345 - Week 2

Microbial implicit factors

Microbe traits such as growth rate and interactions

Intrinsic factors

Food properties such as nutrients pH aw antimicrobials and structure

Extrinsic factors

Storage conditions such as temperature gases and humidity

Cellular microbes

Living microbes including eukaryotes and prokaryotes

Acellular microbes

Non living infectious agents like viruses and prions

Viruses definition

Infectious particles that replicate only inside living host cells

Virus effect on food

Viruses damage plant tissue allowing secondary microbial infections

Fungi types

Molds are filamentous yeasts are single celled

Fungi traits

Eukaryotic slower growing and tolerate acidic foods

Bacteria traits

Prokaryotic fast growing and often cause slime on foods

Food quality vs safety

Quality refers to spoilage safety refers to pathogens

Microbes in food environment

Different parts of food support different microbes due to nutrients oxygen and water

Water activity definition

Measure of available water for microbial use

Effect of sugar on aw

Sugar binds water and lowers aw

Effect of salt on aw

Salt lowers aw strongly because ions bind water molecules

Ice effect on aw

Frozen water is unavailable for microbes and reduces aw

Osmotic stress

Low aw pulls water from cells and inhibits or kills them

Microbial adaptations to low aw

Cells accumulate compatible solutes to prevent water loss

pH in foods

Most microbes prefer near neutral pH and acidic foods spoil slower

Microbial adaptations to low pH

Cells use buffers proton pumps and H plus consuming reactions

Temperature and microbial growth

Cold slows metabolism heat kills microbes

Cold environment adaptations

Cells increase unsaturated fatty acids to keep membranes fluid

Heat environment adaptations

Cells increase saturated fatty acids to prevent membrane melting

Hurdle technology

Using multiple inhibitory factors like pH aw and temperature at once

Requirements for bacterial growth

Bacteria need nutrients water temperature oxygen level and near neutral pH

Requirements for fungal growth

Fungi grow slower tolerate acidity and need moderate aw

Requirements for viral growth

Viruses need host cells not nutrients or aw

Aerobic vs anaerobic microbes

Aerobes use oxygen anaerobes use fermentation or alternate electron acceptors

Food spoilage microbes

Microbes that degrade food and shorten shelf life

Food pathogens

Microbes that cause illness when present in food

Food fermenters

Beneficial microbes that produce desirable changes in food

PCR basic steps

Denature DNA anneal primers and extend strands for amplification

SSU rRNA method

Amplifies and sequences SSU genes to identify and compare microbes

Choosing research method

Select method based on whether you need live cells DNA pH aw or growth characteristics

Predicting microbial changes

Changing aw pH temperature or nutrients alters which microbes can grow

Evaluating experimental design

Assess controls variables and whether results answer the research question