Theoretical and practical considerations in quality control
Theory tells QC what to measure and how to judge it (attributes, standards, sampling, statistics). Practice decides where, by whom and at what cost checks are made.
Theoretical considerations
| Consideration | Meaning | Example |
|---|---|---|
| Quality attributes | Decide which sensory, hidden and quantitative attributes matter for this product | Colour, crispness and oil content of potato chips |
| Standards and specifications | Legal standards (FSSAI), company specifications, grade standards (AGMARK, BIS) and buyers' specifications | FSSAI standard for ghee; the company's own limit for salt in chips |
| Methods of evaluation | Subjective: sensory panels. Objective: physical (colour meter, texture analyser), chemical (moisture, fat, acidity), microbiological (total plate count, coliforms) | Peroxide value of frying oil; a liking test for a new biscuit |
| Sampling | Testing every unit is costly and many tests destroy the sample, so a representative random sample is drawn under a sampling plan | Random samples from a lot of rice bags |
| Statistical quality control (SQC) | Control charts separate chance variation from assignable causes; acceptance sampling decides whether to accept or reject a lot | Control chart for the net weight of 500 g rava packets |
Sensory (subjective) tests
| Test | Question it answers | Example |
|---|---|---|
| Difference tests: paired comparison, duo-trio, triangle | Is there a difference people can detect? | Triangle test: has a new oil changed the taste of murukku? |
| Ranking test | Which sample is more sweet, more salty or more liked? | Rank four payasam samples for sweetness |
| Hedonic scale (9-point) | How much do people like it? | 9 = like extremely, 1 = dislike extremely, for a millet laddu |
| Descriptive analysis | Which attributes does it have, and how strong are they? | A trained panel profiles the texture of idli |
Worked example: control chart for net weight
A unit packs rava in 500 g packets. Over 20 days, the mean of the daily sample means is 502 g and the standard deviation of the sample means is 2 g. Find the control limits, and judge a day whose sample mean is 495 g.
- Centre line (CL) = mean of the sample means = 502 g.
- Upper control limit (UCL) = 502 + (3 × 2) = 502 + 6 = 508 g.
- Lower control limit (LCL) = 502 − (3 × 2) = 502 − 6 = 496 g.
- Today's mean, 495 g, is below the LCL of 496 g.
The control limits are 496 g and 508 g. The 495 g point lies outside them, so the process is out of control: look for an assignable cause (e.g. a worn filler or a wrong setting) and correct it. All values are whole grams, so no rounding was needed.
Practical considerations
- Where to check: fix control points at receiving, at critical process steps, on the finished product, in storage and at dispatch.
- Speed: use rapid tests on the line (platform tests for milk, a refractometer for Brix) so the process can be corrected at once; slower laboratory tests confirm.
- Laboratory and equipment: a clean laboratory, calibrated instruments, standard methods (FSSAI manuals of analysis, BIS, AOAC).
- People: trained analysts, an independent QC head who reports to top management, a screened and trained sensory panel.
- Cost of quality: prevention costs (training, GMP) and appraisal costs (testing) are balanced against failure costs (rejects inside the plant; complaints and recalls outside).
- Records and feedback: results must reach production quickly; records support traceability and audits.
- Legal and customer needs: the law sets the minimum; buyers may ask for more.
Chance vs assignable causes of variation
A point outside the control limits signals an assignable cause.
In a school noon-meal or hospital kitchen, the same ideas work on a small scale: standard recipes are the specification, portion weights are the sample checks, and a simple tasting panel is the sensory test.
2 marks: “What is a control chart?” A graph of a quality measurement over time with a centre line and upper and lower control limits (mean ± 3 SD). Points outside the limits show an assignable cause.
5 marks: “Explain the theoretical considerations in quality control.” Quality attributes; standards and specifications; subjective and objective methods; sampling; statistical quality control with control charts; one example each.
10 marks: “Discuss the theoretical and practical considerations in food quality control.” Theory table; sensory tests; control chart example; practical points: control points, speed, laboratory, people, cost of quality, records, legal needs; close.
Theory is A-S-M-S-S: Attributes, Standards, Methods, Sampling, Statistics. Practice asks: where, how fast, with what, by whom, at what cost?