: high 0.9

Dairy Products Source of Bacteria

• soil

• water

• animals

• plant material

• milking equipment

• utensils

• storage tanks

Dairy Sources of Spoilage Microorganisms

• environmental contaminants represent a significant percentage of spoilage microflora

• spore-forming bactera are abundant in dust, dairy feed concentrates, forages

• often present on the skin and hair of catlle

• storage and transportation

Spoilage Psychrotrophs

• off flavours and odors

• texture and appearance changes

• protein:

  • bitter and putrid flavors

  • coagulation

• fats

  • rancid and fruity flavours

Spoilage Organisms

• some pseudomonas sp can grow at low temps (3-7C)

• hydrolyze and use large molecules of proteins and lipids for growth

• proteolytic and lipolytic

• other important microorganisms: bacillus

Fermentative Spoilage

• lactic acid bacteria (LAB)

  • lactobacillus

  • lactococcus

  • leuconostoc

  • enterococcus

  • pediococcus

  • streptococcus

Milk

• raw milk cannot be sold

• minimum heat requirement for fluid milk: 72 C for 15s

• pasteurization kills all pathogens, not necessarily all spoilage

• pathogens:

  • salmonella

  • listeria monocytogenes

  • brucella

  • C. Coli

Preservation

• pasteurization will eliminate the risk from:

  • most psychrotrophic microbes

  • coliforms

  • leuconostocs and lactobacilli

  • recontamination can occur from the equipment, spoilage occurs at 10^6 -10^7 CFU/ml

Cultured Products

• starter: lactic acid bateria

• unsatisfactory products: imbalance of the culture

• improper temp or ripening time

• infection of the culture

• presence of inhibitors or microbial contamination

Spoilage of Plant Materials

• active spoilage: caused by plant pathogenic microorganisms

• passive spoilage: wound induced, facilitates pathogen colonization

Internal Defense Mechanisms of Plant

• plant immune system (hydrogen peroxide)

• epidermal layer: skin,peel, testa

• internal tissue: pectin acid, pectinic acid

• physiological state: dehydration results in skin breaks

• microbial enzymes: pectinases, cellulases, proteases

Microbial Biota

• natural flora: bacteria, yeast, molds

• post harvest spoilage:

  • land use

  • water sources

  • processing

  • short and long term preservation

Pathogens Associated with Fruits and Vegetables

• salmonella

• shigella

• e.coli

• hepatitis

• norovirus

• cyclospora

• cryptosporidium